bash: Bash Variables
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 (
Job Control
Variables).
'BASH'
The full pathname used to execute the current instance of Bash.
'BASHOPTS'
A colon-separated list of enabled shell options. Each word in the
list is a valid argument for the '-s' option to the 'shopt' builtin
command (The Shopt Builtin). The options appearing in
'BASHOPTS' are those reported as 'on' by 'shopt'. 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.
'BASHPID'
Expands to the process ID of the current Bash process. This
differs from '$$' under certain circumstances, such as subshells
that do not require Bash to be re-initialized.
'BASH_ALIASES'
An associative array variable whose members correspond to the
internal list of aliases as maintained by the 'alias' builtin.
(Bourne Shell Builtins). Elements added to this array
appear in the alias list; however, unsetting array elements
currently does not cause aliases to be removed from the alias list.
If 'BASH_ALIASES' is unset, it loses its special properties, even
if it is subsequently reset.
'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 The Shopt Builtin 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
The Shopt Builtin:: for a description of the 'extdebug' option to
the 'shopt' builtin).
'BASH_CMDS'
An associative array variable whose members correspond to the
internal hash table of commands as maintained by the 'hash' builtin
(Bourne Shell Builtins). Elements added to this array
appear in the hash table; however, unsetting array elements
currently does not cause command names to be removed from the hash
table. If 'BASH_CMDS' is unset, it loses its special properties,
even if it is subsequently reset.
'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_COMPAT'
The value is used to set the shell's compatibility level.
The Shopt Builtin, for a description of the various compatibility
levels and their effects. The value may be a decimal number (e.g.,
4.2) or an integer (e.g., 42) corresponding to the desired
compatibility level. If 'BASH_COMPAT' is unset or set to the empty
string, the compatibility level is set to the default for the
current version. If 'BASH_COMPAT' is set to a value that is not
one of the valid compatibility levels, the shell prints an error
message and sets the compatibility level to the default for the
current version. The valid compatibility levels correspond to the
compatibility options accepted by the 'shopt' builtin described
above (for example, COMPAT42 means that 4.2 and 42 are valid
values). The current version is also a valid value.
'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 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 where each corresponding member of FUNCNAME was invoked.
'${BASH_LINENO[$i]}' is the line number in the source file
('${BASH_SOURCE[$i+1]}') where '${FUNCNAME[$i]}' was called (or
'${BASH_LINENO[$i-1]}' if referenced within another shell
function). Use 'LINENO' to obtain the current line number.
'BASH_LOADABLES_PATH'
A colon-separated list of directories in which the shell looks for
dynamically loadable builtins specified by the 'enable' command.
'BASH_REMATCH'
An array variable whose members are assigned by the '=~' binary
operator to the '[[' conditional command (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 where the
corresponding shell function names in the 'FUNCNAME' array variable
are defined. The shell function '${FUNCNAME[$i]}' is defined in
the file '${BASH_SOURCE[$i]}' and called from
'${BASH_SOURCE[$i+1]}'
'BASH_SUBSHELL'
Incremented by one within each subshell or subshell environment
when the shell begins executing in that environment. The initial
value is 0.
'BASH_VERSINFO'
A readonly array variable (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.
'BASH_XTRACEFD'
If set to an integer corresponding to a valid file descriptor, Bash
will write the trace output generated when 'set -x' is enabled to
that file descriptor. This allows tracing output to be separated
from diagnostic and error messages. The file descriptor is closed
when 'BASH_XTRACEFD' is unset or assigned a new value. Unsetting
'BASH_XTRACEFD' or assigning it the empty string causes the trace
output to be sent to the standard error. Note that setting
'BASH_XTRACEFD' to 2 (the standard error file descriptor) and then
unsetting it will result in the standard error being closed.
'CHILD_MAX'
Set the number of exited child status values for the shell to
remember. Bash will not allow this value to be decreased below a
POSIX-mandated minimum, and there is a maximum value (currently
8192) that this may not exceed. The minimum value is
system-dependent.
'COLUMNS'
Used by the 'select' command to determine the terminal width when
printing selection lists. Automatically set if the 'checkwinsize'
option is enabled (The Shopt Builtin), or in an interactive
shell upon receipt of a 'SIGWINCH'.
'COMP_CWORD'
An index into '${COMP_WORDS}' of the word containing the current
cursor position. This variable is available only in shell
functions invoked by the programmable completion facilities (
Programmable Completion).
'COMP_LINE'
The current command line. This variable is available only in shell
functions and external commands invoked by the programmable
completion facilities (Programmable Completion).
'COMP_POINT'
The index of the current cursor position relative to the beginning
of the current command. If the current cursor position is at the
end of the current command, the value of this variable is equal to
'${#COMP_LINE}'. This variable is available only in shell
functions and external commands invoked by the programmable
completion facilities (Programmable Completion).
'COMP_TYPE'
Set to an integer value corresponding to the type of completion
attempted that caused a completion function to be called: TAB, for
normal completion, '?', for listing completions after successive
tabs, '!', for listing alternatives on partial word completion,
'@', to list completions if the word is not unmodified, or '%', for
menu completion. This variable is available only in shell
functions and external commands invoked by the programmable
completion facilities (Programmable Completion).
