Linux Know-How: Reserved Variables

Bourne shell reserved variables

Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable. The table below gives an overview of these plain shell variables:

Table 3-1. Reserved Bourne shell variables

Variable name	Definition
CDPATH	A colon-separated list of directories used as a search path for the cd built-in command.
HOME	The current user's home directory; the default for the cd built-in. The value of this variable is also used by tilde expansion.
IFS	A list of characters that separate fields; used when the shell splits words as part of expansion.
MAIL	If this parameter is set to a file name and the `MAILPATH` variable is not set, Bash informs the user of the arrival of mail in the specified file.
MAILPATH	A colon-separated list of file names which the shell periodically checks for new mail.
OPTARG	The value of the last option argument processed by the getopts built-in.
OPTIND	The index of the last option argument processed by the getopts built-in.
PATH	A colon-separated list of directories in which the shell looks for commands.
PS1	The primary prompt string. The default value is "'\s-\v\$ '".
PS2	The secondary prompt string. The default value is "'> '".

Bash reserved variables

These variables are set or used by Bash, but other shells do not normally treat them specially.

Table 3-2. Reserved Bash variables

Variable name	Definition
auto_resume	This variable controls how the shell interacts with the user and job control.
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.
BASH_VERSION	The version number of the current instance of Bash.
BASH_VERSINFO	A read-only array variable whose members hold version information for this instance of Bash.
COLUMNS	Used by the select built-in to determine the terminal width when printing selection lists. Automatically set upon receipt of a SIGWINCH signal.
COMP_CWORD	An index into `${COMP_WORDS}` of the word containing the current cursor position.
COMP_LINE	The current command line.
COMP_POINT	The index of the current cursor position relative to the beginning of the current command.
COMP_WORDS	An array variable consisting of the individual words in the current command line.
COMPREPLY	An array variable from which Bash reads the possible completions generated by a shell function invoked by the programmable completion facility.
DIRSTACK	An array variable containing the current contents of the directory stack.
EUID	The numeric effective user ID of the current user.
FCEDIT	The editor used as a default by the `-e` option to the fc built-in command.
FIGNORE	A colon-separated list of suffixes to ignore when performing file name completion.
FUNCNAME	The name of any currently-executing shell function.
GLOBIGNORE	A colon-separated list of patterns defining the set of file names to be ignored by file name expansion.
GROUPS	An array variable containing the list of groups of which the current user is a member.
histchars	Up to three characters which control history expansion, quick substitution, and tokenization.
HISTCMD	The history number, or index in the history list, of the current command.
HISTCONTROL	Defines whether a command is added to the history file.
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, defaults to 500.
HISTIGNORE	A colon-separated list of patterns used to decide which command lines should be saved in the history list.
HISTSIZE	The maximum number of commands to remember on the history list, default 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.
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.
INPUTRC	The name of the Readline initialization file, overriding the default `/etc/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 file name expansion, and determines the behavior of range expressions, equivalence classes, and collating sequences within file name expansion and pattern matching.
LC_CTYPE	This variable determines the interpretation of characters and the behavior of character classes within file name expansion and pattern matching.
LC_MESSAGES	This variable determines the locale used to translate double-quoted strings preceded by a "$" sign.
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 built-in to determine the column length for printing selection lists.
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.
OLDPWD	The previous working directory as set by the cd built-in.
OPTERR	If set to the value 1, Bash displays error messages generated by the getopts built-in.
OSTYPE	A string describing the operating system Bash is running on.
PIPESTATUS	An array variable 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.
PPID	The process ID of the shell's parent process.
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. Defaults to "'#? '"
PS4	The value is the prompt printed before the command line is echoed when the `-x` option is set; defaults to "'+ '".
PWD	The current working directory as set by the cd built-in command.
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 built-in.
SECONDS	This variable expands to the number of seconds since the shell was started.
SHELLOPTS	A colon-separated list of enabled shell options.
SHLVL	Incremented by one each time a new instance of Bash is started.
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.
TMOUT	If set to a value greater than zero, `TMOUT` is treated as the default timeout for the read built-in. In an interative shell, 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.

Check the Bash man, info or doc pages for extended information. Some variables are read-only, some are set automatically and some lose their meaning when set to a different value than the default.