Even since I started writing Bash script, I always use [[ for if conditional statement, the double square brackets, never the single one version unless compatibility has to be taken into account. Why did I choose it? Its because I read [[ is faster. Ive never tried to confirm it by myself, although I do know that [ is a type of shell builtin command and [[ is type of shell keyword, and [ is equivalent to test, which is also a builtin command in Bash for very long time.1
Note
[ and [[ can also be used for arithmetic comparison, such as the use of -gt operator, see another test with Arithmetic Evaluation, ((. This post only focuses on the strings.
From time to time, I often see people still using [ even in the script with a few Bash-only syntaxes or features. They are clearly not writing with compatibility as a requirement. If I have a chance, then I would probably advise the coder to change to [[. I had done so a few times in the past.
However, I never see the numbers, so I used the following to test:
time for ((i = 0; i < 10000; i++)); do <test> -z '' ; done
The result is:
| <test> with | time | % slower |
|---|---|---|
| [ | 00.149s | 00,063.7% |
| [[ | 00.091s | fastest |
| test (builtin) | 00.150s | 00,064.8% |
| /usr/bin/test | 13.798s | 15,063.6% |
[ and test possibly are synonyms since they are pretty close.
As you can see [[ definitely is the winner, and /usr/bin/test external command is the slowest. The problem with /usr/bin/test isnt that is inefficient, but external command is costly.
If you are new to Bash, just use [[.
| [1] | If you dont know the differences between keyword, builtin command, and external command, google them. |