bash.hlp

bash帮助手册

	      \cx    a control-x character

       The  expanded  result  is  single-quoted, as if the dollar sign had not
       been present.

       A double-quoted string preceded by a dollar sign	 ($)  will  cause  the
       string  to  be translated according to the current locale.  If the cur-
       rent locale is C or POSIX, the dollar sign is ignored.  If  the	string
       is translated and replaced, the replacement is double-quoted.

PARAMETERS
       A  parameter is an entity that stores values.  It can be a name, a num-
       ber, or one of the special characters listed below under Special Param-
       eters.	For the shell's purposes, a variable is a parameter denoted by
       a  name.	  A  variable  has  a  value  and  zero	 or  more  attributes.
       Attributes  are assigned using the declare builtin command (see declare
       below in SHELL BUILTIN COMMANDS).

       A parameter is set if it has been assigned a value.  The null string is
       a  valid	 value.	 Once a variable is set, it may be unset only by using
       the unset builtin command (see SHELL BUILTIN COMMANDS below).

       A variable may be assigned to by a statement of the form

	      name=[value]

       If value is not given, the variable is assigned the null	 string.   All
       values  undergo tilde expansion, parameter and variable expansion, com-
       mand substitution, arithmetic expansion, and quote removal (see	EXPAN-
       SION below).  If the variable has its integer attribute set, then value
       is subject to arithmetic expansion even if the  $((...))	 expansion  is
       not  used (see Arithmetic Expansion below).  Word splitting is not per-
       formed, with the exception of "$@" as  explained	 below	under  Special
       Parameters.   Pathname  expansion  is not performed.  Assignment state-
       ments may also appear as arguments to  the  declare,  typeset,  export,
       readonly, and local builtin commands.

   Positional Parameters
       A  positional  parameter	 is a parameter denoted by one or more digits,
       other than the single digit 0.  Positional parameters are assigned from
       the  shell's  arguments when it is invoked, and may be reassigned using
       the set builtin command.	 Positional parameters may not be assigned  to
       with  assignment statements.  The positional parameters are temporarily
       replaced when a shell function is executed (see FUNCTIONS below).

       When a positional parameter consisting of more than a single  digit  is
       expanded, it must be enclosed in braces (see EXPANSION below).

   Special Parameters
       The  shell  treats  several parameters specially.  These parameters may
       only be referenced; assignment to them is not allowed.
       *      Expands to the positional parameters, starting from  one.	  When
	      the  expansion occurs within double quotes, it expands to a sin-
	      gle word with the value of each parameter separated by the first
	      character of the IFS special variable.  That is, "$*" is equiva-
	      lent to "$1c$2c...", where c is the first character of the value
	      of  the IFS variable.  If IFS is unset, the parameters are sepa-
	      rated by spaces.	If IFS is  null,  the  parameters  are	joined
	      without intervening separators.
       @      Expands  to  the positional parameters, starting from one.  When
	      the  expansion  occurs  within  double  quotes,  each  parameter
	      expands to a separate word.  That is, "$@" is equivalent to "$1"
	      "$2" ...	When there are no positional parameters, "$@"  and  $@
	      expand to nothing (i.e., they are removed).
       #      Expands to the number of positional parameters in decimal.
       ?      Expands  to  the status of the most recently executed foreground
	      pipeline.
       -      Expands to the current option flags as  specified	 upon  invoca-
	      tion,  by	 the  set  builtin  command, or those set by the shell
	      itself (such as the -i option).
       $      Expands to the process ID of the shell.  In a  ()	 subshell,  it
	      expands  to  the	process	 ID of the current shell, not the sub-
	      shell.
       !      Expands to the process ID of the most  recently  executed	 back-
	      ground (asynchronous) command.
       0      Expands  to  the name of the shell or shell script.  This is set
	      at shell initialization.	If bash is invoked with a file of com-
	      mands,  $0  is set to the name of that file.  If bash is started
	      with the -c option, then $0 is set to the first  argument	 after
	      the  string to be executed, if one is present.  Otherwise, it is
	      set to the file name used to invoke bash, as given  by  argument
	      zero.
       _      At  shell startup, set to the absolute file name of the shell or
	      shell script being executed as  passed  in  the  argument	 list.
	      Subsequently,  expands to the last argument to the previous com-
	      mand, after expansion.  Also set to the full file name  of  each
	      command  executed and placed in the environment exported to that
	      command.	When checking mail, this parameter holds the  name  of
	      the mail file currently being checked.

   Shell Variables
       The following variables are set by the shell:

       BASH   Expands  to  the	full file name used to invoke this instance of
	      bash.
       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_LINE
	      The current command line.	 This variable is  available  only  in
	      shell  functions	and  external  commands	 invoked  by  the pro-
	      grammable 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 pro-
	      grammable completion  facilities	(see  Programmable  Completion
	      below).

       COMP_WORDS
	      An  array variable (see Arrays below) consisting of the individ-
	      ual words in the current command line.  This variable is	avail-
	      able only in shell functions invoked by the programmable comple-
	      tion facilities (see Programmable Completion below).

       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
	      The  name of any currently-executing shell function.  This vari-
	      able exists only when a shell function  is  executing.   Assign-
	      ments 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.

       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.

       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.

       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