Shell 脚本简要

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主要参考

学习目标: 能写简单脚本, 能看懂长一些的脚本. Shell 脚本不是开发语言, 难以 debug, 不适合写太长.

If you are writing a script that is more than 100 lines long, or that uses non-straightforward control flow logic, you should rewrite it in a more structured language now. Bear in mind that scripts grow. Rewrite your script early to avoid a more time-consuming rewrite at a later date. (Google Shell Style Guide)

Hello world

1. 编写脚本

新建 hello_world 文件.

#!/bin/bash

echo 'Hello world!'  # this is a comment

注释 的语法和 Python 单行注释相同, 当前行 # 后面的内容视为注释.

以 #! 开头的第一行称为 shebang, 告诉系统用什么解释器执行该脚本.

shebang 可以不写 (Show more »)

以 ./hello_world 执行时, usually the parent shell guesses that the script is written for the same shell (minimal Bourne-like shells run the script with /bin/sh, bash runs it as a bash subprocess). (The fact that a program can launch other programs is expressed in the process scheme as a parent process producing a child process.)

另外也可以指定解释器执行, 比如 bash hello_world, python main.py.

sh, bash and dash (Show more »)

The name "bash" is an acronym for "Bourne Again SHell", a reference to the fact bash is an enhanced replacement for sh, the original Unix shell program written by Steve Bourne.

注意现在一般 /bin/sh 软链接到 dash 而不是 bash (以前是). 但默认的 login shell 还是 bash.

2. 使文件可执行

$ chmod 755 hello_world

默认自己只有读写权限 (rw-). 注意可读权限是程序可执行的必要条件, 所以让别人可执行要给 5 (r-x).

3. 将文件放在 shell 可以找到的路径下

$ ./hello_world

如果写成

$ hello_world

会报错. 因为如果不显式指定路径, shell 只在环境变量 PATH 所包含的路径下搜索可执行文件. PATH 默认包括了 /bin, 和 /home/me/bin (创建 ~/bin 目录后重启 shell, 系统一般会自动添加该路径到 PATH) 等.

Good Locations for Scripts (Show more »)

The ~/bin directory is a good place to put scripts intended for personal use. If we write a script that everyone on a system is allowed to use, the traditional location is /usr/local/bin. Scripts intended for use by the system administrator are often located in /usr/local/sbin. In most cases, locally supplied software, whether scripts or compiled programs, should be placed in the /usr/local hierarchy and not in /bin or /usr/bin. These directories are specified by the Linux Filesystem Hierarchy Standard to contain only files supplied and maintained by the Linux distributor.

变量

a=233 TITLE="System Information Report For $HOSTNAME"
b=
d="$(ls -l foo.txt)"  # results of a command
# 可以写成 d=`ls -l foo.txt` 但不推荐
e=$((5 * 7))  # arithmetic expansion
f="\t\ta string\n"  # escape

echo "<html>
    <head><title>${TITLE}</title></head>
    <body><h1>${TITLE}</h1></body>
</html>"
  • 变量名规则同 Python. 习惯用大写字母表示常量, 小写字表示变量.
  • 赋值 = 两侧不能有空格. 一行可以多次赋值.
  • shell 不区分变量类型, 都视为字符串.
  • 单引号不做 string interpolation, 双引号做, 参考 这里.
  • ${} 称为 parameter substitution/expansion (双引号内生效), 类似 Python 的 f-string, 把字符串里面的占位符替换成对应值. 其中 {} 可写可不写, 最好写上避免歧义. $() 称为 command substitution, 见 这里. 单纯的圆括号 () 表示 subshell.
  • 用 " 可以写多行字符串. 另外同 Python, 在行末尾写 \ 为 line continuation.
  • Double quote to prevent globbing and word splitting.
很少用的 declare (Show more »)
  • 声明常量 declare -r TITLE="Page Title"
  • 声明整数变量 `declare -i

如果使用未赋值的变量

$ foo=foo.txt
$ echo $foo1  # 什么都不会打印
$ echo ${foo}1

其中 $foo1 为空 (类似 None/null), 而不是空字符串. 写成 "$foo1" 保证是字符串.

Here documents

少见?

A here document is an additional form of I/O redirection in which we embed a body of text into our script and feed it into the standard input of a command.

command << token
    text
token

where command is the name of command that accepts standard input and token is a string used to indicate the end of the embedded text. Note that the token must appear alone and that there must not be trailing spaces on the line. By default, single and double quotes within here documents lose their special meaning to the shell.

# 例子: cat > ~/foo << _EOF_
# 写成 cat <<- _EOF_ 则忽略 text 中开头的 tab (不忽略空格)
cat << _EOF_
<html>
    <head><title>${TITLE}</title></head>
    <body><h1>${TITLE}</h1></body>
</html>
_EOF_

函数

两种写法. Deprecated, 见 这里

function name {
    commands
    return
}

推荐

name () {
    commands
    return
}

调用方法, 直接写 name, 不要加括号.

局部变量

foo=0

func () {
    local foo
    foo=1
    echo ${foo}
}
  • 不声明局部变量则用的是全局变量.
  • return 可以不写, 默认结尾 return. 如果 return 没写参数 (正整数), 默认 return 最近执行命令的 exit status (exit 命令也是如此, 它写在脚本末尾). 见 这里.
  • shell 函数只能返回 exit status, 见 这里.
  • 不能写参数.

