Either:

chmod 555 scriptname (gives everyone read/execute permission) ^[14]

or

chmod +rx scriptname (gives everyone read/execute permission)

chmod u+rx scriptname (gives only the script owner read/execute permission)

$

Let us carefully distinguish between the name of a variable and its value. If variable1 is the name of a variable, then $variable1 is a reference to its value, the data item it contains. ^[25]

bash$ variable1=23


bash$ echo variable1
variable1

bash$ echo $variable1
23

The only times a variable appears “naked” -- without the $ prefix -- is when declared or assigned, when unset, when exported, in an arithmetic expression within double parentheses (( ... )), or in the special case of a variable representing a signal (see Example 32.5, “Trapping at exit”). Assignment may be with an = (as in var1=27), in a read statement, and at the head of a loop (for var2 in 1 2 3).

Enclosing a referenced value in double quotes (" ... ") does not interfere with variable substitution. This is called partial quoting, sometimes referred to as “weak quoting.” Using single quotes (' ... ') causes the variable name to be used literally, and no substitution will take place. This is full quoting, sometimes referred to as 'strong quoting.' See Chapter 5, Quoting for a detailed discussion.

Note that $variable is actually a simplified form of ${variable}. In contexts where the $variable syntax causes an error, the longer form may work (see Section 2, “Parameter Substitution”, below).

Example 4.1. Variable assignment and substitution

#!/bin/bash
# ex9.sh

# Variables: assignment and substitution

a=375
hello=$a
#   ^ ^

#-------------------------------------------------------------------------
# No space permitted on either side of = sign when initializing variables.
# What happens if there is a space?

#  "VARIABLE =value"
#           ^
#% Script tries to run "VARIABLE" command with one argument, "=value".

#  "VARIABLE= value"
#            ^
#% Script tries to run "value" command with
#+ the environmental variable "VARIABLE" set to "".
#-------------------------------------------------------------------------


echo hello    # hello
# Not a variable reference, just the string "hello" ...

echo $hello   # 375
#    ^          This *is* a variable reference.
echo ${hello} # 375
#               Likewise a variable reference, as above.

# Quoting . . .
echo "$hello"    # 375
echo "${hello}"  # 375

echo

hello="A B  C   D"
echo $hello   # A B C D
echo "$hello" # A B  C   D
# As we see, echo $hello   and   echo "$hello"   give different results.
# =======================================
# Quoting a variable preserves whitespace.
# =======================================

echo

echo '$hello'  # $hello
#    ^      ^
#  Variable referencing disabled (escaped) by single quotes,
#+ which causes the "$" to be interpreted literally.

# Notice the effect of different types of quoting.


hello=    # Setting it to a null value.
echo "\$hello (null value) = $hello"      # $hello (null value) =
#  Note that setting a variable to a null value is not the same as
#+ unsetting it, although the end result is the same (see below).

# --------------------------------------------------------------

#  It is permissible to set multiple variables on the same line,
#+ if separated by white space.
#  Caution, this may reduce legibility, and may not be portable.

var1=21  var2=22  var3=$V3
echo
echo "var1=$var1   var2=$var2   var3=$var3"

# May cause problems with legacy versions of "sh" . . .

# --------------------------------------------------------------

echo; echo

numbers="one two three"
#           ^   ^
other_numbers="1 2 3"
#               ^ ^
#  If there is whitespace embedded within a variable,
#+ then quotes are necessary.
#  other_numbers=1 2 3                  # Gives an error message.
echo "numbers = $numbers"
echo "other_numbers = $other_numbers"   # other_numbers = 1 2 3
#  Escaping the whitespace also works.
mixed_bag=2\ ---\ Whatever
#           ^    ^ Space after escape (\).

echo "$mixed_bag"         # 2 --- Whatever

echo; echo

echo "uninitialized_variable = $uninitialized_variable"
# Uninitialized variable has null value (no value at all!).
uninitialized_variable=   #  Declaring, but not initializing it --
                          #+ same as setting it to a null value, as above.
echo "uninitialized_variable = $uninitialized_variable"
                          # It still has a null value.

uninitialized_variable=23       # Set it.
unset uninitialized_variable    # Unset it.
echo "uninitialized_variable = $uninitialized_variable"
                                # uninitialized_variable =
                                # It still has a null value.
echo

exit 0

Caution

An uninitialized variable has a “null” value -- no assigned value at all (not zero!).

if [ -z "$unassigned" ]
then
  echo "\$unassigned is NULL."
fi     # $unassigned is NULL.

Using a variable before assigning a value to it may cause problems. It is nevertheless possible to perform arithmetic operations on an uninitialized variable.

echo "$uninitialized"                                # (blank line)
let "uninitialized += 5"                             # Add 5 to it.
echo "$uninitialized"                                # 5

#  Conclusion:
#  An uninitialized variable has no value,
#+ however it evaluates as 0 in an arithmetic operation.

See also Example 15.23, “A (useless) script that sources itself”.

=

the assignment operator (no space before and after)

Caution

Do not confuse this with = and -eq, which test, rather than assign!

Note that = can be either an assignment or a test operator, depending on context.

Example 4.2. Plain Variable Assignment

#!/bin/bash
# Naked variables

echo

# When is a variable "naked", i.e., lacking the '$' in front?
# When it is being assigned, rather than referenced.

# Assignment
a=879
echo "The value of \"a\" is $a."

# Assignment using 'let'
let a=16+5
echo "The value of \"a\" is now $a."

echo

# In a 'for' loop (really, a type of disguised assignment):
echo -n "Values of \"a\" in the loop are: "
for a in 7 8 9 11
do
  echo -n "$a "
done

echo
echo

# In a 'read' statement (also a type of assignment):
echo -n "Enter \"a\" "
read a
echo "The value of \"a\" is now $a."

echo

exit 0

Example 4.3. Variable Assignment, plain and fancy

#!/bin/bash

a=23              # Simple case
echo $a
b=$a
echo $b

# Now, getting a little bit fancier (command substitution).

a=`echo Hello!`   # Assigns result of 'echo' command to 'a' ...
echo $a
#  Note that including an exclamation mark (!) within a
#+ command substitution construct will not work from the command-line,
#+ since this triggers the Bash "history mechanism."
#  Inside a script, however, the history functions are disabled by default.

a=`ls -l`         # Assigns result of 'ls -l' command to 'a'
echo $a           # Unquoted, however, it removes tabs and newlines.
echo
echo "$a"         # The quoted variable preserves whitespace.
                  # (See the chapter on "Quoting.")

exit 0

Variable assignment using the $(...) mechanism (a newer method than backquotes). This is likewise a form of command substitution.

# From /etc/rc.d/rc.local
R=$(cat /etc/redhat-release)
arch=$(uname -m)

Local variables

Variables visible only within a code block or function (see also local variables in functions)

Environmental variables

Variables that affect the behavior of the shell and user interface

Note

In a more general context, each process has an “environment”, that is, a group of variables that the process may reference. In this sense, the shell behaves like any other process.

Every time a shell starts, it creates shell variables that correspond to its own environmental variables. Updating or adding new environmental variables causes the shell to update its environment, and all the shell's child processes (the commands it executes) inherit this environment.

Caution

The space allotted to the environment is limited. Creating too many environmental variables or ones that use up excessive space may cause problems.

bash$ eval "`seq 10000 | sed -e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`"

bash$ du
bash: /usr/bin/du: Argument list too long
	          

Note: this “error” has been fixed, as of kernel version 2.6.23.

(Thank you, Stéphane Chazelas for the clarification, and for providing the above example.)

If a script sets environmental variables, they need to be “exported,” that is, reported to the environment local to the script. This is the function of the export command.

Note

A script can export variables only to child processes, that is, only to commands or processes which that particular script initiates. A script invoked from the command-line cannot export variables back to the command-line environment. Child processes cannot export variables back to the parent processes that spawned them.

Definition: A child process is a subprocess launched by another process, its parent.

Positional parameters

Arguments passed to the script from the command line ^[26] : $0, $1, $2, $3 . . .

