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Bash Built-in Variables

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Bash variables are untyped and can be used both for arithmetic operations and string parsing. They can be global (external), local to the shell instance and local to the function and built-in. 

Access can be read-write or read-only.

$
variable substitution. 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. The only time a variable appears "naked", without the $, is when declared or assigned (or when exported). 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".

Note that $variable is actually a simplified alternate form of ${variable}. In contexts where the $variable syntax causes an error, the longer form may work

Example 3-5. Variable assignment and substitution

   1 #!/bin/bash
   2 
   3 # Variables: assignment and substitution
   4 
   5 a=37.5
   6 hello=$a
   7 # No space permitted on either side of = sign when initializing variables.
   8 
   9 echo hello
  10 # Not a reference.
  11 
  12 echo $hello
  13 echo ${hello} #Identical to above.
  14 
  15 echo "$hello"
  16 echo "${hello}"
  17 
  18 echo '$hello'
  19 # Variable referencing disabled by single quotes,
  20 # because $ interpreted literally.
  21 
  22 # Notice the effect of different types of quoting.
  23 
  24 # --------------------------------------------------------------
  25 
  26 # It is permissible to set multiple variables on the same line,
  27 # separated by white space. Careful, this may reduce legibility.
  28 
  29 var1=variable1  var2=variable2  var3=variable3
  30 echo
  31 echo "var1=$var1   var2=$var2  var3=$var3"
  32 
  33 # --------------------------------------------------------------
  34 
  35 echo; echo
  36 
  37 numbers="one two three"
  38 other_numbers="1 2 3"
  39 # If whitespace within variables, then quotes necessary.
  40 echo "numbers = $numbers"
  41 echo "other_numbers = $other_numbers"
  42 echo
  43 
  44 echo "uninitialized variable = $uninitialized_variable"
  45 # Uninitialized variable has null value (no value at all).
  46 
  47 echo
  48 
  49 exit 0
! An uninitialized variable has a "null" value - no assigned value at all (not zero!). Using a variable before assigning a value to it will inevitably cause problems.

Parameter Substitution

${parameter}
Same as $parameter, i.e., value of the variable parameter.

May be used for concatenating variables with strings.

   1 your_id=${USER}-on-${HOSTNAME}
   2 echo "$your_id"
   3 #
   4 echo "Old \$PATH = $PATH"
   5 PATH=${PATH}:/opt/bin  #Add /opt/bin to $PATH for duration of script.
   6 echo "New \$PATH = $PATH"
 

${parameter-default}
If parameter not set, use default.
   1 echo ${username-`whoami`}
   2 # Echoes the result of `whoami`, but variable "username" is still unset.
 
! This is almost equivalent to ${parameter:-default}. The extra : makes a difference only when parameter has been declared, but is null.

 

   1 #!/bin/bash
   2 
   3 username0=
   4 echo "username0 = ${username0-`whoami`}"
   5 # username0 has been declared, but is set to null.
   6 # Will not echo.
   7 
   8 echo "username1 = ${username1-`whoami`}"
   9 # username1 has not been declared.
  10 # Will echo.
  11 
  12 username2=
  13 echo "username2 = ${username2:-`whoami`}"
  14 # username2 has been declared, but is set to null.
  15 # Will echo because of :- rather than just - in condition test.
  16 
  17 exit 0
 

${parameter=default}, ${parameter:=default}
If parameter not set, set it to default.

Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, as above.

 

   1 echo ${username=`whoami`}
   2 # Variable "username" is now set to `whoami`.
 

${parameter+otherwise}, ${parameter:+otherwise}
If parameter set, use 'otherwise", else use null string.

Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, as above.

${parameter?err_msg}, ${parameter:?err_msg}
If parameter set, use it, else print err_msg.

Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, as above.

Example

   1 #!/bin/bash
   2 
   3 # Let's check some of the system's environmental variables.
   4 # If, for example, $USER, the name of the person
   5 # at the console, is not set, the machine will not
   6 # recognize you.
   7 
   8 : ${HOSTNAME?} ${USER?} ${HOME} ${MAIL?}
   9   echo
  10   echo "Name of the machine is $HOSTNAME."
  11   echo "You are $USER."
  12   echo "Your home directory is $HOME."
  13   echo "Your mail INBOX is located in $MAIL."
  14   echo
  15   echo "If you are reading this message,"
  16   echo "critical environmental variables have been set."
  17   echo
  18   echo
  19 
  20 # The ':' operator seems fairly error tolerant.
  21 # This script works even if the '$' omitted in front of
  22 # {HOSTNAME}, {USER?}, {HOME?}, and {MAIL?}. Why?
  23 
  24 # ------------------------------------------------------
  25 
  26 # The ${variablename?} construction can also check
  27 # for variables set within the script.
  28 
  29 ThisVariable=Value-of-ThisVariable
  30 # Note, by the way, that string variables may be set
  31 # to characters disallowed in their names.
  32 : ${ThisVariable?}
  33 echo "Value of ThisVariable is $ThisVariable".
  34 echo
  35 echo
  36 
  37 # If ZZXy23AB has not been set...
  38 : ${ZZXy23AB?}
  39 # This will give you an error message and terminate.
  40 
  41 echo "You will not see this message."
  42 
  43 exit 0
Parameter substitution and/or expansion. The following expressions are the complement to the match in expr string operations (see Example 3-52). These particular ones are used mostly in parsing file path names.
${var#pattern}, ${var##pattern}
Strip off shortest/longest part of pattern if it matches the front end of variable.
${var%pattern}, ${var%%pattern}
Strip off shortest/longest part of pattern if it matches the back end of variable.

