BASH中的内置变量

The following variables are set by the shell:

BASH   Expands  to  the  full file name used to invoke this 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 (see SHELL BUILTIN COMMANDS below).  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 read-only.
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.
Elements added to this array appear in the alias list; unsetting
array elements cause aliases to be removed from the alias list.
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 description of the extdebug
option to the shopt builtin below)
BASH_ARGV
An array variable containing all of the parameters in  the  cur‐
rent 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 exe‐
cuted, the parameters supplied are pushed onto  BASH_ARGV.   The
shell  sets  BASH_ARGV only when in extended debugging mode (see
the description of the extdebug  option  to  the  shopt  builtin
below)
BASH_CMDS
An  associative  array  variable whose members correspond to the
internal hash table  of  commands  as  maintained  by  the  hash
builtin.  Elements added to this array appear in the hash table;
unsetting array elements cause commands to be removed  from  the
hash table.
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_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  func‐
tion).  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.  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 vari‐
able 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 each time a subshell or subshell  environment
is spawned.  The initial value is 0.
BASH_VERSINFO
A readonly array variable 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
Expands  to  a string describing the version of this instance of
bash.
COMP_CWORD
An index into ${COMP_WORDS} of the word containing  the  current
cursor position.  This variable is available only in shell func‐
tions invoked by the  programmable  completion  facilities  (see
Programmable Completion below).
COMP_KEY
The key (or final key of a key sequence) used to invoke the cur‐
rent completion function.
COMP_LINE
The current command line.  This variable is  available  only  in
shell  functions  and  external commands invoked by the program‐
mable completion facilities (see Programmable Completion below).
COMP_POINT
The index of the current cursor position relative to the  begin‐
ning  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  program‐
mable completion facilities (see Programmable Completion below).
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 succes‐
sive tabs, !, for listing alternatives on partial  word  comple‐
tion,  @,  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 program‐
mable completion facilities (see Programmable Completion below).
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  subse‐
quently reset.
COMP_WORDS
An  array variable (see Arrays below) consisting of the individ‐
ual 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 func‐
tions  invoked  by  the  programmable completion facilities (see
Programmable Completion below).
COPROC An array variable (see Arrays below) created to  hold  the  file
descriptors  for  output  from and input to an unnamed coprocess
(see Coprocesses above).
DIRSTACK
An array variable (see Arrays below) containing the current con‐
tents  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 directo‐
ries already in the stack, but the pushd and popd builtins  must
be used to add and remove directories.  Assignment to this vari‐
able will not change the  current  directory.   If  DIRSTACK  is
unset,  it  loses  its  special properties, even if it is subse‐
quently reset.
EUID   Expands to the effective user ID of the current  user,  initial‐
ized at shell startup.  This variable is readonly.
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 bot‐
tom-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 and return an error  sta‐
tus.   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 infor‐
mation.
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.
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.
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically  set  to a string that uniquely describes the type
of machine on which bash is executing.  The default  is  system-
dependent.
LINENO Each  time this parameter is referenced, the shell substitutes a
decimal number representing the current sequential  line  number
(starting  with  1)  within a script or function.  When not in a
script or function, the value substituted is not  guaranteed  to
be meaningful.  If LINENO is unset, it loses its special proper‐
ties, even if it is subsequently reset.
MACHTYPE
Automatically set to a string that fully  describes  the  system
type  on  which  bash is executing, in the standard GNU cpu-com‐
pany-system format.  The default is system-dependent.
MAPFILE
An array variable (see Arrays below) 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 command.
OPTARG The  value  of the last option argument processed by the getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OPTIND The index of the next argument to be processed  by  the  getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OSTYPE Automatically  set to a string that describes the operating sys‐
tem on which bash is executing.  The  default  is  system-depen‐
dent.
PIPESTATUS
An  array  variable (see Arrays below) containing a list of exit
status values from the processes in  the  most-recently-executed
foreground pipeline (which may contain only a single command).
PPID   The  process  ID  of the shell's parent.  This variable is read‐
only.
PWD    The current working directory as set by the cd command.
RANDOM Each time this parameter is referenced, a random integer between
0 and 32767 is generated.  The sequence of random numbers may be
initialized by assigning a value to RANDOM.  If RANDOM is unset,
it  loses  its  special  properties,  even if it is subsequently
reset.