$0 is the name of the script itself, $1 is the first argument, $2 the second, $3 the third, and so forth. ^[27] After $9, the arguments must be enclosed in brackets, for example, ${10}, ${11}, ${12}.

The special variables $* and $@ denote all the positional parameters.

Example 4.5. Positional Parameters

#!/bin/bash

# Call this script with at least 10 parameters, for example
# ./scriptname 1 2 3 4 5 6 7 8 9 10
MINPARAMS=10

echo

echo "The name of this script is \"$0\"."
# Adds ./ for current directory
echo "The name of this script is \"`basename $0`\"."
# Strips out path name info (see 'basename')

echo

if [ -n "$1" ]              # Tested variable is quoted.
then
 echo "Parameter #1 is $1"  # Need quotes to escape #
fi 

if [ -n "$2" ]
then
 echo "Parameter #2 is $2"
fi 

if [ -n "$3" ]
then
 echo "Parameter #3 is $3"
fi 

# ...


if [ -n "${10}" ]  # Parameters > $9 must be enclosed in {brackets}.
then
 echo "Parameter #10 is ${10}"
fi 

echo "-----------------------------------"
echo "All the command-line parameters are: "$*""

if [ $# -lt "$MINPARAMS" ]
then
  echo
  echo "This script needs at least $MINPARAMS command-line arguments!"
fi  

echo

exit 0

Bracket notation for positional parameters leads to a fairly simple way of referencing the last argument passed to a script on the command-line. This also requires indirect referencing.

args=$#           # Number of args passed.
lastarg=${!args}
# Note: This is an *indirect reference* to $args ...


# Or:       lastarg=${!#}             (Thanks, Chris Monson.)
# This is an *indirect reference* to the $# variable.
# Note that lastarg=${!$#} doesn't work.

Some scripts can perform different operations, depending on which name they are invoked with. For this to work, the script needs to check $0, the name it was invoked by. ^[28] There must also exist symbolic links to all the alternate names of the script. See Example 16.2, “Hello or Good-bye”.

Tip

If a script expects a command-line parameter but is invoked without one, this may cause a null variable assignment, generally an undesirable result. One way to prevent this is to append an extra character to both sides of the assignment statement using the expected positional parameter.

variable1_=$1_  # Rather than variable1=$1
# This will prevent an error, even if positional parameter is absent.

critical_argument01=$variable1_

# The extra character can be stripped off later, like so.
variable1=${variable1_/_/}
# Side effects only if $variable1_ begins with an underscore.
# This uses one of the parameter substitution templates discussed later.
# (Leaving out the replacement pattern results in a deletion.)

#  A more straightforward way of dealing with this is
#+ to simply test whether expected positional parameters have been passed.
if [ -z $1 ]
then
  exit $E_MISSING_POS_PARAM
fi


#  However, as Fabian Kreutz points out,
#+ the above method may have unexpected side-effects.
#  A better method is parameter substitution:
#         ${1:-$DefaultVal}
#  See the "Parameter Substition" section
#+ in the "Variables Revisited" chapter.

---

Example 4.6. wh, whois domain name lookup

#!/bin/bash
# ex18.sh

# Does a 'whois domain-name' lookup on any of 3 alternate servers:
#                    ripe.net, cw.net, radb.net

# Place this script -- renamed 'wh' -- in /usr/local/bin

# Requires symbolic links:
# ln -s /usr/local/bin/wh /usr/local/bin/wh-ripe
# ln -s /usr/local/bin/wh /usr/local/bin/wh-apnic
# ln -s /usr/local/bin/wh /usr/local/bin/wh-tucows

E_NOARGS=75


if [ -z "$1" ]
then
  echo "Usage: `basename $0` [domain-name]"
  exit $E_NOARGS
fi

# Check script name and call proper server.
case `basename $0` in    # Or:    case ${0##*/} in
    "wh"       ) whois $1@whois.tucows.com;;
    "wh-ripe"  ) whois $1@whois.ripe.net;;
    "wh-apnic" ) whois $1@whois.apnic.net;;
    "wh-cw"    ) whois $1@whois.cw.net;;
    *          ) echo "Usage: `basename $0` [domain-name]";;
esac 

exit $?

---

The shift command reassigns the positional parameters, in effect shifting them to the left one notch.

$1 <--- $2, $2 <--- $3, $3 <--- $4, etc.

The old $1 disappears, but $0 (the script name) does not change. If you use a large number of positional parameters to a script, shift lets you access those past 10, although {bracket} notation also permits this.

Example 4.7. Using shift

#!/bin/bash
# shft.sh: Using 'shift' to step through all the positional parameters.

#  Name this script something like shft.sh,
#+ and invoke it with some parameters.
#+ For example:
#             sh shft.sh a b c def 83 barndoor

until [ -z "$1" ]  # Until all parameters used up . . .
do
  echo -n "$1 "
  shift
done

echo               # Extra linefeed.

# But, what happens to the "used-up" parameters?
echo "$2"
#  Nothing echoes!
#  When $2 shifts into $1 (and there is no $3 to shift into $2)
#+ then $2 remains empty.
#  So, it is not a parameter *copy*, but a *move*.

exit

#  See also the echo-params.sh script for a "shiftless"
#+ alternative method of stepping through the positional params.

The shift command can take a numerical parameter indicating how many positions to shift.

#!/bin/bash
# shift-past.sh

shift 3    # Shift 3 positions.
#  n=3; shift $n
#  Has the same effect.

echo "$1"

exit 0

# ======================== #


$ sh shift-past.sh 1 2 3 4 5
4

#  However, as Eleni Fragkiadaki, points out,
#+ attempting a 'shift' past the number of
#+ positional parameters ($#) returns an exit status of 1,
#+ and the positional parameters themselves do not change.
#  This means possibly getting stuck in an endless loop. . . .
#  For example:
#      until [ -z "$1" ]
#      do
#         echo -n "$1 "
#         shift 20    #  If less than 20 pos params,
#      done           #+ then loop never ends!
#
# When in doubt, add a sanity check. . . .
#           shift 20 || break
#                    ^^^^^^^^

Note

The shift command works in a similar fashion on parameters passed to a function. See Example 36.18, “Return value trickery”.

Special meanings of certain escaped characters

used with echo and sed

\n

means newline

\r

means return

\t

means tab

\v

means vertical tab

\b

means backspace

\a

means alert (beep or flash)

\0xx

translates to the octal ASCII equivalent of 0nn, where nn is a string of digits

Important

The $' ... ' quoted string-expansion construct is a mechanism that uses escaped octal or hex values to assign ASCII characters to variables, e.g., quote=$'\042'.

Example 5.2. Escaped Characters

#!/bin/bash
# escaped.sh: escaped characters

#############################################################
### First, let's show some basic escaped-character usage. ###
#############################################################

# Escaping a newline.
# ------------------

echo ""

echo "This will print
as two lines."
# This will print
# as two lines.

echo "This will print \
as one line."
# This will print as one line.

echo; echo

echo "============="


echo "\v\v\v\v"      # Prints \v\v\v\v literally.
# Use the -e option with 'echo' to print escaped characters.
echo "============="
echo "VERTICAL TABS"
echo -e "\v\v\v\v"   # Prints 4 vertical tabs.
echo "=============="

echo "QUOTATION MARK"
echo -e "\042"       # Prints " (quote, octal ASCII character 42).
echo "=============="



# The $'\X' construct makes the -e option unnecessary.

echo; echo "NEWLINE and (maybe) BEEP"
echo $'\n'           # Newline.
echo $'\a'           # Alert (beep).
                     # May only flash, not beep, depending on terminal.

# We have seen $'\nnn" string expansion, and now . . .

# =================================================================== #
# Version 2 of Bash introduced the $'\nnn' string expansion construct.
# =================================================================== #

echo "Introducing the \$\' ... \' string-expansion construct . . . "
echo ". . . featuring more quotation marks."

echo $'\t \042 \t'   # Quote (") framed by tabs.
# Note that  '\nnn' is an octal value.

