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GNU Bash Reference Manual
by Chet Ramey and Brian Fox
Paperback (6"x9"), 180 pages
ISBN 0954161777
RRP £19.95 ($29.95)

"An essential resource .... the most detailed coverage available for all aspects of Bash" --- Linux User and Developer Magazine (Issue 37, Mar 2004) Get a printed copy>>>

5.2 Bash Variables

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

A few variables used by Bash are described in different chapters: variables for controlling the job control facilities (see section 7.3 Job Control Variables).

BASH
The full pathname used to execute the current instance of Bash.
BASH_ARGC
An array variable whose values are the number of parameters in each frame of the current bash execution call stack. The number of parameters to the current subroutine (shell function or script executed with . or source) is at the top of the stack. When a subroutine is executed, the number of parameters passed is pushed onto BASH_ARGC. The shell sets BASH_ARGC only when in extended debugging mode (see section 4.2 Bash Builtin Commands for a description of the extdebug option to the shopt builtin).
BASH_ARGV
An array variable containing all of the parameters in the current bash execution call stack. The final parameter of the last subroutine call is at the top of the stack; the first parameter of the initial call is at the bottom. When a subroutine is executed, the parameters supplied are pushed onto BASH_ARGV. The shell sets BASH_ARGV only when in extended debugging mode (see section 4.2 Bash Builtin Commands for a description of the extdebug option to the shopt builtin).
BASH_COMMAND
The command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap.
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 6.2 Bash Startup Files.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in source files corresponding to each member of FUNCNAME. ${BASH_LINENO[$i]} is the line number in the source file where ${FUNCNAME[$i]} was called. The corresponding source file name is ${BASH_SOURCE[$i]}. Use LINENO to obtain the current line number.
BASH_REMATCH
An array variable whose members are assigned by the ‘=~’ binary operator to the [[ conditional command (see section 3.2.4.2 Conditional Constructs). The element with index 0 is the portion of the string matching the entire regular expression. The element with index n is the portion of the string matching the nth parenthesized subexpression. This variable is read-only.
BASH_SOURCE
An array variable whose members are the source filenames corresponding to the elements in the FUNCNAME array variable.
BASH_SUBSHELL
Incremented by one each time a subshell or subshell environment is spawned. The initial value is 0.
BASH_VERSINFO
A readonly array variable (see section 6.7 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.
BASH_VERSION
The version number of the current instance of Bash.
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_WORDBREAKS
The set of characters that the Readline library treats as word separators when performing word completion. If COMP_WORDBREAKS is unset, it loses its special properties, even if it is subsequently reset.
COMP_WORDS
An array variable consisting of the individual words in the current command line. The words are split on shell metacharacters as the shell parser would separate them. 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.
EMACS
If Bash finds this variable in the environment when the shell starts with value ‘t’, it assumes that the shell is running in an emacs shell buffer and disables line editing.
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
An array variable containing the names of all shell functions currently in the execution call stack. The element with index 0 is the name of any currently-executing shell function. The bottom-most element is "main". 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 colon-separated list of values controlling how commands are saved on the history list. If the list of values includes ‘ignorespace’, lines which begin with a space character are not saved in the history list. A value of ‘ignoredups’ causes lines which match the previous history entry to not be saved. A value of ‘ignoreboth’ is shorthand for ‘ignorespace’ and ‘ignoredups’. A value of ‘erasedups’ causes all previous lines matching the current line to be removed from the history list before that line is saved. Any value not in the above list is ignored. If HISTCONTROL is unset, or does not include a valid value, all lines read by the shell parser are saved on the history list, subject to the value of HISTIGNORE. 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, by removing the oldest entries, 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.
HISTTIMEFORMAT
If this variable is set and not null, its value is used as a format string for strftime to print the time stamp associated with each history entry displayed by the history builtin. If this variable is set, time stamps are written to the history file so they may be preserved across shell sessions.
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.
SHELL
The full pathname to the shell is kept in this environment variable. If it is not set when the shell starts, Bash assigns to it the full pathname of the current user's login shell.
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, TMOUT is treated as the default timeout for the read builtin (see section 4.2 Bash Builtin Commands). The select command (see section 3.2.4.2 Conditional Constructs) terminates if input does not arrive after TMOUT seconds when input is coming from a terminal. In an interactive 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.
TMPDIR
If set, Bash uses its value as the name of a directory in which Bash creates temporary files for the shell's use.
UID
The numeric real user id of the current user. This variable is readonly.
ISBN 0954161777GNU Bash Reference ManualSee the print edition