Version 2 of bash adds additional options.

Example 

   1 #!/bin/bash
   2 
   3 #                 rfe
   4 #                 ---
   5 
   6 # Renaming file extensions.
   7 #
   8 #         rfe old_extension new_extension
   9 #
  10 # Example:
  11 # To rename all *.gif files in working directory to *.jpg,
  12 #          rfe gif jpg
  13 
  14 if [ $# -ne 2 ]
  15 then
  16   echo "Usage: `basename $0` old_file_suffix new_file_suffix"
  17   exit 1
  18 fi
  19 
  20 for filename in *.$1
  21 # Traverse list of files ending with 1st argument.
  22 do
  23   mv $filename ${filename%$1}$2
  24   # Strip off part of filename matching 1st argument,
  25   # then append 2nd argument.
  26 done
  27 
  28 exit 0
${var:pos}
Variable var expanded, starting from offset pos.
${var:pos:len}
Expansion to a max of len characters of variable var, from offset pos. See Example A-6 for an example of the creative use of this operator.
${var/patt/replacement}
First match of patt, within var replaced with replacement.

If replacement is omitted, then the first match of patt is replaced by nothing, that is, deleted.

${var//patt/replacement}
All matches of patt, within var replaced with replacement.

Similar to above, if replacement is omitted, then all occurrences patt are replaced by nothing, that is, deleted.

Example 

   1 #!/bin/bash
   2 
   3 var1=abcd-1234-defg
   4 echo "var1 = $var1"
   5 
   6 t=${var1#*-*}
   7 echo "var1 (with everything, up to and including first - stripped out) = $t"
   8 t=${var1%*-*}
   9 echo "var1 (with everything from the last - on stripped out) = $t"
  10 
  11 echo
  12 
  13 path_name=/home/bozo/ideas/thoughts.for.today
  14 echo "path_name = $path_name"
  15 t=${path_name##/*/}
  16 # Same effect as   t=`basename $path_name`
  17 echo "path_name, stripped of prefixes = $t"
  18 t=${path_name%/*.*}
  19 # Same effect as   t=`dirname $path_name`
  20 echo "path_name, stripped of suffixes = $t"
  21 
  22 echo
  23 
  24 t=${path_name:11}
  25 echo "$path_name, with first 11 chars stripped off = $t"
  26 t=${path_name:11:5}
  27 echo "$path_name, with first 11 chars stripped off, length 5 = $t"
  28 
  29 echo
  30 
  31 t=${path_name/bozo/clown}
  32 echo "$path_name with \"bozo\" replaced  by \"clown\" = $t"
  33 t=${path_name/today/}
  34 echo "$path_name with \"today\" deleted = $t"
  35 t=${path_name//o/O}
  36 echo "$path_name with all o's capitalized = $t"
  37 t=${path_name//o/}
  38 echo "$path_name with all o's deleted = $t"
  39 
  40 exit 0

There are several built-in variables which are set or used by Bash:

BASH
The full pathname used to execute the current instance of Bash.
BASH_ENV
If this variable is set when Bash is invoked to execute a shell script, its value is expanded and used as the name of a startup file to read before executing the script. See section Bash Startup Files.
BASH_VERSION
The version number of the current instance of Bash.
BASH_VERSINFO
A readonly array variable (see section Arrays) whose members hold version information for this instance of Bash. The values assigned to the array members are as follows:

BASH_VERSINFO[0]
The major version number (the release).

BASH_VERSINFO[1]
The minor version number (the version).

BASH_VERSINFO[2]
The patch level.

BASH_VERSINFO[3]
The build version.

BASH_VERSINFO[4]
The release status (e.g., beta1).

