Bash Shell Variables

This is a list of special variables that are set in bash(1) to configure it's behavior, or by bash(1) to indicate state. For programming variables, including special ones, see here.

Bash Internals

Limits

$FUNCNEST is the maximum nesting level for functions.

Constants

$BASHPID is the current shell's PID. $PPID is the PID of the shell's parent.

$BASH_VERSION is the current shell's version number. $BASH_VERSINFO is an array representing this information.

$MACHTYPE, $HOSTTYPE, and $OSTYPE describe the system on which bash(1) is running.

$EMACS and $INSIDE_EMACS are used to determine if the shell is operating within Emacs, in which case line editing will be disabled.

Parameters

$RANDOM is a random number between 0 and 32767. If set, the random number generator is seeded with that number.

$SRANDOM is a random 32-bit number that cannot be seeded.

$EPOCHSECONDS and $EPOCHREALTIME are the current time.

$SECONDS is the number of seconds since the bash(1) began running.

Files and Directories

$BASH_ENV (or $ENV) is a startup file used when bash(1) is used as a script interpretter.

$TMPDIR is the location where temporary files are written.

Not Useful

$BASHOPTS and $SHELLOPTS are a colon-delimited list of enabled shell options.

$BASH_COMPAT is the compat level of the shell. Setting to 4.2 or 42 is equivalent to using the compat42 shell option.

$POSIXLY_CORRECT sets POSIX mode.

$BASH_ALIASES is an associative array of aliases. $BASH_CMDS is an associative array of builtins.

$BASH_LOADABLES_PATH is a colon-separated list of directories searched for dynamically loadable builtins.

$SHELL is the full pathname of the shell.

$_ is, at first, the pathname used to call the shell. Thereafter every time a command is called, $_ is the last argument to that command. Furthermore, while a command is executed, $_ is set to the pathname used to call that command. This is exposed when a command inspects its environment.

$BASH_SUBSHELL is the current nesting level of subshells. Similarly, $SHLVL is a counter for number of concurrent shells, which in some circumstances is a shell nesting level.

$BASH_COMMAND is the command currently executing. If bash(1) is executing a trap, $BASH_COMMAND will show the command that triggered the trap.

If bash(1) is called like bash -c '...', $BASH_EXECUTION_STRING is set to the command string.

$BASH_ARGV is an array containing all the arguments in the current shell call stack, while $BASH_ARGC is the count of those arguments. These are only set if extended debugging mode is on.

In a similar manner, $BASH_ARGV0 is the name of the shell. It is identical to $0.

$LINENO is the line number of the currently executing command.

$FUNCNAME is an array of functions currently in the call stack. $BASH_SOURCE is an array of source filenames corresponding to $FUNCNAME. Function ${FUNCNAME[i]} is defined in file ${BASH_SOURCE[i]}. Similarly, $BASH_LINENO is an array of line numbers that corresponds to $FUNCNAME. Function ${FUNCNAME[i]} is called from line number ${BASH_LINENO[i]}.

$COLUMNS and $LINES are used to indicate screen size.

$CHILD_MAX is the number of child process exit statuses to remember.

$PIPESTATUS is an array of exit statuses from each process in the most recent pipeline.

$IGNOREEOF is an integer representing the number of EOF characters that can be received before the shell exits. If set to a value that is not an integer, the interpretted value is 10. If not set, the default value is 1.

Completion

The following are all only useful for programming completion.

$COMP_LINE is the current command line prompt. $COMP_POINT is the current location of the cursor within that line.

$COMP_WORDS is an array of the current command line prompt, tokenized. $COMP_CWORD is the index of $COMP_WORDS where the cursor currently is located. $COMP_WORDBREAKS is the separators that are being used to tokenize the command line prompt.

$COMP_TYPE indicates the type of operation that trigger completion. $COMP_KEY is the key used to trigger completion.

$COMPREPLY is used to pass completions back to the shell.

$READLINE_ARGUMENT, $READLINE_LINE, $READLINE_MARK, and $READLINE_POINT are used to pass information into readline(3).

$INPUTRC is the readline(3) initialization file. The default is ~/.inputrc.

Coprocesses

If a coprocess name is not specified for the coproc builtin, ithe file descriptor is set into the $COPROC array.

Fix Command

$FCEDIT is the editor to use with the fc builtin.

Filename Expansion

$FIGNORE is a colon-delimited list of suffixes to ignore when evaluating filename expansion.

$GLOBIGNORE s a colon-delimited list of patterns to ignore when evaluating filename expansion.

Get Opts

The getopts builtin sets two variables:

• $OPTARG is the most recent options's value

• $OPTIND is the most recent option's index

If $OPTERR is set to 1, error messages will propogate from the getopts builtin.

History

$HISTFILE is the bash(1) history filename.

$HISTFILESIZE is the maximum file size for $HISTFILE, while $HISTSIZE the maximum number of commands for $HISTFILE.

$HISTCONTROL and $HISTIGNORE configure which commands are recorded in the history.

$HISTCMD is the current command's history number.

If $HISTTIMEFORMAT is set, commands are recorded in the history with a timestamp formatted with this value.

$histchars configure the history expansion.

Inter Field Separator

$IFS defines the characters that should be interpreted as separating tokens.

Frequently, this is only manipulated when writing a pipeline that specially delimits items with the null byte.

Locale

$LC_COLLATE, $LC_CTYPE, $LC_MESSAGES, $LC_NUMERIC, and $LC_TIME are all used in different contexts to determine the current locale.

$LANG is the fallback value. $LC_ALL overrides $LANG and all of the above if set.

Mail

If $MAIL is set, bash(1) monitors it (assuming it is a mail file or Maildir directory) and informs the user of changes.

Similarly, $MAILPATH is a colon-delimited list of locations to monitor. If set, $MAIL is not monitored.

$MAILCHECK is the number of seconds bash(1) will wait between checks. The default is 60.

Map File

If a variable name is not specified for the mapfile builtin, $MAPFILE is set as an array with that output.

Name Resolution

$HOSTNAME is the name of the current system.

$HOSTFILE is the hosts file that should be referenced for resolving names.

Paths

$PWD is the current working directory. $OLDPWD is the previous working directory.

$PATH is a colon-delimited list of directories searched for commands.

$EXECIGNORE is a a colon-delimited list of patterns to ignore when searching for commands.

$CDPATH is similarly searched by the cd builtin.

$DIRSTACK is an array containing the directory stack, as manipulated by the popd and pushd builtins.

Prompts

$PS1 is printed at the beginning of every command prompt.

$PS2 is printed at the beginning of run-on lines.

$PS3 is printed at the prompt in select menus.

$PS0

$PS4

$PROMPT_COMMAND is either a command string or an array of command strings. These commands are executed before printing the prompt.

$PROMPT_DIRTRIM sets the number of directory names to retain when expanding the \w and \W escapes in a prompt.

Read

If a variable name is not specified for the read builtin, $REPLY is set with that output.

$TMOUT sets a timeout for the read and select builtins.

Regular Expressions

$BASH_REMATCH is an array set by the =~ operator. The first item is the substring matching the entire regular expression. ${BASH_REMATCH[i]} is the substring matching the ith subexpression.

Time

$TIMEFORMAT sets the format for how the time builtin will display.

Tracing

$BASH_XTRACEFD will, if set to an integer corresponding to a valid file descriptor, cause trace output to be written to that file descriptor.

User

$UID is the user's ID. $EUID is the effective EID.

$GROUPS is an array of groups the user is a member of.

$HOME is the user's home directory.

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