Linux 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
helpis a shell builtin
<== 表明help是内建命令

[root@web ~]#
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

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

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

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]
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

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

whileCOMMANDS; 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
ashort 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 helptopics match `error'. Try `help help' or `man -k error' or `info error'.
[root@web ~]#
help ls
-bash: help: no helptopics 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 $HOME is the
default DIR. The variable CDPATH defines the search path for
the directory containing DIR. Alternative directory names in CDPATH
are separated by a colon (:). A null directory name is the same as
the current directory, i.e. `.'. If DIR begins with a slash (/),
then CDPATH is not used. If the directory is not found, and the
shell option `cdable_vars' is set, then try the word as a variable
name. If that variable has a value, then cd to the value of that
variable. The -P option says to use the physical directory structure
instead of following symbolic links; the -L option forces symbolic links
to be followed.
[root@web ~]#
help pwd
pwd: pwd [-LP]
Print the current working directory. With the -P option, pwd prints
the physical directory, without any symbolic links; the -L option
makes pwd follow symbolic links.
[root@web ~]#
help if
if: if COMMANDS;then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [ else COMMANDS; ] fi
The `if COMMANDS' list is executed. If its exit status is zero, then the
`then COMMANDS' list is executed. Otherwise, each `elif COMMANDS' list is
executed in turn, and if its exit status is zero, the corresponding
`then COMMANDS' list is executed and the if command completes. Otherwise,
the `else COMMANDS' list is executed, if present. The exit status of the
entire construct is the exit status of the last command executed, or zero
if no condition tested true.
[root@web ~]#
help for
for: for NAME [inWORDS ... ;] do COMMANDS; done
The `for' loop executes a sequence of commands for each member in a
list of items. If `in WORDS ...;' is not present, then `in"$@"' 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: whileCOMMANDS; 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 ~]# helpcase
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 canbe
0 to 3 octal digits

You can explicitly turn off the interpretation of the above characters
with the -E option.
[root@web ~]#