BASH_VERSINFO[5]
The value of MACHTYPE.
COLUMNS
Used by the select builtin command to determine the terminal width when printing selection lists. Automatically set upon receipt of a SIGWINCH.
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the current cursor position. This variable is available only in shell functions invoked by the programmable completion facilities (see section 8.6 Programmable Completion).
COMP_LINE
The current command line. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see section 8.6 Programmable Completion).
COMP_POINT
The index of the current cursor position relative to the beginning of the current command. If the current cursor position is at the end of the current command, the value of this variable is equal to ${#COMP_LINE}. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see section 8.6 Programmable Completion)
COMP_WORDS
An array variable consisting of the individual words in the current command line. This variable is available only in shell functions invoked by the programmable completion facilities (see section 8.6 Programmable Completion).
COMPREPLY
An array variable from which Bash reads the possible completions generated by a shell function invoked by the programmable completion facility (see section 8.6 Programmable Completion).
DIRSTACK
An array variable containing the current contents of the directory stack. Directories appear in the stack in the order they are displayed by the dirs builtin. Assigning to members of this array variable may be used to modify directories already in the stack, but the pushd and popd builtins must be used to add and remove directories. Assignment to this variable will not change the current directory. If DIRSTACK is unset, it loses its special properties, even if it is subsequently reset.
EUID
The numeric effective user id of the current user. This variable is readonly.
FCEDIT
The editor used as a default by the `-e' option to the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when performing filename completion. A file name whose suffix matches one of the entries in FIGNORE is excluded from the list of matched file names. A sample value is `.o:~'
FUNCNAME
The name of any currently-executing shell function. This variable exists only when a shell function is executing. Assignments to FUNCNAME have no effect and return an error status. If FUNCNAME is unset, it loses its special properties, even if it is subsequently reset.
GLOBIGNORE
A colon-separated list of patterns defining the set of filenames to be ignored by filename expansion. If a filename matched by a filename expansion pattern also matches one of the patterns in GLOBIGNORE, it is removed from the list of matches.
GROUPS
An array variable containing the list of groups of which the current user is a member. Assignments to GROUPS have no effect and return an error status. If GROUPS is unset, it loses its special properties, even if it is subsequently reset.
histchars
Up to three characters which control history expansion, quick substitution, and tokenization (see section 9.3 History Expansion). The first character is the history expansion character, that is, the character which signifies the start of a history expansion, normally `!'. The second character is the character which signifies `quick substitution' when seen as the first character on a line, normally `^'. The optional third character is the character which indicates that the remainder of the line is a comment when found as the first character of a word, usually `#'. The history comment character causes history substitution to be skipped for the remaining words on the line. It does not necessarily cause the shell parser to treat the rest of the line as a comment.
HISTCMD
The history number, or index in the history list, of the current command. If HISTCMD is unset, it loses its special properties, even if it is subsequently reset.
HISTCONTROL
A value of `ignorespace' means to not enter lines which begin with a space or tab into the history list. A value of `ignoredups' means to not enter lines which match the last entered line. A value of `ignoreboth' combines the two options. Unset, or set to any other value than those above, means to save all lines on the history list. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of HISTCONTROL.
HISTFILE
The name of the file to which the command history is saved. The default value is `~/.bash_history'.
HISTFILESIZE
The maximum number of lines contained in the history file. When this variable is assigned a value, the history file is truncated, if necessary, to contain no more than that number of lines. The history file is also truncated to this size after writing it when an interactive shell exits. The default value is 500.
HISTIGNORE
A colon-separated list of patterns used to decide which command lines should be saved on the history list. Each pattern is anchored at the beginning of the line and must match the complete line (no implicit `*' is appended). Each pattern is tested against the line after the checks specified by HISTCONTROL are applied. In addition to the normal shell pattern matching characters, `&' matches the previous history line. `&' may be escaped using a backslash; the backslash is removed before attempting a match. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of HISTIGNORE.

HISTIGNORE subsumes the function of HISTCONTROL. A pattern of `&' is identical to ignoredups, and a pattern of `[ ]*' is identical to ignorespace. Combining these two patterns, separating them with a colon, provides the functionality of ignoreboth.

