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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 nameDefinition
CDPATHA colon-separated list of directories used as a search path for the cd built-in command.
HOMEThe current user's home directory; the default for the cd built-in. The value of this variable is also used by tilde expansion.
IFSA list of characters that separate fields; used when the shell splits words as part of expansion.
MAILIf 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.
MAILPATHA colon-separated list of file names which the shell periodically checks for new mail.
OPTARGThe value of the last option argument processed by the getopts built-in.
OPTINDThe index of the last option argument processed by the getopts built-in.
PATHA colon-separated list of directories in which the shell looks for commands.
PS1The primary prompt string. The default value is "'\s-\v\$ '".
PS2The 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 nameDefinition
auto_resumeThis variable controls how the shell interacts with the user and job control.
BASHThe full pathname used to execute the current instance of Bash.
BASH_ENVIf 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_VERSIONThe version number of the current instance of Bash.
BASH_VERSINFOA read-only array variable whose members hold version information for this instance of Bash.
COLUMNSUsed by the select built-in to determine the terminal width when printing selection lists. Automatically set upon receipt of a SIGWINCH signal.
COMP_CWORDAn index into ${COMP_WORDS} of the word containing the current cursor position.
COMP_LINEThe current command line.
COMP_POINTThe index of the current cursor position relative to the beginning of the current command.
COMP_WORDSAn array variable consisting of the individual words in the current command line.
COMPREPLYAn array variable from which Bash reads the possible completions generated by a shell function invoked by the programmable completion facility.
DIRSTACKAn array variable containing the current contents of the directory stack.
EUIDThe numeric effective user ID of the current user.
FCEDITThe editor used as a default by the -e option to the fc built-in command.
FIGNOREA colon-separated list of suffixes to ignore when performing file name completion.
FUNCNAMEThe name of any currently-executing shell function.
GLOBIGNOREA colon-separated list of patterns defining the set of file names to be ignored by file name expansion.
GROUPSAn array variable containing the list of groups of which the current user is a member.
histcharsUp to three characters which control history expansion, quick substitution, and tokenization.
HISTCMDThe history number, or index in the history list, of the current command.
HISTCONTROLDefines whether a command is added to the history file.
HISTFILEThe name of the file to which the command history is saved. The default value is ~/.bash_history.
HISTFILESIZEThe maximum number of lines contained in the history file, defaults to 500.
HISTIGNOREA colon-separated list of patterns used to decide which command lines should be saved in the history list.
HISTSIZEThe maximum number of commands to remember on the history list, default is 500.
HOSTFILEContains the name of a file in the same format as /etc/hosts that should be read when the shell needs to complete a hostname.
HOSTNAMEThe name of the current host.
HOSTTYPEA string describing the machine Bash is running on.
IGNOREEOFControls the action of the shell on receipt of an EOF character as the sole input.
INPUTRCThe name of the Readline initialization file, overriding the default /etc/inputrc.
LANGUsed to determine the locale category for any category not specifically selected with a variable starting with LC_.
LC_ALLThis variable overrides the value of LANG and any other LC_ variable specifying a locale category.
LC_COLLATEThis 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_CTYPEThis variable determines the interpretation of characters and the behavior of character classes within file name expansion and pattern matching.
LC_MESSAGESThis variable determines the locale used to translate double-quoted strings preceded by a "$" sign.
LC_NUMERICThis variable determines the locale category used for number formatting.
LINENOThe line number in the script or shell function currently executing.
LINESUsed by the select built-in to determine the column length for printing selection lists.
MACHTYPEA string that fully describes the system type on which Bash is executing, in the standard GNU CPU-COMPANY-SYSTEM format.
MAILCHECKHow often (in seconds) that the shell should check for mail in the files specified in the MAILPATH or MAIL variables.
OLDPWDThe previous working directory as set by the cd built-in.
OPTERRIf set to the value 1, Bash displays error messages generated by the getopts built-in.
OSTYPEA string describing the operating system Bash is running on.
PIPESTATUSAn 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_CORRECTIf this variable is in the environment when bash starts, the shell enters POSIX mode.
PPIDThe process ID of the shell's parent process.
PROMPT_COMMANDIf set, the value is interpreted as a command to execute before the printing of each primary prompt (PS1).
PS3The value of this variable is used as the prompt for the select command. Defaults to "'#? '"
PS4The value is the prompt printed before the command line is echoed when the -x option is set; defaults to "'+ '".
PWDThe current working directory as set by the cd built-in command.
RANDOMEach 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.
REPLYThe default variable for the read built-in.
SECONDSThis variable expands to the number of seconds since the shell was started.
SHELLOPTSA colon-separated list of enabled shell options.
SHLVLIncremented by one each time a new instance of Bash is started.
TIMEFORMATThe 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.
TMOUTIf 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.
UIDThe 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.

Last Update: 2010-12-16