shell 之 if 判断

一、shell 示例

#!/bin/bash

str="Hello"
if [ -n "$str" ]; then
    echo "The string is not empty."
else
    echo "The string is empty."
fi

一、if 表达式说明

在 shell 脚本中，-n 选项用于条件表达式，以检查字符串是否为非空（non-empty）。

if [ -n "$str" ]; 的分解：

• -n：测试字符串是否不为空（即，其长度大于零）。
• "$str"：正在测试的变量。将其括在双引号中可防止变量为空或未设置时出现错误。
• [ 和 ]：这些是 test 命令的一部分，用于评估表达式。确保括号和括号内的内容之间有空格。

二、常见陷阱

务必确保 [ 之后和 ] 之前有一个空格，否则您会遇到语法错误。
正确的写法：

if [ -n "$str" ];   # 正确
if [ -n "$str"];    # 错误 (空格缺失)

三、Option 拓展

文件判断：

-e file exists
-f file is a regular file (not a directory or device file)
-s file is not zero size
-d file is a directory
-b file is a block device
-c file is a character device
-p file is a pipe
-L file is a symbolic link
-S file is a socket
-t file (descriptor) is associated with a terminal device
-r file has read permission (for the user running the test)
-w file has write permission (for the user running the test)
-x file has execute permission (for the user running the test)

字符串判断：

[ -z STRING ]  
- True if the length of "STRING" is zero.

[ -n STRING ] or [ STRING ] 
- True if the length of "STRING" is non-zero.


[ STRING1 == STRING2 ] 
- True if the strings are equal. 
  "=" may be used instead of "==" for strict POSIX compliance.

[ STRING1 != STRING2 ] 
- True if the strings are not equal.

[ STRING1 < STRING2 ] 
- True if "STRING1" sorts before "STRING2" lexicographically in the current locale.

[ STRING1 > STRING2 ] 
- True if "STRING1" sorts after "STRING2" lexicographically in the current locale.

数字判断：

[ ARG1 OP ARG2 ] 
- "ARG1" and "ARG2" are integers.
- "OP" is one of:

  -eq : equal to,
  -ne : not equal to,
  -lt : less than,
  -le : less than or equal to,
  -gt : greater than,
  -ge : or greater than or equal to.

These arithmetic binary operators return true if "ARG1" is [OP] "ARG2", respectively. 

Ref

• https://unix.stackexchange.com/a/628130/237356
• https://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html
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