# It also works with hexadecimal values, in an $'\xhhh' construct.
echo $'\t \x22 \t'  # Quote (") framed by tabs.
# Thank you, Greg Keraunen, for pointing this out.
# Earlier Bash versions allowed '\x022'.

echo


# Assigning ASCII characters to a variable.
# ----------------------------------------
quote=$'\042'        # " assigned to a variable.
echo "$quote Quoted string $quote and this lies outside the quotes."

echo

# Concatenating ASCII chars in a variable.
triple_underline=$'\137\137\137'  # 137 is octal ASCII code for '_'.
echo "$triple_underline UNDERLINE $triple_underline"

echo

ABC=$'\101\102\103\010'           # 101, 102, 103 are octal A, B, C.
echo $ABC

echo

escape=$'\033'                    # 033 is octal for escape.
echo "\"escape\" echoes as $escape"
#                                   no visible output.

echo

exit 0

A more elaborate example:

Example 5.3. Detecting key-presses

#!/bin/bash
# Author: Sigurd Solaas, 20 Apr 2011
# Used in ABS Guide with permission.
# Requires version 4.2+ of Bash.

key="no value yet"
while true; do
  clear
  echo "Bash Extra Keys Demo. Keys to try:"
  echo
  echo "* Insert, Delete, Home, End, Page_Up and Page_Down"
  echo "* The four arrow keys"
  echo "* Tab, enter, escape, and space key"
  echo "* The letter and number keys, etc."
  echo
  echo "    d = show date/time"
  echo "    q = quit"
  echo "================================"
  echo

 # Convert the separate home-key to home-key_num_7:
 if [ "$key" = $'\x1b\x4f\x48' ]; then
  key=$'\x1b\x5b\x31\x7e'
  #   Quoted string-expansion construct. 
 fi

 # Convert the separate end-key to end-key_num_1.
 if [ "$key" = $'\x1b\x4f\x46' ]; then
  key=$'\x1b\x5b\x34\x7e'
 fi

 case "$key" in
  $'\x1b\x5b\x32\x7e')  # Insert
   echo Insert Key
  ;;
  $'\x1b\x5b\x33\x7e')  # Delete
   echo Delete Key
  ;;
  $'\x1b\x5b\x31\x7e')  # Home_key_num_7
   echo Home Key
  ;;
  $'\x1b\x5b\x34\x7e')  # End_key_num_1
   echo End Key
  ;;
  $'\x1b\x5b\x35\x7e')  # Page_Up
   echo Page_Up
  ;;
  $'\x1b\x5b\x36\x7e')  # Page_Down
   echo Page_Down
  ;;
  $'\x1b\x5b\x41')  # Up_arrow
   echo Up arrow
  ;;
  $'\x1b\x5b\x42')  # Down_arrow
   echo Down arrow
  ;;
  $'\x1b\x5b\x43')  # Right_arrow
   echo Right arrow
  ;;
  $'\x1b\x5b\x44')  # Left_arrow
   echo Left arrow
  ;;
  $'\x09')  # Tab
   echo Tab Key
  ;;
  $'\x0a')  # Enter
   echo Enter Key
  ;;
  $'\x1b')  # Escape
   echo Escape Key
  ;;
  $'\x20')  # Space
   echo Space Key
  ;;
  d)
   date
  ;;
  q)
  echo Time to quit...
  echo
  exit 0
  ;;
  *)
   echo You pressed: \'"$key"\'
  ;;
 esac

 echo
 echo "================================"

 unset K1 K2 K3
 read -s -N1 -p "Press a key: "
 K1="$REPLY"
 read -s -N2 -t 0.001
 K2="$REPLY"
 read -s -N1 -t 0.001
 K3="$REPLY"
 key="$K1$K2$K3"

done

exit $?

See also Example 37.1, “String expansion”.

\"

gives the quote its literal meaning

echo "Hello"                     # Hello
echo "\"Hello\" ... he said."    # "Hello" ... he said.

\$

gives the dollar sign its literal meaning (variable name following \$ will not be referenced)

echo "\$variable01"           # $variable01
echo "The book cost \$7.98."  # The book cost $7.98.

\\

gives the backslash its literal meaning

echo "\\"  # Results in \

# Whereas . . .

echo "\"   # Invokes secondary prompt from the command-line.
           # In a script, gives an error message.

# However . . .

echo '\'   # Results in \

Else if and elif

elif

elif is a contraction for else if. The effect is to nest an inner if/then construct within an outer one.

if [ condition1 ]
then
   command1
   command2
   command3
elif [ condition2 ]
# Same as else if
then
   command4
   command5
else
   default-command
fi

Returns true if...

-e

file exists

-a

file exists

This is identical in effect to -e. It has been “deprecated,” ^[34] and its use is discouraged.

-f

file is a regular file (not a directory or device file)

-s

file is not zero size

-d

file is a directory

-b

file is a block device

-c

file is a character device

device0="/dev/sda2"    # /   (root directory)
if [ -b "$device0" ]
then
  echo "$device0 is a block device."
fi

# /dev/sda2 is a block device.



device1="/dev/ttyS1"   # PCMCIA modem card.
if [ -c "$device1" ]
then
  echo "$device1 is a character device."
fi

# /dev/ttyS1 is a character device.

-p

file is a pipe

function show_input_type()
{
   [ -p /dev/fd/0 ] && echo PIPE || echo STDIN
}

show_input_type "Input"                           # STDIN
echo "Input" | show_input_type                    # PIPE

# This example courtesy of Carl Anderson.

-h

file is a symbolic link

-L

file is a symbolic link

-S

file is a socket

-t

file (descriptor) is associated with a terminal device

This test option may be used to check whether the stdin [ -t 0 ] or stdout [ -t 1 ] in a given script is a terminal.

-r

file has read permission (for the user running the test)

-w

file has write permission (for the user running the test)

-x

file has execute permission (for the user running the test)

-g

set-group-id (sgid) flag set on file or directory

If a directory has the sgid flag set, then a file created within that directory belongs to the group that owns the directory, not necessarily to the group of the user who created the file. This may be useful for a directory shared by a workgroup.

-u

set-user-id (suid) flag set on file

A binary owned by root with set-user-id flag set runs with root privileges, even when an ordinary user invokes it. ^[35] This is useful for executables (such as pppd and cdrecord) that need to access system hardware. Lacking the suid flag, these binaries could not be invoked by a non-root user.

	      -rwsr-xr-t    1 root       178236 Oct  2  2000 /usr/sbin/pppd
	      

A file with the suid flag set shows an s in its permissions.

-k

sticky bit set

Commonly known as the sticky bit, the save-text-mode flag is a special type of file permission. If a file has this flag set, that file will be kept in cache memory, for quicker access. ^[36] If set on a directory, it restricts write permission. Setting the sticky bit adds a t to the permissions on the file or directory listing. This restricts altering or deleting specific files in that directory to the owner of those files.

	      drwxrwxrwt    7 root         1024 May 19 21:26 tmp/
	      

If a user does not own a directory that has the sticky bit set, but has write permission in that directory, she can only delete those files that she owns in it. This keeps users from inadvertently overwriting or deleting each other's files in a publicly accessible directory, such as /tmp. (The owner of the directory or root can, of course, delete or rename files there.)

-O

you are owner of file

-G

group-id of file same as yours

-N

file modified since it was last read

f1 -nt f2

file f1 is newer than f2

f1 -ot f2

file f1 is older than f2

f1 -ef f2

files f1 and f2 are hard links to the same file

!

“not” -- reverses the sense of the tests above (returns true if condition absent).

integer comparison

-eq

is equal to

if [ "$a" -eq "$b" ]

-ne

is not equal to

if [ "$a" -ne "$b" ]

-gt

is greater than

if [ "$a" -gt "$b" ]

-ge

is greater than or equal to

if [ "$a" -ge "$b" ]

-lt

is less than

if [ "$a" -lt "$b" ]

-le

is less than or equal to

if [ "$a" -le "$b" ]

<

is less than (within double parentheses)

(("$a" < "$b"))

<=

is less than or equal to (within double parentheses)

(("$a" <= "$b"))

>

is greater than (within double parentheses)

(("$a" > "$b"))

>=

is greater than or equal to (within double parentheses)

(("$a" >= "$b"))

string comparison

=

is equal to

if [ "$a" = "$b" ]

Caution

Note the whitespace framing the =.

if [ "$a"="$b" ] is not equivalent to the above.

