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9.1. Internal Variables

Builtin variables:

variables affecting bash script behavior

The path to the Bash binary itself
bash$ echo $BASH

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


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

See Example 20-1 for usage.


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
  echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}"

# 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).

The version of Bash installed on the system

bash$ echo $BASH_VERSION
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.


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.


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


"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.


Name of the current function

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


                                   # Null value outside a function.

See also Example A-49.


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


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

root# echo ${GROUPS[1]}

root# echo ${GROUPS[5]}

Home directory of the user, usually /home/username (see Example 9-16)


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 9-16.


host type

Like $MACHTYPE, identifies the system hardware.

bash$ echo $HOSTTYPE

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"

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

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

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

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

Example 9-1. $IFS and whitespace

# ifs.sh

# $IFS treats whitespace differently than other characters.

  for arg
    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 "-----"

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.



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

See also Example 15-41, Example 10-7, and Example 18-14 for instructive examples of using $IFS.


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


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.


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


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.

last_cmd_arg=$_  # Save it.

echo "At line number $LINENO, variable \"v1\" = $v1"
echo "Last command argument processed = $last_cmd_arg"

machine type

Identifies the system hardware.

bash$ echo $MACHTYPE

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


operating system type

bash$ echo $OSTYPE

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

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.


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

bash$ echo $PIPESTATUS

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

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

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[*]}

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.

The $PIPESTATUS variable gives unexpected results in some contexts.

bash$ echo $BASH_VERSION

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.

$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[@]}

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


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

Compare this with the pidof command.


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


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


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


The tertiary prompt, displayed in a select loop (see Example 10-29).


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


Working directory (directory you are in at the time)

This is the analog to the pwd builtin command.



clear # Clear screen.


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!"

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.

echo "Done."
echo "Old files deleted in $TargetDirectory."

# Various other operations here, as necessary.

exit $?

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.

# reply.sh

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

echo -n "What is your favorite vegetable? "

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

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.


exit 0

The number of seconds the script has been running.



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

while [ "$SECONDS" -le "$TIME_LIMIT" ]
  if [ "$SECONDS" -eq 1 ]

  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

echo -e "\a"  # Beep!

exit 0
The list of enabled shell options, a readonly variable.
bash$ echo $SHELLOPTS

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

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


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" ]
  song="(no answer)"
  # Default response.

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

# 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.

  if [ "$answer" = TIMEOUT ]
    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.


  sleep $TIMELIMIT && kill -s 14 $$ &
  # Waits 3 seconds, then sends sigalarm to script.


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

echo "What is your favorite vegetable "
read answer

#  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

# timeout.sh

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

INTERVAL=5                # timeout interval

timedout_read() {
  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).


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


# 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

# t-out.sh
# 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.x.


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

exit 0

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?

# 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?
  echo "You are root."
  echo "You are just an ordinary user (but mom loves you just the same)."

exit 0

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

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


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

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
tcsh% echo $SHELL
tcsh% echo $TERM

bash$ echo $LOGNAME
bash$ echo $SHELL
bash$ echo $TERM

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 14-16)


Number of command-line arguments [3] 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 $@

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


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


index=1          # Initialize count.

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


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

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


index=1          # Reset count.

echo "Listing args with \$* (unquoted):"
for arg in $*
  echo "Arg #$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.
# Invoke with ./scriptname 1 2 3 4 5

echo "$@"    # 1 2 3 4 5
echo "$@"    # 2 3 4 5
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 15-24 and Example 15-25.

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

Example 9-7. Inconsistent $* and $@ behavior


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


# 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


#  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.


Other Special Parameters


Flags passed to script (using set). See Example 14-16.

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


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"
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

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

Special variable set to final argument of previous command executed.

Example 9-9. Underscore variable


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 15-31, and Example 14-27). This is usually simpler than invoking mktemp.



The PID of the currently running script is $$, of course.


Somewhat analogous to recursion, in this context nesting refers to a pattern embedded within a larger pattern. One of the definitions of nest, according to the 1913 edition of Webster's Dictionary, illustrates this beautifully: "A collection of boxes, cases, or the like, of graduated size, each put within the one next larger."


The words "argument" and "parameter" are often used interchangeably. In the context of this document, they have the same precise meaning: a variable passed to a script or function.

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