退出状态

命令（包括我们编写的脚本和 shell 函数）在终止时向系统发出一个值，称为退出状态。该值是 0 到 255 范围内的整数，表示命令执行成功或失败。按照惯例，值为零表示成功，任何其他值表示失败。

IF语句

在 shell 脚本中，通过上一个命令的退出状态决定是否执行下一个命令，保证多条命令能按顺序正确执行。

;：是命令分割符，可以在一行放置多个命令。

if commands; then
commands
[elif commands; then
commands...]
[else
commands]
fi

IF ELIF ELSE

#!/bin/bash

echo -n "Enter a number between 1 and 3 inclusive > "
read character
if [ "$character" = "1" ]; then
    echo "You entered one."
elif [ "$character" = "2" ]; then
    echo "You entered two."
elif [ "$character" = "3" ]; then
    echo "You entered three."
else
    echo "You did not enter a number between 1 and 3."
fi

CASE

语法格式：

case word in
  patterns ) commands ;;
esac

case可以拥有任意数量的模式和陈述。模式可以使文本文字或通配符。可以使用"|"分割多个模式。

"*"：可以匹配所有的字符，放在最后校验无效的输入

#!/bin/bash
echo -n "Type a digit or a letter > "
read character
case $character in
  # Check for letters
  [[:lower:]] | [[:upper:]] ) echo "You typed the letter $character"
          ;;
  # Check for digits
  [0-9] )      echo "You typed the digit $character"
          ;;
  # Check for anything else
  * )       echo "You did not type a letter or a digit"
esac

TEST使用

一般和if联合使用，用来做校验。

有两种表达形式：

  • test expression
  • [ expression ]
if [ -f .bash_profile ]
then
  echo "You have a .bash_profile. Things are fine."
else
  echo "Yikes! You have no .bash_profile!"
fi

常用的判断表达式：

Expression Description Expression Description
-d file True if file is a directory. -r file True if file is a file readable by you.
-e file True if fileexists. -w file True if file is a file writable by you.
-f file True if file exists and is a regular file. -x file True if file is a file executable by you.
-L file True if file is a symbolic link. file1 -nt file2 True if file1 is newer than file2.
-z string True if string is empty. file1 -ot file2 True if file1 is older than file2
-n string True if string is not empty. string1 !=string2 True if string1 does not equal string2.
string1 = string2 True if string1 equals string2.

EXIT

在脚本完成时设置退出状态

  • exit 0：成功
  • exit 1：失败

跟踪

可以在脚本中使用set命令打开和关闭跟踪。

  • set -x：打开跟踪
  • set +x：关闭跟踪

循环

在没有接收到退出命令前，一直循环执行。注意循环条件，避免死循环。

while

#!/bin/bash
number=0
while [ "$number" -lt 10 ]; 
do
  echo "Number = $number"
  number=$((number + 1))
done

until

#!/bin/bash
number=0
until [ "$number" -ge 10 ]; 
do
  echo "Number = $number"
  number=$((number + 1))
done

for

for variable in words; 
do
  commands
done