==

is equal to

if [ "$a" == "$b" ]

This is a synonym for =.

Note

The == comparison operator behaves differently within a double-brackets test than within single brackets.

[[ $a == z* ]]   # True if $a starts with an "z" (pattern matching).
[[ $a == "z*" ]] # True if $a is equal to z* (literal matching).

[ $a == z* ]     # File globbing and word splitting take place.
[ "$a" == "z*" ] # True if $a is equal to z* (literal matching).

# Thanks, Stéphane Chazelas

!=

is not equal to

if [ "$a" != "$b" ]

This operator uses pattern matching within a [[ ... ]] construct.

<

is less than, in ASCII alphabetical order

if [[ "$a" < "$b" ]]

if [ "$a" \< "$b" ]

Note that the “<” needs to be escaped within a [ ] construct.

>

is greater than, in ASCII alphabetical order

if [[ "$a" > "$b" ]]

if [ "$a" \> "$b" ]

Note that the “>” needs to be escaped within a [ ] construct.

See Example 27.11, “The Bubble Sort” for an application of this comparison operator.

-z

string is null, that is, has zero length

 String=''   # Zero-length ("null") string variable.

if [ -z "$String" ]
then
  echo "\$String is null."
else
  echo "\$String is NOT null."
fi     # $String is null.

-n

string is not null.

Caution

The -n test requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets (see Example 7.6, “Testing whether a string is null”) normally works, however, this is an unsafe practice. Always quote a tested string. ^[37]

compound comparison

-a

logical and

exp1 -a exp2 returns true if both exp1 and exp2 are true.

-o

logical or

exp1 -o exp2 returns true if either exp1 or exp2 is true.

assignment

variable assignment

Initializing or changing the value of a variable

=

All-purpose assignment operator, which works for both arithmetic and string assignments.

var=27
category=minerals  # No spaces allowed after the "=".

Caution

Do not confuse the “=” assignment operator with the = test operator.

#   =  as a test operator

if [ "$string1" = "$string2" ]
then
   command
fi

#  if [ "X$string1" = "X$string2" ] is safer,
#+ to prevent an error message should one of the variables be empty.
#  (The prepended "X" characters cancel out.)

arithmetic operators

+

plus

-

minus

*

multiplication

/

division

**

exponentiation

# Bash, version 2.02, introduced the "**" exponentiation operator.

let "z=5**3"    # 5 * 5 * 5
echo "z = $z"   # z = 125

%

modulo, or mod (returns the remainder of an integer division operation)

bash$ expr 5 % 3
2
	      

5/3 = 1, with remainder 2

This operator finds use in, among other things, generating numbers within a specific range (see Example 9.11, “Generating random numbers” and Example 9.15, “Rolling a single die with RANDOM”) and formatting program output (see Example 27.16, “Complex array application: Exploring a weird mathematical series” and Example A.6, “Collatz series”). It can even be used to generate prime numbers, (see Example A.15, “Generating prime numbers using the modulo operator”). Modulo turns up surprisingly often in numerical recipes.

Example 8.1. Greatest common divisor

#!/bin/bash
# gcd.sh: greatest common divisor
#         Uses Euclid's algorithm

#  The "greatest common divisor" (gcd) of two integers
#+ is the largest integer that will divide both, leaving no remainder.

#  Euclid's algorithm uses successive division.
#    In each pass,
#+      dividend <---  divisor
#+      divisor  <---  remainder
#+   until remainder = 0.
#    The gcd = dividend, on the final pass.
#
#  For an excellent discussion of Euclid's algorithm, see
#+ Jim Loy's site, http://www.jimloy.com/number/euclids.htm.


# ------------------------------------------------------
# Argument check
ARGS=2
E_BADARGS=85

if [ $# -ne "$ARGS" ]
then
  echo "Usage: `basename $0` first-number second-number"
  exit $E_BADARGS
fi
# ------------------------------------------------------


gcd ()
{

  dividend=$1             #  Arbitrary assignment.
  divisor=$2              #! It doesn't matter which of the two is larger.
                          #  Why not?

  remainder=1             #  If an uninitialized variable is used inside
                          #+ test brackets, an error message results.

  until [ "$remainder" -eq 0 ]
  do    #  ^^^^^^^^^^  Must be previously initialized!
    let "remainder = $dividend % $divisor"
    dividend=$divisor     # Now repeat with 2 smallest numbers.
    divisor=$remainder
  done                    # Euclid's algorithm

}                         # Last $dividend is the gcd.


gcd $1 $2

echo; echo "GCD of $1 and $2 = $dividend"; echo


# Exercises :
# ---------
# 1) Check command-line arguments to make sure they are integers,
#+   and exit the script with an appropriate error message if not.
# 2) Rewrite the gcd () function to use local variables.

exit 0

+=

plus-equal (increment variable by a constant) ^[38]

let "var += 5" results in var being incremented by 5.

-=

minus-equal (decrement variable by a constant)

*=

times-equal (multiply variable by a constant)

let "var *= 4" results in var being multiplied by 4.

/=

slash-equal (divide variable by a constant)

%=

mod-equal (remainder of dividing variable by a constant)

Arithmetic operators often occur in an expr or let expression.

Example 8.2. Using Arithmetic Operations

#!/bin/bash
# Counting to 11 in 10 different ways.

n=1; echo -n "$n "

let "n = $n + 1"   # let "n = n + 1"  also works.
echo -n "$n "


: $((n = $n + 1))
#  ":" necessary because otherwise Bash attempts
#+ to interpret "$((n = $n + 1))" as a command.
echo -n "$n "

(( n = n + 1 ))
#  A simpler alternative to the method above.
#  Thanks, David Lombard, for pointing this out.
echo -n "$n "

n=$(($n + 1))
echo -n "$n "

: $[ n = $n + 1 ]
#  ":" necessary because otherwise Bash attempts
#+ to interpret "$[ n = $n + 1 ]" as a command.
#  Works even if "n" was initialized as a string.
echo -n "$n "

n=$[ $n + 1 ]
#  Works even if "n" was initialized as a string.
#* Avoid this type of construct, since it is obsolete and nonportable.
#  Thanks, Stephane Chazelas.
echo -n "$n "

# Now for C-style increment operators.
# Thanks, Frank Wang, for pointing this out.

let "n++"          # let "++n"  also works.
echo -n "$n "

(( n++ ))          # (( ++n ))  also works.
echo -n "$n "

: $(( n++ ))       # : $(( ++n )) also works.
echo -n "$n "

: $[ n++ ]         # : $[ ++n ] also works
echo -n "$n "

echo

exit 0

bitwise operators

<<

bitwise left shift (multiplies by 2 for each shift position)

<<=

left-shift-equal

let "var <<= 2" results in var left-shifted 2 bits (multiplied by 4)

>>

bitwise right shift (divides by 2 for each shift position)

>>=

right-shift-equal (inverse of <<=)

&

bitwise AND

&=

bitwise AND-equal

|

bitwise OR

|=

bitwise OR-equal

~

bitwise NOT

^

bitwise XOR

^=

bitwise XOR-equal

logical (boolean) operators

!

NOT

if [ ! -f $FILENAME ]
then
  ...

&&

AND

if [ $condition1 ] && [ $condition2 ]
#  Same as:  if [ $condition1 -a $condition2 ]
#  Returns true if both condition1 and condition2 hold true...

if [[ $condition1 && $condition2 ]]    # Also works.
#  Note that && operator not permitted inside brackets
#+ of [ ... ] construct.

Note

&& may also be used, depending on context, in an and list to concatenate commands.

||

OR

if [ $condition1 ] || [ $condition2 ]
# Same as:  if [ $condition1 -o $condition2 ]
# Returns true if either condition1 or condition2 holds true...

if [[ $condition1 || $condition2 ]]    # Also works.
#  Note that || operator not permitted inside brackets
#+ of a [ ... ] construct.

