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 "=".

Do not confuse the "=" assignment operator with the = test operator. # = as a test operator if [ "$string1" = "$string2" ] # 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.) then command fi

arithmetic operators

+

plus

-

minus

*

multiplication

/

division

**

exponentiation

# Bash, version 2.02, introduced the "**" exponentiation operator. let "z=5**3" echo "z = $z" # z = 125

%

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

bash$ echo `expr 5 % 3` 2

This operator finds use in, among other things, generating numbers within a specific range (see Example 9-24 and Example 9-27) and formatting program output (see Example 26-15 and Example A-6). It can even be used to generate prime numbers, (see Example A-16). Modulo turns up surprisingly often in various 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=65 if [ $# -ne "$ARGS" ] then echo "Usage: `basename $0` first-number second-number" exit $E_BADARGS fi # ------------------------------------------------------ gcd () { # Arbitrary assignment. dividend=$1 # It does not matter divisor=$2 #+ which of the two is larger. # Why? remainder=1 # If uninitialized variable used in loop, #+ it results in an error message #+ on first pass through loop. until [ "$remainder" -eq 0 ] do 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 # Exercise : # -------- # Check command-line arguments to make sure they are integers, #+ and exit the script with an appropriate error message if not. exit 0

+=

"plus-equal" (increment variable by a constant)

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 negate

!

bitwise NOT

^

bitwise XOR

^=

"bitwise XOR-equal"

logical operators

&&

and (logical)

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 within [ ... ] construct.

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

||

or (logical)

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 within [ ... ] construct.

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), but only the last operation is returned.

let "t1 = ((5 + 3, 7 - 1, 15 - 4))" echo "t1 = $t1" # t1 = 11 let "t2 = ((a = 9, 15 / 3))" # Set "a" and calculate "t2". echo "t2 = $t2 a = $a" # t2 = 5 a = 9

Numerical Constants

A shell script interprets a number as decimal (base 10), unless that number has a special prefix or notation. A number preceded by a 0 is octal (base 8). A number preceded by 0x is hexadecimal (base 16). A number with an embedded # evaluates as BASE#NUMBER (with range and notational restrictions).

Example 8-4. Representation of numerical constants

#!/bin/bash # numbers.sh: Representation of numbers in different bases. # Decimal: the default let "dec = 32" echo "decimal number = $dec" # 32 # Nothing out of the ordinary here. # Octal: numbers preceded by '0' (zero) let "oct = 032" echo "octal number = $oct" # 26 # Expresses result in decimal. # --------- ------ -- ------- # Hexadecimal: numbers preceded by '0x' or '0X' let "hex = 0x32" echo "hexadecimal number = $hex" # 50 # Expresses result in decimal. # Other bases: BASE#NUMBER # BASE between 2 and 64. # NUMBER must use symbols within the BASE range, see below. let "bin = 2#111100111001101" echo "binary number = $bin" # 31181 let "b32 = 32#77" echo "base-32 number = $b32" # 231 let "b64 = 64#@_" echo "base-64 number = $b64" # 4031 # This notation only works for a limited range (2 - 64) of ASCII characters. # 10 digits + 26 lowercase characters + 26 uppercase characters + @ + _ echo echo $((36#zz)) $((2#10101010)) $((16#AF16)) $((53#1aA)) # 1295 170 44822 3375 # Important note: # -------------- # Using a digit out of range of the specified base notation #+ gives an error message. let "bad_oct = 081" # (Partial) error message output: # bad_oct = 081: value too great for base (error token is "081") # Octal numbers use only digits in the range 0 - 7. exit 0 # Thanks, Rich Bartell and Stephane Chazelas, for clarification.

Internal Variables

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 20-1 for usage.

$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 3.00.14(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.

$DIRSTACK

the top value in the directory stack (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.

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.

$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.

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 9-14)

$HOSTNAME

The hostname command assigns the system name at bootup in an init script. However, the gethostname() function sets the Bash internal variable $HOSTNAME. See also Example 9-14.

$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.

bash$ echo $IFS | cat -vte $ bash$ bash -c 'set w x y z; IFS=":-;"; echo "$*"' w:x:y:z

$IFS does not handle whitespace the same as it does other characters. Example 9-1. $IFS and whitespace #!/bin/bash # $IFS treats whitespace differently than other characters. output_args_one_per_line() { for arg do echo "[$arg]" done } 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 as above, but substitute ":" for " ". output_args_one_per_line $var # # [] # [a] # [] # [b] # [c] # [] # [] # [] # The same thing happens with the "FS" field separator in awk. # Thank you, Stephane Chazelas. echo exit 0

(Thanks, S. C., for clarification and examples.)

