Linux命令之help - 显示Bash内建命令的帮助信息

用途说明

help命令顾名思义就是显示帮助信息的，它是个Bash内建命令，也只是用来显示Bash内建命令的帮助信息的（Display  helpful  information about builtin commands）。如果要显示外部命令的帮助信息，可以使用man命令或者info命令。在使用Linux或Unix系统时，我们始终要记住，所有的命令几乎都可以在系统中找到帮助信息，而通过互联网找来的信息只是帮助你迅速入门，而且英文版的资料比中文版的资料可能更靠谱一些，本文也不例外。

常用参数

help命令如果不带任何参数，则显示Bash常用内建命令的列表。

格式：help

help命令如果跟上命令名称，则显示此命令的详细帮助信息。

格式：help COMMAND

如果只想显示简单的帮助信息，加上-s即可。

格式：help -s COMMAND

 

使用示例

示例一 显示内建命令列表

[root@web ~]# type -a help 
help is a shell builtin      <== 表明help是内建命令 
[root@web ~]# help 
GNU bash, version 3.2.25(1)-release (i686-redhat-linux-gnu)
These shell commands are defined internally.  Type `help' to see this list.
Type `help name' to find out more about the function `name'.
Use `info bash' to find 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 a name 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] [-a name] file [redirec exit [n]
 export [-nf] [name[=value] ...] or false
 fc [-e ename] [-nlr] [first] [last fg [job_spec]
 for NAME [in WORDS ... ;] do COMMA for (( exp1; exp2; exp3 )); do COM
 function NAME { COMMANDS ; } or NA getopts optstring name [arg]
 hash [-lr] [-p pathname] [-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 [-v var] 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
 source filename [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]
 while COMMANDS; do COMMANDS; done  { COMMANDS ; }
[root@web ~]# 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
    a short usage synopsis.
[root@web ~]# help -s help 
help: help [-s] [pattern ...]
[root@web ~]#

示例二 非内建命令的帮助信息

[root@web ~]# help error 
-bash: help: no help topics match `error'.  Try `help help' or `man -k error' or `info error'.
[root@web ~]# help ls 
-bash: help: no help topics match `ls'.  Try `help help' or `man -k ls' or `info ls'.
[root@web ~]# man ls

示例三 一些常用内建命令的帮助信息

[root@web ~]# help cd 
cd: cd [-L|-P] [dir]
    Change the current directory to DIR.  The variable $HOME is the
    default DIR.  The variable CDPATH defines the search path for
    the directory containing DIR.  Alternative directory names in CDPATH
    are separated by a colon (:).  A null directory name is the same as
    the current directory, i.e. `.'.  If DIR begins with a slash (/),
    then CDPATH is not used.  If the directory is not found, and the
    shell option `cdable_vars' is set, then try the word as a variable
    name.  If that variable has a value, then cd to the value of that
    variable.  The -P option says to use the physical directory structure
    instead of following symbolic links; the -L option forces symbolic links
    to be followed.
[root@web ~]# help pwd 
pwd: pwd [-LP]
    Print the current working directory.  With the -P option, pwd prints
    the physical directory, without any symbolic links; the -L option
    makes pwd follow symbolic links.
[root@web ~]# help if 
if: if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [ else COMMANDS; ] fi
    The `if COMMANDS' list is executed.  If its exit status is zero, then the
    `then COMMANDS' list is executed.  Otherwise, each `elif COMMANDS' list is
    executed in turn, and if its exit status is zero, the corresponding
    `then COMMANDS' list is executed and the if command completes.  Otherwise,
    the `else COMMANDS' list is executed, if present.  The exit status of the
    entire construct is the exit status of the last command executed, or zero
    if no condition tested true.
[root@web ~]# help for 
for: for NAME [in WORDS ... ;] do COMMANDS; done
    The `for' loop executes a sequence of commands for each member in a
    list of items.  If `in WORDS ...;' is not present, then `in "$@"' 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.
[root@web ~]# help while 
while: while COMMANDS; do COMMANDS; done
    Expand and execute COMMANDS as long as the final command in the
    `while' COMMANDS has an exit status of zero.
[root@web ~]# help case
case: case WORD in [PATTERN [| PATTERN]...) COMMANDS ;;]... esac
    Selectively execute COMMANDS based upon WORD matching PATTERN.  The
    `|' is used to separate multiple patterns.
[root@web ~]# 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 can be
                0 to 3 octal digits
    
    You can explicitly turn off the interpretation of the above characters
    with the -E option.

[root@web ~]#

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