Note

Bash tests the exit status of each statement linked with a logical operator.

Example 8.3. Compound Condition Tests Using && and ||

#!/bin/bash

a=24
b=47

if [ "$a" -eq 24 ] && [ "$b" -eq 47 ]
then
  echo "Test #1 succeeds."
else
  echo "Test #1 fails."
fi

# ERROR:   if [ "$a" -eq 24 && "$b" -eq 47 ]
#+         attempts to execute  ' [ "$a" -eq 24 '
#+         and fails to finding matching ']'.
#
#  Note:  if [[ $a -eq 24 && $b -eq 24 ]]  works.
#  The double-bracket if-test is more flexible
#+ than the single-bracket version.       
#    (The "&&" has a different meaning in line 17 than in line 6.)
#    Thanks, Stephane Chazelas, for pointing this out.


if [ "$a" -eq 98 ] || [ "$b" -eq 47 ]
then
  echo "Test #2 succeeds."
else
  echo "Test #2 fails."
fi


#  The -a and -o options provide
#+ an alternative compound condition test.
#  Thanks to Patrick Callahan for pointing this out.


if [ "$a" -eq 24 -a "$b" -eq 47 ]
then
  echo "Test #3 succeeds."
else
  echo "Test #3 fails."
fi


if [ "$a" -eq 98 -o "$b" -eq 47 ]
then
  echo "Test #4 succeeds."
else
  echo "Test #4 fails."
fi


a=rhino
b=crocodile
if [ "$a" = rhino ] && [ "$b" = crocodile ]
then
  echo "Test #5 succeeds."
else
  echo "Test #5 fails."
fi

exit 0

The && and || operators also find use in an arithmetic context.

bash$ echo $(( 1 && 2 )) $((3 && 0)) $((4 || 0)) $((0 || 0))
1 0 1 0
	      

miscellaneous operators

,

Comma operator

The comma operator chains together two or more arithmetic operations. All the operations are evaluated (with possible side effects. ^[39]

let "t1 = ((5 + 3, 7 - 1, 15 - 4))"
echo "t1 = $t1"           ^^^^^^  # t1 = 11
# Here t1 is set to the result of the last operation. Why?

let "t2 = ((a = 9, 15 / 3))"      # Set "a" and calculate "t2".
echo "t2 = $t2    a = $a"         # t2 = 5    a = 9

The comma operator finds use mainly in for loops. See Example 11.13, “A C-style for loop”.

Builtin variables:

variables affecting bash script behavior

$BASH

The path to the Bash binary itself

bash$ echo $BASH
/bin/bash

$BASH_ENV

An environmental variable pointing to a Bash startup file to be read when a script is invoked

$BASH_SUBSHELL

A variable indicating the subshell level. This is a new addition to Bash, version 3.

See Example 21.1, “Variable scope in a subshell” for usage.

$BASHPID

Process ID of the current instance of Bash. This is not the same as the $$ variable, but it often gives the same result.

bash4$ echo $$
11015


bash4$ echo $BASHPID
11015


bash4$ ps ax | grep bash4
11015 pts/2    R      0:00 bash4
	      

But ...

#!/bin/bash4

echo "\$\$ outside of subshell = $$"                              # 9602
echo "\$BASH_SUBSHELL  outside of subshell = $BASH_SUBSHELL"      # 0
echo "\$BASHPID outside of subshell = $BASHPID"                   # 9602

echo

( echo "\$\$ inside of subshell = $$"                             # 9602
  echo "\$BASH_SUBSHELL inside of subshell = $BASH_SUBSHELL"      # 1
  echo "\$BASHPID inside of subshell = $BASHPID" )                # 9603
  # Note that $$ returns PID of parent process.

$BASH_VERSINFO[n]

A 6-element array containing version information about the installed release of Bash. This is similar to $BASH_VERSION, below, but a bit more detailed.

# Bash version info:

for n in 0 1 2 3 4 5
do
  echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}"
done  

# BASH_VERSINFO[0] = 3                      # Major version no.
# BASH_VERSINFO[1] = 00                     # Minor version no.
# BASH_VERSINFO[2] = 14                     # Patch level.
# BASH_VERSINFO[3] = 1                      # Build version.
# BASH_VERSINFO[4] = release                # Release status.
# BASH_VERSINFO[5] = i386-redhat-linux-gnu  # Architecture
                                            # (same as $MACHTYPE).

$BASH_VERSION

The version of Bash installed on the system

bash$ echo $BASH_VERSION
3.2.25(1)-release
	      

tcsh% echo $BASH_VERSION
BASH_VERSION: Undefined variable.
	      

Checking $BASH_VERSION is a good method of determining which shell is running. $SHELL does not necessarily give the correct answer.

$CDPATH

A colon-separated list of search paths available to the cd command, similar in function to the $PATH variable for binaries. The $CDPATH variable may be set in the local ~/.bashrc file.

bash$ cd bash-doc
bash: cd: bash-doc: No such file or directory


bash$ CDPATH=/usr/share/doc
bash$ cd bash-doc
/usr/share/doc/bash-doc


bash$ echo $PWD
/usr/share/doc/bash-doc
	      

$DIRSTACK

The top value in the directory stack ^[41] (affected by pushd and popd)

This builtin variable corresponds to the dirs command, however dirs shows the entire contents of the directory stack.

$EDITOR

The default editor invoked by a script, usually vi or emacs.

$EUID

“effective” user ID number

Identification number of whatever identity the current user has assumed, perhaps by means of su.

Caution

The $EUID is not necessarily the same as the $UID.

$FUNCNAME

Name of the current function

xyz23 ()
{
  echo "$FUNCNAME now executing."  # xyz23 now executing.
}

xyz23

echo "FUNCNAME = $FUNCNAME"        # FUNCNAME =
                                   # Null value outside a function.

See also Example A.50, “An alternate version of the getopt-simple.sh script”.

$GLOBIGNORE

A list of filename patterns to be excluded from matching in globbing.

$GROUPS

Groups current user belongs to

This is a listing (array) of the group id numbers for current user, as recorded in /etc/passwd and /etc/group.

root# echo $GROUPS
0


root# echo ${GROUPS[1]}
1


root# echo ${GROUPS[5]}
6
	      

$HOME

Home directory of the user, usually /home/username (see Example 10.7, “Using parameter substitution and error messages”)

$HOSTNAME

The hostname command assigns the system host name at bootup in an init script. However, the gethostname() function sets the Bash internal variable $HOSTNAME. See also Example 10.7, “Using parameter substitution and error messages”.

$HOSTTYPE

host type

Like $MACHTYPE, identifies the system hardware.

bash$ echo $HOSTTYPE
i686

$IFS

internal field separator

This variable determines how Bash recognizes fields, or word boundaries, when it interprets character strings.

$IFS defaults to whitespace (space, tab, and newline), but may be changed, for example, to parse a comma-separated data file. Note that $* uses the first character held in $IFS. See Example 5.1, “Echoing Weird Variables”.

bash$ echo "$IFS"



(With $IFS set to default, a blank line displays.)
	      


bash$ echo "$IFS" | cat -vte
 ^I$
 $
(Show whitespace: here a single space, ^I [horizontal tab],
  and newline, and display "$" at end-of-line.)



bash$ bash -c 'set w x y z; IFS=":-;"; echo "$*"'
w:x:y:z
(Read commands from string and assign any arguments to pos params.)
	      

Set $IFS to eliminate whitespace in pathnames.

IFS="$(printf '\n\t')"   # Per David Wheeler.

Caution

$IFS does not handle whitespace the same as it does other characters.

