Shell Scripting Knowledge Chaos Set

For example, comparing strings, judging whether the file exists and whether it is readable, etc., usually uses "[]" to represent the condition test.

Note: The space here is important. The space to ensure the square brackets. I have wasted a lot of precious time because the space is missing or the position is wrong.

If ....; Then
....
Elif ...; Then
....
Else
....
Fi
[-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 scripts with conditional selection are generally competent for simple shell scripts 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.

Shell Scripting Knowledge Chaos Set