Bash Logic 2

Jan 21, 2024 · 2 min read · bash ·

Doing logic with [[ and ]]

(this topic is covered in the bash manual here)

Return a status of 0 or 1 depending on the evaluation of the bash conditional expressions
example 1: is this a file?

 1$> 
 2$> ls -l
 3total 4
 4-rw-rw-r-- 1 osboxes osboxes 6 Jan 21 08:39 myfile
 5$> 
 6$> [[ -f myfile ]]
 7$> echo $?
 80
 9$> [[ -f myfile5 ]]
10$> echo $?
111
12$>

example 2: is this the correct lexicographical order?

1$> 
2$> [[ camera > computer ]]
3$> echo $?
41
5$> [[ server > bannana ]]
6$> echo $?
70
8$>

example 3: arithmetic comparing

1$> 
2$> [[ 15  -lt  19 ]]
3$> echo $?
40
5$> [[ 15  -lt 8 ]]
6$> echo $?
71
8$>

You can combine this with shell arithmetic:

1$> 
2$> [[ $(( 4+5 ))  -gt 7 ]] 
3$> echo $?
40
5$> echo $(( 4+5 ))
69
7$>

The older [ and ]

(this is covered here)

The original Bourne shell used a command called test.
Bash keep supporting this because it is part of the POSIX standard that tries to make UNIX system work the same.
(see Bourne shell builting)
It works the verys similar to the [ and [[ commands:

 1> 
 2$> ls -l
 3total 4
 4-rw-rw-r-- 1 osboxes osboxes 6 Jan 21 08:39 myfile
 5$> test -f myfile 
 6$> echo $?
 70
 8$> test -f file5
 9$> echo $?
101
11$>

Another example:

1$> 
2$> test $(( 5 + 2 )) -gt 9
3$> echo $?
41
5$> 
6$> test $(( 5 + 2 )) -eq 7
7$> echo $?
80
9$>

Both test and [ are executable commands.
There is also a shell builtin for [.