Example 9.1. $IFS and whitespace

#!/bin/bash
# ifs.sh


var1="a+b+c"
var2="d-e-f"
var3="g,h,i"

IFS=+
# The plus sign will be interpreted as a separator.
echo $var1     # a b c
echo $var2     # d-e-f
echo $var3     # g,h,i

echo

IFS="-"
# The plus sign reverts to default interpretation.
# The minus sign will be interpreted as a separator.
echo $var1     # a+b+c
echo $var2     # d e f
echo $var3     # g,h,i

echo

IFS=","
# The comma will be interpreted as a separator.
# The minus sign reverts to default interpretation.
echo $var1     # a+b+c
echo $var2     # d-e-f
echo $var3     # g h i

echo

IFS=" "
# The space character will be interpreted as a separator.
# The comma reverts to default interpretation.
echo $var1     # a+b+c
echo $var2     # d-e-f
echo $var3     # g,h,i

# ======================================================== #

# However ...
# $IFS treats whitespace differently than other characters.

output_args_one_per_line()
{
  for arg
  do
    echo "[$arg]"
  done #  ^    ^   Embed within brackets, for your viewing pleasure.
}

echo; echo "IFS=\" \""
echo "-------"

IFS=" "
var=" a  b c   "
#    ^ ^^   ^^^
output_args_one_per_line $var  # output_args_one_per_line `echo " a  b c   "`
# [a]
# [b]
# [c]


echo; echo "IFS=:"
echo "-----"

IFS=:
var=":a::b:c:::"               # Same pattern as above,
#    ^ ^^   ^^^                #+ but substituting ":" for " "  ...
output_args_one_per_line $var
# []
# [a]
# []
# [b]
# [c]
# []
# []

# Note "empty" brackets.
# The same thing happens with the "FS" field separator in awk.


echo

exit

(Many thanks, Stéphane Chazelas, for clarification and above examples.)

See also Example 16.41, “Analyzing a spam domain”, Example 11.8, “A grep replacement for binary files”, and Example 19.14, “Parsing a mailbox” for instructive examples of using $IFS.

$IGNOREEOF

Ignore EOF: how many end-of-files (control-D) the shell will ignore before logging out.

$LC_COLLATE

Often set in the .bashrc or /etc/profile files, this variable controls collation order in filename expansion and pattern matching. If mishandled, LC_COLLATE can cause unexpected results in filename globbing.

Note

As of version 2.05 of Bash, filename globbing no longer distinguishes between lowercase and uppercase letters in a character range between brackets. For example, ls [A-M]* would match both File1.txt and file1.txt. To revert to the customary behavior of bracket matching, set LC_COLLATE to C by an export LC_COLLATE=C in /etc/profile and/or ~/.bashrc.

$LC_CTYPE

This internal variable controls character interpretation in globbing and pattern matching.

$LINENO

This variable is the line number of the shell script in which this variable appears. It has significance only within the script in which it appears, and is chiefly useful for debugging purposes.

# *** BEGIN DEBUG BLOCK ***
last_cmd_arg=$_  # Save it.

echo "At line number $LINENO, variable \"v1\" = $v1"
echo "Last command argument processed = $last_cmd_arg"
# *** END DEBUG BLOCK ***

$MACHTYPE

machine type

Identifies the system hardware.

bash$ echo $MACHTYPE
i686

$OLDPWD

Old working directory (“OLD-Print-Working-Directory”, previous directory you were in).

$OSTYPE

operating system type

bash$ echo $OSTYPE
linux

$PATH

Path to binaries, usually /usr/bin/, /usr/X11R6/bin/, /usr/local/bin, etc.

When given a command, the shell automatically does a hash table search on the directories listed in the path for the executable. The path is stored in the environmental variable, $PATH, a list of directories, separated by colons. Normally, the system stores the $PATH definition in /etc/profile and/or ~/.bashrc (see Appendix H, Important Files).

bash$ echo $PATH
/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin:/sbin:/usr/sbin

PATH=${PATH}:/opt/bin appends the /opt/bin directory to the current path. In a script, it may be expedient to temporarily add a directory to the path in this way. When the script exits, this restores the original $PATH (a child process, such as a script, may not change the environment of the parent process, the shell).

Note

The current “working directory”, ./, is usually omitted from the $PATH as a security measure.

$PIPESTATUS

Array variable holding exit status(es) of last executed foreground pipe.

bash$ echo $PIPESTATUS
0

bash$ ls -al | bogus_command
bash: bogus_command: command not found
bash$ echo ${PIPESTATUS[1]}
127

bash$ ls -al | bogus_command
bash: bogus_command: command not found
bash$ echo $?
127
	      

The members of the $PIPESTATUS array hold the exit status of each respective command executed in a pipe. $PIPESTATUS[0] holds the exit status of the first command in the pipe, $PIPESTATUS[1] the exit status of the second command, and so on.

Caution

The $PIPESTATUS variable may contain an erroneous 0 value in a login shell (in releases prior to 3.0 of Bash).

tcsh% bash

bash$ who | grep nobody | sort
bash$ echo ${PIPESTATUS[*]}
0
	      

The above lines contained in a script would produce the expected 0 1 0 output.

Thank you, Wayne Pollock for pointing this out and supplying the above example.

Note

The $PIPESTATUS variable gives unexpected results in some contexts.

bash$ echo $BASH_VERSION
3.00.14(1)-release

bash$ $ ls | bogus_command | wc
bash: bogus_command: command not found
 0       0       0

bash$ echo ${PIPESTATUS[@]}
141 127 0
	      

Chet Ramey attributes the above output to the behavior of ls. If ls writes to a pipe whose output is not read, then SIGPIPE kills it, and its exit status is 141. Otherwise its exit status is 0, as expected. This likewise is the case for tr.

Note

$PIPESTATUS is a “volatile” variable. It needs to be captured immediately after the pipe in question, before any other command intervenes.

bash$ $ ls | bogus_command | wc
bash: bogus_command: command not found
 0       0       0

bash$ echo ${PIPESTATUS[@]}
0 127 0

bash$ echo ${PIPESTATUS[@]}
0
	      

Note

The pipefail option may be useful in cases where $PIPESTATUS does not give the desired information.

$PPID

The $PPID of a process is the process ID (pid) of its parent process. ^[42]

Compare this with the pidof command.

$PROMPT_COMMAND

A variable holding a command to be executed just before the primary prompt, $PS1 is to be displayed.

$PS1

This is the main prompt, seen at the command-line.

$PS2

The secondary prompt, seen when additional input is expected. It displays as “>”.

$PS3

The tertiary prompt, displayed in a select loop (see Example 11.30, “Creating menus using select”).

$PS4

The quartenary prompt, shown at the beginning of each line of output when invoking a script with the -x [verbose trace] option. It displays as “+”.

As a debugging aid, it may be useful to embed diagnostic information in $PS4.

P4='$(read time junk < /proc/$$/schedstat; echo "@@@ $time @@@ " )'
# Per suggestion by Erik Brandsberg.
set -x
# Various commands follow ...

$PWD

Working directory (directory you are in at the time)

This is the analog to the pwd builtin command.

#!/bin/bash

E_WRONG_DIRECTORY=85

clear # Clear the screen.

TargetDirectory=/home/bozo/projects/GreatAmericanNovel

cd $TargetDirectory
echo "Deleting stale files in $TargetDirectory."

if [ "$PWD" != "$TargetDirectory" ]
then    # Keep from wiping out wrong directory by accident.
  echo "Wrong directory!"
  echo "In $PWD, rather than $TargetDirectory!"
  echo "Bailing out!"
  exit $E_WRONG_DIRECTORY
fi  

rm -rf *
rm .[A-Za-z0-9]*    # Delete dotfiles.
# rm -f .[^.]* ..?*   to remove filenames beginning with multiple dots.
# (shopt -s dotglob; rm -f *)   will also work.
# Thanks, S.C. for pointing this out.

#  A filename (`basename`) may contain all characters in the 0 - 255 range,
#+ except "/".
#  Deleting files beginning with weird characters, such as -
#+ is left as an exercise. (Hint: rm ./-weirdname or rm -- -weirdname)
result=$?   # Result of delete operations. If successful = 0.

echo
ls -al              # Any files left?
echo "Done."
echo "Old files deleted in $TargetDirectory."
echo

# Various other operations here, as necessary.

exit $result

$REPLY

The default value when a variable is not supplied to read. Also applicable to select menus, but only supplies the item number of the variable chosen, not the value of the variable itself.

