Please explain the syntax of this 'if' statement

[DavidEwan]

I'm looking at this page:
https://docs.github.com/en/actions/learn-github-actions/contexts

and this statement in particular:
if [[ ${{ steps.generate_number.outputs.random_number }} == 0 ]]; then exit 0; else exit 1; fi

Why are there two square brackets? What purpose do they serve and why are there two of them??
If you could point me to a docs page that explains that would be great. Thanks.

[Shivansh-Mittal]

I searched on google for this query and found this discussion:
https://unix.stackexchange.com/questions/99185/what-do-square-brackets-mean-without-the-if-on-the-left

I hope this may help.

[thuongtruong1009]

Hi @DavidEwan
Because this is the basic syntax standard set by GitHub, to avoid the syntax overlap with template_literals in some languages like javascript `${something}`
So it is a common standard to set and call variable names in some commands like Github Action
See Two square brackets syntax in Github Action example syntax

[airtower-luna]

That line is part of a run step running in Bash. The double square brackets are part of Bash's syntax for conditional expressions, and double square brackets support some extensions (e.g. regular expression matching) relative to the single square bracket conditionals. See Conditional Constructs in the Bash Manual for details.