用途說明

help命令顧名思義就是顯示幫助資訊的，它是個Bash內建命令，也只是用來顯示Bash內建命令的幫助資訊的（Display helpful  information about builtin commands）。如果要顯示外部命令的幫助資訊，可以使用man命令或者info命令。在使用Linux或Unix系統時，我們始終要記住，所有的命令幾乎都可以在系統中找到幫助資訊，而通過網際網路找來的資訊只是幫助你迅速入門，而且英文版的資料比中文版的資料可能更靠譜一些，本文也不例外。

常用引數

help命令如果不帶任何引數，則顯示Bash常用內建命令的列表。

格式：help

help命令如果跟上命令名稱，則顯示此命令的詳細幫助資訊。

格式：help COMMAND

如果只想顯示簡單的幫助資訊，加上-s即可。

格式：help -s COMMAND

使用示例

示例一 顯示內建命令列表

[[email protected] ~]# type -a help

helpis a shell builtin      <== 表明help是內建命令

[[email protected] ~]# help

GNU bash, version3.2.25(1)-release (i686-redhat-linux-gnu)

These shell commandsare defined internally.  Type `help' to see this list.

Type `help name' tofind out more about the function `name'.

Use `info bash' tofind out more about the shell in general.

Use `man -k' or`info' to find out more about commands not in this list.

A star (*) next to aname means that the command is disabled.

 JOB_SPEC[&]                      (( expression ))

 . filename[arguments]            :

 [ arg...]                        [[ expression ]]

 alias [-p][name[=value] ... ]     bg [job_spec ...]

 bind[-lpvsPVS] [-m keymap] [-f fi break [n]

 builtin[shell-builtin [arg ...]]  caller [EXPR]

 case WORD in[PATTERN [| PATTERN]. cd [-L|-P] [dir]

 command [-pVv]command [arg ...]   compgen [-abcdefgjksuv] [-o option

 complete[-abcdefgjksuv] [-pr] [-o continue [n]

 declare[-afFirtx] [-p] [name[=val dirs [-clpv] [+N] [-N]

 disown [-h][-ar] [jobspec ...]    echo [-neE] [arg ...]

 enable [-pnds][-a] [-f filename]  eval [arg ...]

 exec [-cl] [-aname] file [redirec exit [n]

 export [-nf][name[=value] ...] or false

 fc [-e ename][-nlr] [first] [last fg [job_spec]

 for NAME [inWORDS ... ;] do COMMA for (( exp1; exp2; exp3 )); do COM

 function NAME{ COMMANDS ; } or NA getopts optstring name [arg]

 hash [-lr] [-ppathname] [-dt] [na help [-s] [pattern ...]

 history [-c][-d offset] [n] or hi if COMMANDS; then COMMANDS; [ elif

 jobs [-lnprs][jobspec ...] or job kill [-s sigspec | -n signum | -si

 let arg [arg...]                 local name[=value] ...

 logout                            popd [+N | -N] [-n]

 printf [-vvar] format [arguments] pushd [dir | +N | -N] [-n]

 pwd[-LP]                         read [-ers] [-u fd] [-t timeout] [

 readonly [-af][name[=value] ...]  return [n]

 select NAME[in WORDS ... ;] do CO set [--abefhkmnptuvxBCHP] [-o opti

 shift[n]                         shopt [-pqsu] [-o long-option] opt

 sourcefilename [arguments]        suspend [-f]

 test[expr]                       time [-p] PIPELINE

 times                             trap [-lp] [arg signal_spec ...]

 true                              type [-afptP] name [name ...]

 typeset[-afFirtx] [-p] name[=valu ulimit [-SHacdfilmnpqstuvx] [limit

 umask [-p][-S][mode]            unalias [-a] name [name ...]

 unset [-f][-v] [name ...]         until COMMANDS;do COMMANDS; done

 variables -Some variable names an wait [n]

 whileCOMMANDS; do COMMANDS; done  { COMMANDS ; }

[[email protected] ~]# help help

help: help [-s][pattern ...]

