Conditional expressions of shell programming

There are three forms of statements that can be used as expressions of if statements in linux

1. Command form

If the command is executed successfully, it returns 0 (such as grep, a match is found) and if
the execution fails, it returns non-zero (grep, no match is found)

if commend
then codes
fi

example

read -p"输入用户名：" name
if id -u $name > /dev/null 2>&1#
then echo "$name exist"
else 
echo "$name does not exist!"

Enter username: adins
adins does not exist

2. Functional form

If the function executes successfully, it returns true, and if it fails, it returns false.

#!/bin/bash
fun(){
    
    
   echo "adins"
}
if fun
then echo test
fi

Three, test, [] form

In fact, the expressions in the form of test and [] have the same effect.
Recommend a blog: Linux Bash Shell Programming (8): Conditional Judgment and Examples