On Thu, Nov 21, 2024 at 08:14:35 +0100, Erwan David wrote:
> On Thu, Nov 21, 2024 at 07:39:33AM CET, Bitfox <h...@bitfox.ddns.net> said:
> >
> > BTW, what’s the difference between [[ ]] and [ ] here? I know only the
> > latter.
>
> IIRC, [[ ]] is a bash/zsh builtin, [ ] is /bin/[ other name of
> /bin/test
That's partly correct. [[ is a "shell keyword" in bash, meaning it can
be parsed differently. For example, you can put ( ) && || inside it,
which normally wouldn't be allowed. [ is a "shell builtin" in bash,
which means it has the parsing behavior of an ordinary command.
There are /usr/bin/[ and /usr/bin/test commands as well, but you'll
almost never use those. The shell builtin commands take precedence,
unless you disable them, or explicitly type out /usr/bin/[ etc.
The behavior of [ and test is dictated by POSIX, which is a set of minimal
standards for shell commands (and other things). Shells are free to add
their own extensions on top of what POSIX requires, and *all* of them do.
The behavior of [[ is "whatever the shell's developers want it to be",
since it's not a POSIX command at all.
You'll often find that people writing bash script (as opposed to portable
sh scripts) prefer to use [[ because it handles certain things in a
clearer way, as well as providing additional features. For example,
[[ $a = 1 ]] # You don't need quotes around "$a" here.
[[ $file = *.mp3 ]] # The right hand side of = uses glob matching.
[[ $file =~ $myregex ]] # =~ does a regular expression (ERE) match.
[[ ( $x -ge 1 ) && ( $y -le 5 ) ]]
The nearest sh equivalents of these would be:
[ "$a" = 1 ]
case $file in *.mp3) ...;; esac
if printf %s "$file" | grep -q -E -- "$myregex"
[ "$x" -ge 1 ] && [ "$y" -le 5 ] # Two separate [ commands.
# DO NOT use [ "$x" -ge 1 -a "$y" -le 5 ]. That's unspecified.
In the specific example code that was posted earlier in this thread,
the [[ command didn't offer any real advantage over [. However, since
the script was already a bash script, one presumes the author was simply
following the "use [[ in bash, [ in sh" convention.
