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.

Contents

  1. Bash Shell Variables
    1. Bash Internals
      1. History
      2. Useful
      3. Not Useful
    2. Get Opts
    3. Home
    4. Inter Field Separator
    5. Locale
    6. Mail
    7. Paths
    8. Prompts
    9. User

Bash Internals

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.

Useful

$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.

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

$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.

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

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

Not Useful

$BASHOPTS is 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.

$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.

$_ 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_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.

$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]}.

$BASH_SUBSHELL is the current nesting level of shells.

Get Opts

The getopts builtin sets two variables:

Home

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.

Paths

$PWD is the current working directory.

$PATH is a colon-delimited list of directories searched 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

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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