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.