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44. help

获取有关 bash 内置命令的快速帮助。 例如： 

help set
help test
help if
help for

一行帮助说明： 

$ help -d if
if - Execute commands based on conditional.

一行命令的用法概要： 

$ help -s if
if: if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [ else COMMANDS; ] fi

显示类似于 man 的内置帮助 

$ help -m if
NAME
    if - Execute commands based on conditional.

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

DESCRIPTION
    Execute commands based on conditional.

    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.

    Exit Status:
    Returns the status of the last command executed.

SEE ALSO
    bash(1)

IMPLEMENTATION
    GNU bash, version 4.2.46(2)-release (x86_64-redhat-linux-gnu)
    Copyright (C) 2011 Free Software Foundation, Inc.
    License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>