READLINE_LINE
The contents of the readline line buffer, for use with "bind -x"
(see SHELL BUILTIN COMMANDS below).
READLINE_POINT
The position of the insertion point in the readline line buffer,
for use with "bind -x" (see SHELL BUILTIN COMMANDS below).
REPLY  Set to the line of input read by the read builtin  command  when
no arguments are supplied.
SECONDS
Each  time  this  parameter is referenced, the number of seconds
since shell invocation is returned.  If a value is  assigned  to
SECONDS,  the  value  returned upon subsequent references is the
number of seconds since the assignment plus the value  assigned.
If SECONDS is unset, it loses its special properties, even if it
is subsequently reset.
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 SHELL BUILTIN COMMANDS below).  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 read-only.
SHLVL  Incremented by one each time an instance of bash is started.
UID    Expands to the user ID of the current user, initialized at shell
startup.  This variable is readonly.

The  following  variables  are  used by the shell.  In some cases, bash
assigns a default value to a variable; these cases are noted below.

BASH_ENV
If this parameter is set when bash is executing a shell  script,
its  value  is  interpreted as a filename containing commands to
initialize the shell, as in ~/.bashrc.  The value of BASH_ENV is
subjected  to  parameter  expansion,  command  substitution, and
arithmetic expansion before being interpreted as  a  file  name.
PATH is not used to search for the resultant file name.
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.  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.
CDPATH The search path for the cd command.  This is  a  colon-separated
list  of  directories  in  which the shell looks for destination
directories specified by the cd  command.   A  sample  value  is
".:~:/usr".
COLUMNS
Used  by  the  select compound command to determine the terminal
width when printing selection  lists.   Automatically  set  upon
receipt of a SIGWINCH.
COMPREPLY
An array variable from which bash reads the possible completions
generated by a shell function invoked by the  programmable  com‐
pletion facility (see Programmable Completion below).
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.
FCEDIT The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes  to  ignore  when  performing
filename completion (see READLINE below).  A filename whose suf‐
fix matches one of the entries in FIGNORE is excluded  from  the
list of matched filenames.  A sample value is ".o:~" (Quoting is
needed when assigning a value to this variable,  which  contains
tildes).
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 pathname expansion.  If a filename matched by a
pathname  expansion  pattern also matches one of the patterns in
GLOBIGNORE, it is removed from the list of matches.
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
matching 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 in which command history is saved (see HIS‐
TORY below).  The default value is ~/.bash_history.   If  unset,
the  command  history  is  not  saved  when an interactive shell
exits.
HISTFILESIZE
The maximum number of lines contained in the history file.  When
this  variable  is  assigned  a value, the history file is trun‐
cated, if necessary, by removing the oldest entries, to  contain
no  more  than  that number of lines.  The default value is 500.
The history file is also truncated to this size after writing it
when an interactive shell exits.
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 com‐
plete line (no implicit  `*'  is  appended).   Each  pattern  is
tested  against  the line after the checks specified by HISTCON‐
TROL 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.
HISTSIZE
The  number  of commands to remember in the command history (see
HISTORY below).  The default value is 500.
HISTTIMEFORMAT
If this variable is set and not null, its value  is  used  as  a
format string for strftime(3) 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.
HOME   The home directory of the current user; the default argument for
the cd builtin command.  The value of this variable is also used
when performing tilde expansion.
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 comple‐
tions.  When HOSTFILE is unset, the hostname list is cleared.
IFS    The Internal Field Separator that is  used  for  word  splitting
after  expansion  and  to  split  lines into words with the read
builtin  command.   The  default  value  is  ``‐
line>''.
IGNOREEOF
Controls the action of an interactive shell on receipt of an EOF
character as the sole input.  If set, the value is the number of
consecutive  EOF  characters  which  must  be typed as the first
characters on an input line before bash exits.  If the  variable
exists  but  does not have a numeric value, or has no value, the
default value is 10.  If it does not exist,  EOF  signifies  the
end of input to the shell.
INPUTRC
The  filename  for  the  readline  startup  file, overriding the
default of ~/.inputrc (see READLINE below).
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 pathname expansion, and determines the behavior
of  range  expressions,  equivalence  classes,   and   collating
sequences within pathname expansion and pattern matching.
LC_CTYPE
This  variable  determines  the interpretation of characters and
the behavior of character classes within pathname expansion  and
pattern matching.