条件语句

两种写法, 见 这里.

x=5
y=~/foo.txt

if [ "$x" -eq 5 -a ! \( -e "$y" \) ]; then
    echo "equal"
elif [ "$x" -lt 5 ]; then
    echo "less than"
else
    echo "greater than"
fi

# 或者不写分号, 但是把 then 写在下一行
# 因为分号只是分隔命令用
# if [ ... ]
# then

Using the quotes around the parameter ensures that the operator is always followed by a string, even if the string is empty.

Exit status

Commands (including the scripts and shell functions we write) issue a value to the system when they terminate, called an exit status. This value, which is an integer in the range of 0 to 255 (没有负数, 一般 0 表示成功), indicates the success or failure of the command’s execution.

执行命令后, 执行 $? 可得上一条命令的 exit status. shell 有两个 bulitin 命令 (不是变量), true 的 exit status 为 0, false 为 1.

test

The command used most frequently with if is test. 两种写法

test expression

第二种更流行

[ expression ]

当 expression 为真时返回 exit status 0, 否则 1. 注意 test 和 [ 都是命令 (后者参数以 ] 结尾, 也因此 [ 后与 ] 前需要空格).

下面详细的要查表, 随便列几个.

File expressions

  • -e file: file exists
  • -d file: file exists and is a directory, -f regular file
  • -x file: 存在且有执行权限, 类似地, -r, -w

String expressions

  • string: string is not null.
  • -n string: the length of string is greater than zero, -z 表示长度为 0
  • string1 == string2: 相等. 在 bash 推荐双等号, 但是 POSIX 只能用单等号. 不相等用 !=

Integer expressions

  • int1 -eq int2: 相等, -ne 不相等. 可以直接用双等号?
  • int1 -le int2: 小于等于, -lt 为小于. 字符串比较用 "<", 记得双引号, 否则会视为 redirection operators. 注意字符串比较大小与整数比较大小方法不同.

Modern test

现代 bash 提供了下述语法 (推荐使用)

[[ expression ]]

比单个方括号增加的功能是正则匹配.

string =~ regex

例如

if [[ "$INT" =~ ^-?[0-9]+$ ]]

其他可参考 这里.

Since all expressions and operators used by test are treated as command arguments by the shell (unlike [[]] and (( ))), characters that have special meaning to bash, such as <, >, (, and ), must be quoted or escaped.

逻辑运算符

  test [[]] and (())
AND -a &&
OR -o ||
NOT ! !

此外 shell 本身可以用 && 或者 || 拼接命令, 短路执行. 可以作为 if 的 one liner.

$ mkdir temp && cd temp
$ [[ -d temp ]] || mkdir temp  # 不存在才创建

(()) for integers

bash 的语法, 少见?

if ((1))  # true
if ((0))  # false
if ((INT == 0))
if ((INT < 0))
if (( ((INT % 2)) == 0 ))

读取键盘输入

The read builtin command is used to read a single line of standard input. This command can be used to read keyboard input or, when redirection is employed, a line of data from a file.

read [-options] [variable...]

If no variable name is supplied, the shell variable REPLY contains the line of data.

少见? 鸽了, 直接看书.

循环

while

foo=1
while [ "$foo" -lt 5 ]; do
    echo "$foo"
    foo=$((foo+1))
done

同 if 可以用双方括号. 此外还有 break 和 continue.

for

for foo in 1 2 3 4; do
    echo "$foo"
done

brace expansion

for foo in {1..4}

此外还有 C 语言形式的 for 循环, 略.

Debug

书上列了一些典型错误, 直接看.

直接 print 大法 (指 echo) 或者 bash 提供了 tracing

#!/bin/bash -x

或者

#!/bin/bash

# blahblah

set -x # Turn on tracing
# blahblah
set +x # Turn off tracing

位置参数

Executing

./script.sh Hello World

Will make

$0 = ./script.sh
$1 = Hello
$2 = World

当参数很多时, The shift command causes all the parameters to “move down one” each time it is executed.

#!/bin/bash

# posit-param2: script to display all arguments

count=1
# $# 参数数量, 不算 $0
while [[ $# -gt 0 ]]; do
    echo "Argument $count = $1"
    count=$((count + 1))
    shift
done

可以在函数里使用这些位置参数.

可以结合 case 写位置参数, 略.

数组

只支持一维数组. 使用场景可以参考 SC2086#exceptions.

# index 从 0 开始, 但是赋值时中间可以不赋值
a[1]=foo
echo ${a[1]}

days=(Sun Mon Tue Wed Thu Fri Sat)
days=([0]=Sun [1]=Mon [2]=Tue [3]=Wed [4]=Thu
[5]=Fri [6]=Sat)

for i in "${days[@]}"; do echo $i; done

Tips

shellcheck 插件提供了 很多建议

  • Use cd ... || exit in case cd fails. See SC2164. 因为默认情况下 shell 脚本遇到错误会继续执行下一句, 而不是退出.
  • Check exit code directly with e.g. if mycmd;, not indirectly with $?. See SC2181.

Google 也有 style guide.