   Display helpful information about builtin commands.  If PATTERN is

   specified, gives detailed help on all commands matching PATTERN,

   otherwise a list of the builtins is printed.  The -s option

   restricts the output for each builtin command matching PATTERN to

    ashort usage synopsis.

[[email protected] ~]# help -s help

help: help [-s][pattern ...]

[[email protected] ~]#

示例二 非內建命令的幫助資訊

[[email protected] ~]# help error

-bash: help: no helptopics match `error'.  Try `help help' or `man -k error' or `info error'.

[[email protected]b ~]# help ls

-bash: help: no helptopics match `ls'.  Try `help help' or `man -k ls' or `info ls'.

[[email protected] ~]# man ls

示例三 一些常用內建命令的幫助資訊

[[email protected] ~]# help cd

cd: cd [-L|-P] [dir]

   Change the current directory to DIR.  The variable $HOME is the

   default DIR.  The variable CDPATH defines the search path for

   the directory containing DIR.  Alternative directory names in CDPATH

   are separated by a colon (:).  A null directory name is the same as

   the current directory, i.e. `.'.  If DIR begins with a slash (/),

   then CDPATH is not used.  If the directory is not found, and the

   shell option `cdable_vars' is set, then try the word as a variable

   name.  If that variable has a value, then cd to the value of that

   variable.  The -P option says to use the physical directory structure

   instead of following symbolic links; the -L option forces symbolic links

   to be followed.

[[email protected] ~]# help pwd

pwd: pwd [-LP]

   Print the current working directory.  With the -P option, pwd prints

   the physical directory, without any symbolic links; the -L option

   makes pwd follow symbolic links.

[[email protected] ~]# help if

if: if COMMANDS;then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [ else COMMANDS; ] fi

   The `if COMMANDS' list is executed.  If its exit status is zero, then the

   `then COMMANDS' list is executed.  Otherwise, each `elif COMMANDS' list is

   executed in turn, and if its exit status is zero, the corresponding

   `then COMMANDS' list is executed and the if command completes.  Otherwise,

   the `else COMMANDS' list is executed, if present.  The exit status of the

   entire construct is the exit status of the last command executed, or zero

   if no condition tested true.

[[email protected] ~]# help for

for: for NAME [inWORDS ... ;] do COMMANDS; done

   The `for' loop executes a sequence of commands for each member in a

   list of items.  If `in WORDS ...;' is not present, then `in"[email protected]"' is

   assumed.  For each element in WORDS, NAME is set to that element, and

   the COMMANDS are executed.

for ((: for (( exp1;exp2; exp3 )); do COMMANDS; done

   Equivalent to

       (( EXP1 ))

       while (( EXP2 )); do

               COMMANDS

               (( EXP3 ))

       done

   EXP1, EXP2, and EXP3 are arithmetic expressions.  If any expression is

   omitted, it behaves as if it evaluates to 1.

[[email protected] ~]# help while

while: whileCOMMANDS; do COMMANDS; done

   Expand and execute COMMANDS as long as the final command in the

   `while' COMMANDS has an exit status of zero.

[[email protected] ~]# helpcase

case: case WORD in[PATTERN [| PATTERN]...) COMMANDS ;;]... esac

   Selectively execute COMMANDS based upon WORD matching PATTERN.  The

   `|' is used to separate multiple patterns.

[[email protected] ~]# help echo

echo: echo [-neE][arg ...]

   Output the ARGs.  If -n is specified, the trailing newline is

   suppressed.  If the -e option is given, interpretation of the

   following backslash-escaped characters is turned on:

       \a      alert (bell)

       \b      backspace

       \c      suppress trailing newline

       \E      escape character

       \f      form feed

       \n      new line

       \r      carriage return

       \t      horizontal tab

       \v      vertical tab

       \\      backslash

       \0nnn   the character whose ASCII code is NNN (octal).  NNN canbe

               0 to 3 octal digits

   You can explicitly turn off the interpretation of the above characters

   with the -E option.

[[email protected] ~]#