LC_MESSAGES
This  variable  determines  the locale used to translate double-
quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale  category  used  for  number
formatting.
LINES  Used  by  the  select  compound  command to determine the column
length for printing selection  lists.   Automatically  set  upon
receipt of a SIGWINCH.
MAIL   If  this  parameter  is  set to a file or directory name and the
MAILPATH variable is not set,  bash  informs  the  user  of  the
arrival  of  mail in the specified file or Maildir-format direc‐
tory.
MAILCHECK
Specifies how often (in seconds)  bash  checks  for  mail.   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.
MAILPATH
A colon-separated list of file names to  be  checked  for  mail.
The message to be printed when mail arrives in a particular file
may be specified by separating the file name  from  the  message
with a `?'.  When used in the text of the message, $_ expands to
the name of the current mailfile.  Example:
MAILPATH='/var/mail/bfox?"You  have  mail":~/shell-mail?"$_  has
mail!"'
Bash  supplies  a default value for this variable, but the loca‐
tion of the user mail files that it  uses  is  system  dependent
(e.g., /var/mail/$USER).
OPTERR If set to the value 1, bash displays error messages generated by
the getopts builtin command (see SHELL BUILTIN COMMANDS  below).
OPTERR  is  initialized to 1 each time the shell is invoked or a
shell script is executed.
PATH   The search path for commands.  It is a colon-separated  list  of
directories  in  which the shell looks for commands (see COMMAND
EXECUTION below).  A zero-length (null) directory  name  in  the
value of PATH indicates the current directory.  A null directory
name may appear as two adjacent colons,  or  as  an  initial  or
trailing  colon.   The  default path is system-dependent, and is
set by the administrator who installs bash.  A common  value  is
``/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin''.
POSIXLY_CORRECT
If  this  variable  is  in the environment when bash starts, the
shell enters 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.
PROMPT_COMMAND
If set, the value is executed as a command prior to issuing each
primary prompt.
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  (see  PROMPTING  below).
Characters removed are replaced with an ellipsis.
PS1    The  value  of  this parameter is expanded (see PROMPTING below)
and used as the primary prompt string.   The  default  value  is
``\s-\v\$ ''.
PS2    The  value of this parameter is expanded as with PS1 and used as
the secondary prompt string.  The default is ``> ''.
PS3    The value of this parameter is used as the prompt for the select
command (see SHELL GRAMMAR above).
PS4    The  value  of  this  parameter  is expanded as with PS1 and the
value is printed before each command  bash  displays  during  an
execution  trace.  The first character of PS4 is replicated mul‐
tiple times, as necessary, to indicate multiple levels of  indi‐
rection.  The default is ``+ ''.
SHELL  The full pathname to the shell is kept in this environment vari‐
able.  If it is not set when the shell starts, bash  assigns  to
it the full pathname of the current user's login shell.
TIMEFORMAT
The  value of this parameter is used as a format string specify‐
ing how the timing information for pipelines prefixed  with  the
time  reserved word should be displayed.  The % character intro‐
duces 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%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.  The select command termi‐
nates 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.  Bash terminates after waiting for
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.
auto_resume
This variable controls how the shell interacts with the user and
job control.  If this variable is set, single word  simple  com‐
mands without redirections are treated as candidates for resump‐
tion of an existing stopped job.  There is no ambiguity allowed;
if  there  is more than one job beginning with the string typed,
the job most recently accessed  is  selected.   The  name  of  a
stopped  job, in this context, is the command line used to start
it.  If set to the value exact, the string supplied  must  match
the  name  of  a  stopped  job exactly; if set to substring, the
string supplied needs to match a substring  of  the  name  of  a
stopped  job.  The substring value provides functionality analo‐
gous to the %?  job identifier (see JOB CONTROL below).  If  set
to  any  other  value, the supplied string must be a prefix of a
stopped job's name; this provides functionality analogous to the
%string job identifier.
histchars
The  two or three characters which control history expansion and
tokenization (see HISTORY EXPANSION below).  The first character
is  the history expansion character, the character which signals
the start of a history  expansion,  normally  `!'.   The  second
character  is the quick substitution character, which is used as
shorthand for re-running the previous command  entered,  substi‐
tuting  one  string  for another in the command.  The default is
`^'.  The optional third character is the character which  indi‐
cates  that the remainder of the line is a comment when found as
the first character of a word, normally `#'.  The  history  com‐
ment 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.