Shell Script

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Outline:

  • Intro
  • Shebang
  • Execute Shell Script
  • I/O redirection

Intro

常见Shell指令

scripts vs shell functions

Some differences between shell functions and scripts that you should keep in mind are:

  • Functions have to be in the same language as the shell, while scripts can be written in any language. This is why including a shebang for scripts is important.
  • Functions are loaded once when their definition is read. Scripts are loaded every time they are executed. This makes functions slightly faster to load, but whenever you change them you will have to reload their definition.
  • Functions are executed in the current shell environment( 可以简单理解为， function的所在路径是当前路径 ) whereas scripts execute in their own process. Thus, functions can modify environment variables, e.g. change your current directory, whereas scripts can’t. Scripts will be passed by value environment variables that have been exported using export
  • As with any programming language, functions are a powerful construct to achieve modularity, code reuse, and clarity of shell code. Often shell scripts will include their own function definitions.

Shell Programming Style

https://google.github.io/styleguide/shellguide.html

Shebang

Note that scripts need not necessarily be written in bash to be called from the terminal. For instance, here’s a simple Python script that outputs its arguments in reversed order:

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#!/usr/local/bin/python
import sys
for arg in reversed(sys.argv[1:]):
    print(arg)

  • shebang: the character sequence consisting of #! at the beginning of a script in a Unix-like operating system

    • shell会将shebang中#!之后的内容作为一个程序的路径，打开该程序， 将本script的路径当作参数传入( 即: 将整个script当作input传入shebang所指定的程序 ）

    • For example, if a script is named with the path path/to/script, and it starts with the following line, #!/bin/sh, then the program loader is instructed to run the program /bin/sh, passing path/to/script as the first argument. In Linux, this behavior is the result of both kernel and user-space code.[9]

    • The shebang line is usually ignored by the interpreter, because the "#" character is a comment marker in many scripting languages; some language interpreters that do not use the hash mark to begin comments still may ignore the shebang line in recognition of its purpose.

shebang with env

The shebang expects a full path to the interpreter to use so the following syntax would be incorrect:

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#!python

Setting a full path like this might work:

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#!/usr/local/bin/python

but would be non portable as python might be installed in /bin, /opt/python/bin, or wherever other location.

Using env

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#!/usr/bin/env python

is a method allowing a portable way to specify to the OS a full path equivalent to the one where python is first located in the PATH.

Execute Shell Script

执行脚本文件：

  1. 直接指定sh来执行该脚本，不需要shebang，也不需要脚本有执行权限（因为该脚本直接作为参数传给了sh).

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    sh script_file_path

  2. 需要写shebang来指定解释器, 并且要指定脚本路径.

    必须加上./使得该command name被识别为一个路径名. 否则shell会继续在alias, builtin和PATH中搜索该command name. 1

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    chmod +x script_file ##(chown, chgrp optionally)
    ./script_file

  3. 使用.或source在当前shell session中执行该脚本

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    source script_file

    or

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    . script_file

    因此, 该方法可以用于刷新当前shell环境:

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    source ~/.bashrc

  4. Shell script是能在命令行直接输入的, 但仅会作用一次

注意: 方法1,2都是新开一个子shell session，在其中执行脚本，而方法3, 4是在当前shell session中执行脚本

I/O redirection

一般情况下，每个 Linux 命令运行时都会打开三个文件：

  • 标准输入文件(stdin)：stdin的文件描述符为0，Unix程序默认从stdin读取数据。

  • 标准输出文件(stdout)：stdout 的文件描述符为1，Unix程序默认向stdout输出数据。

  • 标准错误文件(stderr)：stderr的文件描述符为2，Unix程序会向stderr流中写入错误信息

  • Discard the output

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    $ command > /dev/null

    The file /dev/null is a special file that automatically discards all its input.

  • Discard both output of a command and its error output,

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    $ command > /dev/null 2>&1
    • a command normally writes its output to STDOUT
    • use standard redirection to redirect STDERR to STDOUT
    • 这里2>&1将标准错误（2）合并到标准输出（1）， 而标准输出已经被重定向到了/dev/null, 因此总体效果是，标准错误和输出都被重定向到了/dev/null

  • file descriptor:

    • 0 : STDIN
    • 1: STDOUT
    • 2: STDERR
Sr.No. Command & Description
1 pgm > file
Output of pgm is redirected to file
会覆盖目标文件中原有的数据
2 pgm < file
Program pgm reads its input from file
3 pgm >> file
Output of pgm is appended to file
4 n > file
Output from stream with descriptor n redirected to file
5 n >> file
Output from stream with descriptor n appended to file
6 n >& m
Merges output from stream n with stream m
7 n <& m
Merges input from stream n with stream m
8 << tag
Standard input comes from here through next tag at the start of line
9 |
Takes output from one program, or process, and sends it to another

Here Document

Here Document 目前没有统一的翻译，这里暂译为”嵌入文档“。Here Document 是 Shell 中的一种特殊的重定向方式，它的基本的形式如下：

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command << delimiter
document
delimiter

它的作用是将两个 delimiter 之间的内容(document) 作为输入传递给 command

注意：

  • 结尾的delimiter 一定要顶格写，前面不能有任何字符，后面也不能有任何字符，包括空格和 tab 缩进。
  • 开始的delimiter前后的空格会被忽略掉。

下面的例子，通过 wc -l 命令计算 document 的行数：

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wc -l << EOF
    This is a simple lookup program
    for good (and bad) restaurants
    in Cape Town.
EOF

也可将 Here Document 用在脚本中，例如：

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#!/bin/bash

cat << EOF
    This is a simple lookup program
    for good (and bad) restaurants
    in Cape Town.
EOF

  1. https://lyk-love.cn/2023/04/24/Shell-Script-Searching-Path/↩︎