'COMP_KEY'
The key (or final key of a key sequence) used to invoke the current
completion function.
'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 line is split into words as Readline would split
it, using 'COMP_WORDBREAKS' as described above. This variable is
available only in shell functions invoked by the programmable
completion facilities (Programmable Completion).
'COMPREPLY'
An array variable from which Bash reads the possible completions
generated by a shell function invoked by the programmable
completion facility (Programmable Completion). Each array
element contains one possible completion.
'COPROC'
An array variable created to hold the file descriptors for output
from and input to an unnamed coprocess (Coprocesses).
'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.
'ENV'
Similar to 'BASH_ENV'; used when the shell is invoked in POSIX Mode
(Bash POSIX Mode).
'EUID'
The numeric effective user id of the current user. This variable
is readonly.
'EXECIGNORE'
A colon-separated list of shell patterns (Pattern Matching)
defining the list of filenames to be ignored by command search
using 'PATH'. Files whose full pathnames match one of these
patterns are not considered executable files for the purposes of
completion and command execution via 'PATH' lookup. This does not
affect the behavior of the '[', 'test', and '[[' commands. Full
pathnames in the command hash table are not subject to
'EXECIGNORE'. Use this variable to ignore shared library files
that have the executable bit set, but are not executable files.
The pattern matching honors the setting of the 'extglob' shell
option.
'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 filename whose suffix matches one of the
entries in 'FIGNORE' is excluded from the list of matched
filenames. 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 (the one with the highest index) is '"main"'.
This variable exists only when a shell function is executing.
Assignments to 'FUNCNAME' have no effect. If 'FUNCNAME' is unset,
it loses its special properties, even if it is subsequently reset.
This variable can be used with 'BASH_LINENO' and 'BASH_SOURCE'.
Each element of 'FUNCNAME' has corresponding elements in
'BASH_LINENO' and 'BASH_SOURCE' to describe the call stack. For
instance, '${FUNCNAME[$i]}' was called from the file
'${BASH_SOURCE[$i+1]}' at line number '${BASH_LINENO[$i]}'. The
'caller' builtin displays the current call stack using this
information.
'FUNCNEST'
If set to a numeric value greater than 0, defines a maximum
function nesting level. Function invocations that exceed this
nesting level will cause the current command to abort.
'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. The pattern
matching honors the setting of the 'extglob' shell option.
'GROUPS'
An array variable containing the list of groups of which the
current user is a member. Assignments to 'GROUPS' have no effect.
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 (History Interaction). 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, to contain no more than that number of lines by
removing the oldest entries. The history file is also truncated to
this size after writing it when a shell exits. If the value is 0,
the history file is truncated to zero size. Non-numeric values and
numeric values less than zero inhibit truncation. The shell sets
the default value to the value of 'HISTSIZE' after reading any
startup files.
'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'. The pattern matching honors the setting of the
'extglob' shell option.
'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. If
the value is 0, commands are not saved in the history list.
Numeric values less than zero result in every command being saved
on the history list (there is no limit). The shell sets the
default value to 500 after reading any startup files.
'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. This uses the history comment
character to distinguish timestamps from other history lines.
'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, or
does not name a readable file, 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 (Filename
Expansion).
'LC_CTYPE'
This variable determines the interpretation of characters and the
behavior of character classes within filename expansion and pattern
matching (Filename Expansion).
'LC_MESSAGES'
This variable determines the locale used to translate double-quoted
strings preceded by a '$' (Locale Translation).
'LC_NUMERIC'
This variable determines the locale category used for number
formatting.
'LC_TIME'
This variable determines the locale category used for data and time
formatting.
'LINENO'
The line number in the script or shell function currently
executing.
'LINES'
Used by the 'select' command to determine the column length for
printing selection lists. Automatically set if the 'checkwinsize'
option is enabled (The Shopt Builtin), or in an interactive
shell 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.
'MAPFILE'
An array variable created to hold the text read by the 'mapfile'
builtin when no variable name is supplied.
'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 (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 (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').
'PROMPT_DIRTRIM'
If set to a number greater than zero, the value is used as the
number of trailing directory components to retain when expanding
the '\w' and '\W' prompt string escapes (Controlling the
Prompt). Characters removed are replaced with an ellipsis.
'PS0'
The value of this parameter is expanded like PS1 and displayed by
interactive shells after reading a command and before the command
is executed.
'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 (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.
'READLINE_LINE'
The contents of the Readline line buffer, for use with 'bind -x'
(Bash Builtins).
'READLINE_POINT'
The position of the insertion point in the Readline line buffer,
for use with 'bind -x' (Bash Builtins).
'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 (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 (Bash Builtins).
The 'select' command (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 a line of input after issuing the primary
prompt. Bash terminates after waiting for that number of seconds
if a complete line of 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.
Info Catalog
bash: Bourne Shell Variables
bash: Shell Variables