CODEWITHSUNDEEP
linuxbash
Vamsi Prabhala
Vamsi Prabhala

Reputation: 49260

Why does excluding a ] to the test command [ not stop script execution?

I have a Bash script as the following.

#!/bin/bash
if [ "1" -eq "1"
then echo "something"
else echo "some other thing"
fi

When i execute the script, I get the following output

test.sh: line 2: [: missing `]'
some other thing

which is unclear to me, as I excepted the script to exit with an error. Can this be explained in more detail if it is an expected behavior?

However when i use the compound command [[ 

#!/bin/bash
if [[ "1" == "1"
then echo "something"
else echo "some other thing"
fi

i do get the error and the script stops executing further.

test.sh: line 2: syntax error in conditional expression
test.sh: line 3: syntax error near `then'
test.sh: line 3: `then echo "something"'

Upvotes: 1

Views: 69

Answers (2)

Charles Duffy
Charles Duffy

Reputation: 295619

[ is not bash syntax, it's just a command (a command with a shell-builtin implementation available as a performance optimization, yes, but subject to the same parse-time behavior as the standard /bin/[ external implementation or any other command). Putting an invalid ping command or an invalid grep command in the same position doesn't make your script's syntax invalid either.

By contrast, [[ is actual syntax, which is what makes the features it has over [ (a set which includes recognizing patterns without expanding them as globs, suppressing string-splitting and glob expansion, letting && and || to be used to combine tests, etc) possible, and also makes invalid usage able to cause a parse error.

If you want to reduce confusion to readers, consider using the name test in place of [. While [ and test are typically aliases to the same command (with the only difference in behavior being whether ] is required), it's much more visually clear to readers that if test 1 -eq 1; then is running a command named test and not some kind of special shell syntax.

Upvotes: 3

muru
muru

Reputation: 4896

$ type [[ [
[[ is a shell keyword
[ is a shell builtin

[ is an ordinary built-in. [[ is a shell keyword, and it has special syntactic significance (e.g., variables do not undergo word splitting). So an invalid usage of [[ can be a parsing error, stopping the script from being parsed correctly, whereas similar usage of [ is just the built-in returning a non-zero exit code with possibly some error output. It makes no more difference to the shell than does, say, false returning a non-zero exit code.

Upvotes: 1

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