另一种方法，这里还没有提到的是在无限while循环中初始化参数。下面不是最清晰的示例，但它达到了它的目的。

#!/usr/bin/env bash
[ "$1" ] && foo=0 && bar="baz"
while : "${foo=2}" "${bar:=qux}"; do
    echo "$foo"
    (( foo == 3 )) && echo "$bar" && break
    (( foo=foo+1 ))
done

自我记录的功能

您还可以使用:将文档嵌入到函数中。

假设您有一个库脚本mylib.sh，它提供了各种函数。您可以将库(。Mylib.sh)，然后直接调用函数(lib_function1 arg1 arg2)，或者避免使您的命名空间混乱，并使用函数参数调用库(Mylib.sh lib_function1 arg1 arg2)。

如果您还可以输入mylib.sh——help并获得可用函数及其用法的列表，而不必在帮助文本中手动维护函数列表，这不是很好吗?

#!/bin/bash

# all "public" functions must start with this prefix
LIB_PREFIX='lib_'

# "public" library functions
lib_function1() {
    : This function does something complicated with two arguments.
    :
    : Parameters:
    : '   arg1 - first argument ($1)'
    : '   arg2 - second argument'
    :
    : Result:
    : "   it's complicated"

    # actual function code starts here
}

lib_function2() {
    : Function documentation

    # function code here
}

# help function
--help() {
    echo MyLib v0.0.1
    echo
    echo Usage: mylib.sh [function_name [args]]
    echo
    echo Available functions:
    declare -f | sed -n -e '/^'$LIB_PREFIX'/,/^}$/{/\(^'$LIB_PREFIX'\)\|\(^[ \t]*:\)/{
        s/^\('$LIB_PREFIX'.*\) ()/\n=== \1 ===/;s/^[ \t]*: \?['\''"]\?/    /;s/['\''"]\?;\?$//;p}}'
}

# main code
if [ "${BASH_SOURCE[0]}" = "${0}" ]; then
    # the script was executed instead of sourced
    # invoke requested function or display help
    if [ "$(type -t - "$1" 2>/dev/null)" = function ]; then
        "$@"
    else
        --help
    fi
fi

关于代码的一些注释:

All "public" functions have the same prefix. Only these are meant to be invoked by the user, and to be listed in the help text. The self-documenting feature relies on the previous point, and uses declare -f to enumerate all available functions, then filters them through sed to only display functions with the appropriate prefix. It is a good idea to enclose the documentation in single quotes, to prevent undesired expansion and whitespace removal. You'll also need to be careful when using apostrophes/quotes in the text. You could write code to internalize the library prefix, i.e. the user only has to type mylib.sh function1 and it gets translated internally to lib_function1. This is an exercise left to the reader. The help function is named "--help". This is a convenient (i.e. lazy) approach that uses the library invoke mechanism to display the help itself, without having to code an extra check for $1. At the same time, it will clutter your namespace if you source the library. If you don't like that, you can either change the name to something like lib_help or actually check the args for --help in the main code and invoke the help function manually.

它类似于Python中的传递。

一种用法是将一个函数存根，直到它被写入:

future_function () { :; }

其他答案中没有提到的两个用法:

日志记录

以这个示例脚本为例:

set -x
: Logging message here
example_command

第一行set -x使shell在运行命令之前打印出该命令。这是一个非常有用的结构。缺点是通常的echo Log消息类型的语句现在打印两次消息。冒号方法避开了这个问题。请注意，您仍然必须转义特殊字符，就像您对echo所做的那样。

Cron职位名称

我看到它在cron作业中被使用，像这样:

45 10 * * * : Backup for database ; /opt/backup.sh

这是一个cron作业，每天在10:45运行脚本/opt/backup.sh。这种技术的优点是，当/opt/backup.sh打印一些输出时，它使电子邮件主题看起来更好看。

:也可以是块注释(类似于C语言中的/* */)。例如，如果你想在脚本中跳过一段代码，你可以这样做:

: << 'SKIP'

your code block here

SKIP

如果你想把一个文件截断为零字节，这对清除日志很有用，试试这个:

:> file.log