See also Example 12-37 for an instructive example 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.

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 G).

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).

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. Interestingly enough, this does not necessarily give the same result as the exit status of the last executed command.

bash$ echo $PIPESTATUS 0 bash$ ls -al | bogus_command bash: bogus_command: command not found bash$ echo $PIPESTATUS 141 bash$ 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.

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.

In the version 2.05b.0(1)-release of Bash, and possibly earlier versions as well, the $PIPESTATUS variable appears to be broken. This is a bug that has been (mostly) fixed in Bash, version 3.0 and later. bash$ echo $BASH_VERSION 2.05b.0(1)-release bash$ $ ls | bogus_command | wc bash: bogus_command: command not found 0 0 0 bash$ echo ${PIPESTATUS[@]} 141 127 0 bash$ echo $BASH_VERSION 3.00.0(1)-release bash$ $ ls | bogus_command | wc bash: bogus_command: command not found 0 0 0 bash$ echo ${PIPESTATUS[@]} 0 127 0

$PIPESTATUS is a "volatile" variable. It needs to be captured immediately after the pipe in question, before any other command intervenes. bash$ $ ls | bogus_command | wc bash: bogus_command: command not found 0 0 0 bash$ echo ${PIPESTATUS[@]} 0 127 0 bash$ echo ${PIPESTATUS[@]} 0

$PPID

The $PPID of a process is the process ID (pid) of its parent process. [1]

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 10-29).

$PS4

The quartenary prompt, shown at the beginning of each line of output when invoking a script with the -x option. It displays as "+".

$PWD

working directory (directory you are in at the time)

This is the analog to the pwd builtin command.

#!/bin/bash E_WRONG_DIRECTORY=73 clear # Clear 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. # Filenames may contain all characters in the 0 - 255 range, except "/". # Deleting files beginning with weird characters is left as an exercise. # Various other operations here, as necessary. echo echo "Done." echo "Old files deleted in $TargetDirectory." echo exit 0

$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 "$TIME_LIMIT" ] do if [ "$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. If, at the command line, $SHLVL is 1, then in a script it will increment to 2.

$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 29-5) 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. 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 14 } trap Int14Vector 14 # 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 "t-out.sh", below. # 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 # Inspired by a suggestion from "syngin seven" (thanks). TIMELIMIT=4 # 4 seconds read -t $TIMELIMIT variable <&1 echo if [ -z "$variable" ] then echo "Timed out, variable still unset." else echo "variable = $variable" fi exit 0 # Exercise for the reader: # ----------------------- # Why is the redirection (<&1) necessary in line 8? # What happens if it is omitted?

$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.

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 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 and Example 11-15)

$#

number of command line arguments [2] or positional parameters (see Example 33-2)

$*

All of the positional parameters, seen as a single word

"$*" 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.

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=65 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 12-21 and Example 12-22.

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 they are quoted or not. # Inconsistent handling of word splitting and linefeeds. set -- "First one" "second" "third:one" "" "Fifth: :one" # Setting the script arguments, $1, $2, etc. echo echo 'IFS unchanged, using "$*"' 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 by Stephane Chazelas, # and slightly modified by the document author.

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 whatever Bash or sh version being run. # It is therefore inadvisable to depend on this "feature" in a script. # Thanks, Stephane Chazelas. exit 0

Other Special Parameters

$-

Flags passed to script (using set). See Example 11-15.

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.

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.

$_

Special variable set to last argument of previous command executed.

Example 9-9. Underscore variable

#!/bin/bash echo $_ # /bin/bash # Just called /bin/bash to run the script. 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 23-7)

$$

Process ID of the script itself. The $$ variable often finds use in scripts to construct "unique" temp file names (see Example A-13, Example 29-6, Example 12-28, and Example 11-25). This is usually simpler than invoking mktemp.

Notes

[1]	The PID of the currently running script is $$, of course.
[2]	The words "argument" and "parameter" are often used interchangeably. In the context of this document, they have the same precise meaning, that of a variable passed to a script or function.

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