#!/bin/bash
# reply.sh

# REPLY is the default value for a 'read' command.

echo
echo -n "What is your favorite vegetable? "
read

echo "Your favorite vegetable is $REPLY."
#  REPLY holds the value of last "read" if and only if
#+ no variable supplied.

echo
echo -n "What is your favorite fruit? "
read fruit
echo "Your favorite fruit is $fruit."
echo "but..."
echo "Value of \$REPLY is still $REPLY."
#  $REPLY is still set to its previous value because
#+ the variable $fruit absorbed the new "read" value.

echo

exit 0

$SECONDS

The number of seconds the script has been running.

#!/bin/bash

TIME_LIMIT=10
INTERVAL=1

echo
echo "Hit Control-C to exit before $TIME_LIMIT seconds."
echo

while [ "$SECONDS" -le "$TIME_LIMIT" ]
do   #   $SECONDS is an internal shell variable.
  if [ "$SECONDS" -eq 1 ]
  then
    units=second
  else  
    units=seconds
  fi

  echo "This script has been running $SECONDS $units."
  #  On a slow or overburdened machine, the script may skip a count
  #+ every once in a while.
  sleep $INTERVAL
done

echo -e "\a"  # Beep!

exit 0

$SHELLOPTS

The list of enabled shell options, a readonly variable.

bash$ echo $SHELLOPTS
braceexpand:hashall:histexpand:monitor:history:interactive-comments:emacs
	      

$SHLVL

Shell level, how deeply Bash is nested. ^[43] If, at the command-line, $SHLVL is 1, then in a script it will increment to 2.

Note

This variable is not affected by subshells. Use $BASH_SUBSHELL when you need an indication of subshell nesting.

$TMOUT

If the $TMOUT environmental variable is set to a non-zero value time, then the shell prompt will time out after $time seconds. This will cause a logout.

As of version 2.05b of Bash, it is now possible to use $TMOUT in a script in combination with read.

# Works in scripts for Bash, versions 2.05b and later.

TMOUT=3    # Prompt times out at three seconds.

echo "What is your favorite song?"
echo "Quickly now, you only have $TMOUT seconds to answer!"
read song

if [ -z "$song" ]
then
  song="(no answer)"
  # Default response.
fi

echo "Your favorite song is $song."

There are other, more complex, ways of implementing timed input in a script. One alternative is to set up a timing loop to signal the script when it times out. This also requires a signal handling routine to trap (see Example 32.5, “Trapping at exit”) the interrupt generated by the timing loop (whew!).

Example 9.2. Timed Input

#!/bin/bash
# timed-input.sh

# TMOUT=3    Also works, as of newer versions of Bash.

TIMER_INTERRUPT=14
TIMELIMIT=3  # Three seconds in this instance.
             # May be set to different value.

PrintAnswer()
{
  if [ "$answer" = TIMEOUT ]
  then
    echo $answer
  else       # Don't want to mix up the two instances. 
    echo "Your favorite veggie is $answer"
    kill $!  #  Kills no-longer-needed TimerOn function
             #+ running in background.
             #  $! is PID of last job running in background.
  fi

}  


TimerOn()
{
  sleep $TIMELIMIT && kill -s 14 $$ &
  # Waits 3 seconds, then sends sigalarm to script.
}  


Int14Vector()
{
  answer="TIMEOUT"
  PrintAnswer
  exit $TIMER_INTERRUPT
}  

trap Int14Vector $TIMER_INTERRUPT
# Timer interrupt (14) subverted for our purposes.

echo "What is your favorite vegetable "
TimerOn
read answer
PrintAnswer


#  Admittedly, this is a kludgy implementation of timed input.
#  However, the "-t" option to "read" simplifies this task.
#  See the "t-out.sh" script.
#  However, what about timing not just single user input,
#+ but an entire script?

#  If you need something really elegant ...
#+ consider writing the application in C or C++,
#+ using appropriate library functions, such as 'alarm' and 'setitimer.'

exit 0

An alternative is using stty.

Example 9.3. Once more, timed input

#!/bin/bash
# timeout.sh

#  Written by Stephane Chazelas,
#+ and modified by the document author.

INTERVAL=5                # timeout interval

timedout_read() {
  timeout=$1
  varname=$2
  old_tty_settings=`stty -g`
  stty -icanon min 0 time ${timeout}0
  eval read $varname      # or just  read $varname
  stty "$old_tty_settings"
  # See man page for "stty."
}

echo; echo -n "What's your name? Quick! "
timedout_read $INTERVAL your_name

#  This may not work on every terminal type.
#  The maximum timeout depends on the terminal.
#+ (it is often 25.5 seconds).

echo

if [ ! -z "$your_name" ]  # If name input before timeout ...
then
  echo "Your name is $your_name."
else
  echo "Timed out."
fi

echo

# The behavior of this script differs somewhat from "timed-input.sh."
# At each keystroke, the counter resets.

exit 0

Perhaps the simplest method is using the -t option to read.

Example 9.4. Timed read

#!/bin/bash
# t-out.sh [time-out]
# Inspired by a suggestion from "syngin seven" (thanks).


TIMELIMIT=4         # 4 seconds

read -t $TIMELIMIT variable <&1
#                           ^^^
#  In this instance, "<&1" is needed for Bash 1.x and 2.x,
#  but unnecessary for Bash 3+.

echo

if [ -z "$variable" ]  # Is null?
then
  echo "Timed out, variable still unset."
else  
  echo "variable = $variable"
fi  

exit 0

$UID

User ID number

Current user's user identification number, as recorded in /etc/passwd

This is the current user's real id, even if she has temporarily assumed another identity through su. $UID is a readonly variable, not subject to change from the command line or within a script, and is the counterpart to the id builtin.

Example 9.5. Am I root?

#!/bin/bash
# am-i-root.sh:   Am I root or not?

ROOT_UID=0   # Root has $UID 0.

if [ "$UID" -eq "$ROOT_UID" ]  # Will the real "root" please stand up?
then
  echo "You are root."
else
  echo "You are just an ordinary user (but mom loves you just the same)."
fi

exit 0


# ============================================================= #
# Code below will not execute, because the script already exited.

# An alternate method of getting to the root of matters:

ROOTUSER_NAME=root

username=`id -nu`              # Or...   username=`whoami`
if [ "$username" = "$ROOTUSER_NAME" ]
then
  echo "Rooty, toot, toot. You are root."
else
  echo "You are just a regular fella."
fi

See also Example 2.3, “cleanup: An enhanced and generalized version of above scripts.”.

Note

The variables $ENV, $LOGNAME, $MAIL, $TERM, $USER, and $USERNAME are not Bash builtins. These are, however, often set as environmental variables in one of the Bash or login startup files. $SHELL, the name of the user's login shell, may be set from /etc/passwd or in an “init” script, and it is likewise not a Bash builtin.

tcsh% echo $LOGNAME
bozo
tcsh% echo $SHELL
/bin/tcsh
tcsh% echo $TERM
rxvt

bash$ echo $LOGNAME
bozo
bash$ echo $SHELL
/bin/tcsh
bash$ echo $TERM
rxvt
	      

Positional Parameters

$0, $1, $2, etc.

Positional parameters, passed from command line to script, passed to a function, or set to a variable (see Example 4.5, “Positional Parameters” and Example 15.16, “Using set with positional parameters”)

$#

Number of command-line arguments ^[44] or positional parameters (see Example 36.2, “ A slightly more complex shell wrapper”)

$*

All of the positional parameters, seen as a single word, "$*" is equivalent to "$1${IFS:0:1}$2${IFS:0:1}$3..."

Note

“$*” must be quoted.

$@

Same as $*, but each parameter is a quoted string, that is, the parameters are passed on intact, without interpretation or expansion. This means, among other things, that each parameter in the argument list is seen as a separate word, "$@" is equivalent to "$1" "$2" ...

Note

Of course, “$@” should be quoted.

