Bash Bracket Cheat Sheet (By Julia Evans)

[ Is a Builtin, But [[ Is Part of the Language

http://www.oilshell.org/blog/2016/10/12.html

There are lots of subtleties about it, which I don't really recommend learning. Oil fixes a bunch of these problems:

http://www.oilshell.org/blog/2020/01/simplest-explanation.ht...

But if you really want to know:

* It also has == which does NOT mean equality. It means fnmatch (glob) the right-hand side.

* Similarly, it has =~ to match a regex on the RHS. However you should avoid using a literal regex, and use a var $pat instead, because bash's lexing/escaping/quoting of the pattern is busted, and it even says so in the bash manual.

Summary:

    [ "$a" == "$b" ] # string equality
    [ "$a" = "$b" ]  # string equality
    [[ $a == $b ]]   # fnmatch
    [[ $a = $b ]]    # fnmatch
    [[ $a =~ $b ]]   # regex match
    [[ $a -eq $b ]]  # numeric equality
    (( a == b ))     # numeric equality
    (( a = b ))      # numeric assignment!

It has dynamic types and operators like Python or JS. There's only one expression language, which replaces all these inconsistent constructs.

In other words, could a competent engineer write shell scripts well yet not have encountered that specific feature?