HISTSIZE
The maximum number of commands to remember on the history list. The default value is 500.
HOSTFILE
Contains the name of a file in the same format as `/etc/hosts' that should be read when the shell needs to complete a hostname. The list of possible hostname completions may be changed while the shell is running; the next time hostname completion is attempted after the value is changed, Bash adds the contents of the new file to the existing list. If HOSTFILE is set, but has no value, Bash attempts to read `/etc/hosts' to obtain the list of possible hostname completions. When HOSTFILE is unset, the hostname list is cleared.
HOSTNAME
The name of the current host.
HOSTTYPE
A string describing the machine Bash is running on.
IGNOREEOF
Controls the action of the shell on receipt of an EOF character as the sole input. If set, the value denotes the number of consecutive EOF characters that can be read as the first character on an input line before the shell will exit. If the variable exists but does not have a numeric value (or has no value) then the default is 10. If the variable does not exist, then EOF signifies the end of input to the shell. This is only in effect for interactive shells.
INPUTRC
The name of the Readline initialization file, overriding the default of `~/.inputrc'.
LANG
Used to determine the locale category for any category not specifically selected with a variable starting with LC_.
LC_ALL
This variable overrides the value of LANG and any other LC_ variable specifying a locale category.
LC_COLLATE
This variable determines the collation order used when sorting the results of filename expansion, and determines the behavior of range expressions, equivalence classes, and collating sequences within filename expansion and pattern matching (see section 3.5.8 Filename Expansion).
LC_CTYPE
This variable determines the interpretation of characters and the behavior of character classes within filename expansion and pattern matching (see section 3.5.8 Filename Expansion).
LC_MESSAGES
This variable determines the locale used to translate double-quoted strings preceded by a `$' (see section 3.1.2.5 Locale-Specific Translation).
LC_NUMERIC
This variable determines the locale category used for number formatting.
LINENO
The line number in the script or shell function currently executing.
LINES
Used by the select builtin command to determine the column length for printing selection lists. Automatically set upon receipt of a SIGWINCH.
MACHTYPE
A string that fully describes the system type on which Bash is executing, in the standard GNU cpu-company-system format.
MAILCHECK
How often (in seconds) that the shell should check for mail in the files specified in the MAILPATH or MAIL variables. The default is 60 seconds. When it is time to check for mail, the shell does so before displaying the primary prompt. If this variable is unset, or set to a value that is not a number greater than or equal to zero, the shell disables mail checking.
OLDPWD
The previous working directory as set by the cd builtin.
OPTERR
If set to the value 1, Bash displays error messages generated by the getopts builtin command.
OSTYPE
A string describing the operating system Bash is running on.
PIPESTATUS
An array variable (see section 6.7 Arrays) containing a list of exit status values from the processes in the most-recently-executed foreground pipeline (which may contain only a single command).
POSIXLY_CORRECT
If this variable is in the environment when bash starts, the shell enters POSIX mode (see section 6.11 Bash POSIX Mode) before reading the startup files, as if the `--posix' invocation option had been supplied. If it is set while the shell is running, bash enables POSIX mode, as if the command
  
set -o posix
had been executed.
PPID
The process ID of the shell's parent process. This variable is readonly.
PROMPT_COMMAND
If set, the value is interpreted as a command to execute before the printing of each primary prompt ($PS1).
PS3
The value of this variable is used as the prompt for the select command. If this variable is not set, the select command prompts with `#? '
PS4
The value is the prompt printed before the command line is echoed when the `-x' option is set (see section 4.3 The Set Builtin). The first character of PS4 is replicated multiple times, as necessary, to indicate multiple levels of indirection. The default is `+ '.
PWD
The current working directory as set by the cd builtin.
RANDOM
Each time this parameter is referenced, a random integer between 0 and 32767 is generated. Assigning a value to this variable seeds the random number generator.
REPLY
The default variable for the read builtin.
SECONDS
This variable expands to the number of seconds since the shell was started. Assignment to this variable resets the count to the value assigned, and the expanded value becomes the value assigned plus the number of seconds since the assignment.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in the list is a valid argument for the `-o' option to the set builtin command (see section 4.3 The Set Builtin). The options appearing in SHELLOPTS are those reported as `on' by `set -o'. If this variable is in the environment when Bash starts up, each shell option in the list will be enabled before reading any startup files. This variable is readonly.
SHLVL
Incremented by one each time a new instance of Bash is started. This is intended to be a count of how deeply your Bash shells are nested.
TIMEFORMAT
The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time reserved word should be displayed. The `%' character introduces an escape sequence that is expanded to a time value or other information. The escape sequences and their meanings are as follows; the braces denote optional portions.

%%
A literal `%'.

%[p][l]R
The elapsed time in seconds.

%[p][l]U
The number of CPU seconds spent in user mode.

%[p][l]S
The number of CPU seconds spent in system mode.

%P
The CPU percentage, computed as (%U + %S) / %R.

The optional p is a digit specifying the precision, the number of fractional digits after a decimal point. A value of 0 causes no decimal point or fraction to be output. At most three places after the decimal point may be specified; values of p greater than 3 are changed to 3. If p is not specified, the value 3 is used.

The optional l specifies a longer format, including minutes, of the form MMmSS.FFs. The value of p determines whether or not the fraction is included.

If this variable is not set, Bash acts as if it had the value

  
$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
If the value is null, no timing information is displayed. A trailing newline is added when the format string is displayed.

TMOUT
If set to a value greater than zero, the value is interpreted as the number of seconds to wait for input after issuing the primary prompt when the shell is interactive. Bash terminates after that number of seconds if input does not arrive.
UID
The numeric real user id of the current user. This variable is readonly.

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Last modified: August 15, 2009