Example 9.6. arglist: Listing arguments with $* and $@

#!/bin/bash
# arglist.sh
# Invoke this script with several arguments, such as "one two three" ...

E_BADARGS=85

if [ ! -n "$1" ]
then
  echo "Usage: `basename $0` argument1 argument2 etc."
  exit $E_BADARGS
fi  

echo

index=1          # Initialize count.

echo "Listing args with \"\$*\":"
for arg in "$*"  # Doesn't work properly if "$*" isn't quoted.
do
  echo "Arg #$index = $arg"
  let "index+=1"
done             # $* sees all arguments as single word. 
echo "Entire arg list seen as single word."

echo

index=1          # Reset count.
                 # What happens if you forget to do this?

echo "Listing args with \"\$@\":"
for arg in "$@"
do
  echo "Arg #$index = $arg"
  let "index+=1"
done             # $@ sees arguments as separate words. 
echo "Arg list seen as separate words."

echo

index=1          # Reset count.

echo "Listing args with \$* (unquoted):"
for arg in $*
do
  echo "Arg #$index = $arg"
  let "index+=1"
done             # Unquoted $* sees arguments as separate words. 
echo "Arg list seen as separate words."

exit 0

Following a shift, the $@ holds the remaining command-line parameters, lacking the previous $1, which was lost.

#!/bin/bash
# Invoke with ./scriptname 1 2 3 4 5

echo "$@"    # 1 2 3 4 5
shift
echo "$@"    # 2 3 4 5
shift
echo "$@"    # 3 4 5

# Each "shift" loses parameter $1.
# "$@" then contains the remaining parameters.

The $@ special parameter finds use as a tool for filtering input into shell scripts. The cat "$@" construction accepts input to a script either from stdin or from files given as parameters to the script. See Example 16.24, “rot13: ultra-weak encryption.” and Example 16.25, “Generating “Crypto-Quote” Puzzles”.

Caution

The $* and $@ parameters sometimes display inconsistent and puzzling behavior, depending on the setting of $IFS.

Example 9.7. Inconsistent $* and $@ behavior

#!/bin/bash

#  Erratic behavior of the "$*" and "$@" internal Bash variables,
#+ depending on whether or not they are quoted.
#  Demonstrates inconsistent handling of word splitting and linefeeds.


set -- "First one" "second" "third:one" "" "Fifth: :one"
# Setting the script arguments, $1, $2, $3, etc.

echo

echo 'IFS unchanged, using "$*"'
c=0
for i in "$*"               # quoted
do echo "$((c+=1)): [$i]"   # This line remains the same in every instance.
                            # Echo args.
done
echo ---

echo 'IFS unchanged, using $*'
c=0
for i in $*                 # unquoted
do echo "$((c+=1)): [$i]"
done
echo ---

echo 'IFS unchanged, using "$@"'
c=0
for i in "$@"
do echo "$((c+=1)): [$i]"
done
echo ---

echo 'IFS unchanged, using $@'
c=0
for i in $@
do echo "$((c+=1)): [$i]"
done
echo ---

IFS=:
echo 'IFS=":", using "$*"'
c=0
for i in "$*"
do echo "$((c+=1)): [$i]"
done
echo ---

echo 'IFS=":", using $*'
c=0
for i in $*
do echo "$((c+=1)): [$i]"
done
echo ---

var=$*
echo 'IFS=":", using "$var" (var=$*)'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo ---

echo 'IFS=":", using $var (var=$*)'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo ---

var="$*"
echo 'IFS=":", using $var (var="$*")'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo ---

echo 'IFS=":", using "$var" (var="$*")'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo ---

echo 'IFS=":", using "$@"'
c=0
for i in "$@"
do echo "$((c+=1)): [$i]"
done
echo ---

echo 'IFS=":", using $@'
c=0
for i in $@
do echo "$((c+=1)): [$i]"
done
echo ---

var=$@
echo 'IFS=":", using $var (var=$@)'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo ---

echo 'IFS=":", using "$var" (var=$@)'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo ---

var="$@"
echo 'IFS=":", using "$var" (var="$@")'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo ---

echo 'IFS=":", using $var (var="$@")'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done

echo

# Try this script with ksh or zsh -y.

exit 0

#  This example script written by Stephane Chazelas,
#+ and slightly modified by the document author.

Note

The $@ and $* parameters differ only when between double quotes.

Example 9.8. $* and $@ when $IFS is empty

#!/bin/bash

#  If $IFS set, but empty,
#+ then "$*" and "$@" do not echo positional params as expected.

mecho ()       # Echo positional parameters.
{
echo "$1,$2,$3";
}


IFS=""         # Set, but empty.
set a b c      # Positional parameters.

mecho "$*"     # abc,,
#                   ^^
mecho $*       # a,b,c

mecho $@       # a,b,c
mecho "$@"     # a,b,c

#  The behavior of $* and $@ when $IFS is empty depends
#+ on which Bash or sh version being run.
#  It is therefore inadvisable to depend on this "feature" in a script.


# Thanks, Stephane Chazelas.

exit

Other Special Parameters

$-

Flags passed to script (using set). See Example 15.16, “Using set with positional parameters”.

Caution

This was originally a ksh construct adopted into Bash, and unfortunately it does not seem to work reliably in Bash scripts. One possible use for it is to have a script self-test whether it is interactive.

$!

PID (process ID) of last job run in background

LOG=$0.log

COMMAND1="sleep 100"

echo "Logging PIDs background commands for script: $0" >> "$LOG"
# So they can be monitored, and killed as necessary.
echo >> "$LOG"

# Logging commands.

echo -n "PID of \"$COMMAND1\":  " >> "$LOG"
${COMMAND1} &
echo $! >> "$LOG"
# PID of "sleep 100":  1506

# Thank you, Jacques Lederer, for suggesting this.

Using $! for job control:

possibly_hanging_job & { sleep ${TIMEOUT}; eval 'kill -9 $!' &> /dev/null; }
# Forces completion of an ill-behaved program.
# Useful, for example, in init scripts.

# Thank you, Sylvain Fourmanoit, for this creative use of the "!" variable.

Or, alternately:

# This example by Matthew Sage.
# Used with permission.

TIMEOUT=30   # Timeout value in seconds
count=0

possibly_hanging_job & {
        while ((count < TIMEOUT )); do
                eval '[ ! -d "/proc/$!" ] && ((count = TIMEOUT))'
                # /proc is where information about running processes is found.
                # "-d" tests whether it exists (whether directory exists).
                # So, we're waiting for the job in question to show up.
                ((count++))
                sleep 1
        done
        eval '[ -d "/proc/$!" ] && kill -15 $!'
        # If the hanging job is running, kill it.
}

#  -------------------------------------------------------------- #

#  However, this may not work as specified if another process
#+ begins to run after the "hanging_job" . . .
#  In such a case, the wrong job may be killed.
#  Ariel Meragelman suggests the following fix.

TIMEOUT=30
count=0
# Timeout value in seconds
possibly_hanging_job & {

while ((count < TIMEOUT )); do
  eval '[ ! -d "/proc/$lastjob" ] && ((count = TIMEOUT))'
  lastjob=$!
  ((count++))
  sleep 1
done
eval '[ -d "/proc/$lastjob" ] && kill -15 $lastjob'

}

exit

$_

Special variable set to final argument of previous command executed.

Example 9.9. Underscore variable

#!/bin/bash

echo $_              #  /bin/bash
                     #  Just called /bin/bash to run the script.
                     #  Note that this will vary according to
                     #+ how the script is invoked.

du >/dev/null        #  So no output from command.
echo $_              #  du

ls -al >/dev/null    #  So no output from command.
echo $_              #  -al  (last argument)

:
echo $_              #  :

$?

Exit status of a command, function, or the script itself (see Example 24.7, “Maximum of two numbers”)

$$

Process ID (PID) of the script itself. ^[45] The $$ variable often finds use in scripts to construct “unique” temp file names (see Example 32.6, “Cleaning up after Control-C”, Example 16.31, “Unpacking an rpm archive”, and Example 15.27, “A script that kills itself”). This is usually simpler than invoking mktemp.