Shell-Judgment statement

The process controls the "if" expression if the condition is true then the following part is executed: if ...; Then

....
Elif ...; Then
....
Else
....
Fi
In most cases, you can use test commands to test the condition. 　　For example, you can compare strings, determine whether the file exists and whether it is readable, etc... The condition test is usually represented by "[]". Note that the space here is important. The space to ensure the square brackets.
[-F "somefile"]: Determine if it is a file
[-X "/bin/ls"]: Determine if/bin/ls exists and has executable permissions
[-N ' $var]: Determine if the $var variable has a value
["$a" = "$b"]: Determine if $ A and $b are equal-r file user readable as True
-W file user can write as true
-X file user can execute as true
-F file is true for regular files
-d file files are directory-True
-C File file is true for character special files
-B file files are true for block special files
-S file files non-0 o'clock True
-T file is true when the specified device is terminal (default = 1)

#########################################################
Shell script with conditional selection
Simple shell scripts are generally competent for tasks that do not contain variables. However, when you perform some decision-making tasks, you need to include the if/then criteria to judge. Shell scripting supports such operations, including comparison operations, determining whether a file exists, and so on. The basic if Condition command options are:-eq-compare two parameters for equality (for example, if [2–eq 5])
-ne-comparison of two parameters is not equal
-lt-parameter 1 is less than parameter 2
-le-parameter 1 is less than or equal to parameter 2
-gt-parameter 1 is greater than parameter 2
-ge-parameter 1 is greater than or equal to parameter 2
-f-Check if a file exists (for example, if [-F "filename"])
-D Check if directory exists
Almost all judgments can be implemented with these comparison operators. The common-f command option in a script checks to see if it exists before executing a file.

################################################################## determine if a file exists
#!/bin/sh
Today= ' date-d yesterday +%y%m%d '
File= "Apache_$today.tar.gz"
Cd/home/chenshuo/shell
If [-F "$file"];then
echo "OK"
Else
echo "Error $file" >error.log
Mail-s "fail backup from test" [email protected] fi

Shell-Judgment statement