Add defer keyword

[yassinebenaid]

What will be the impact of having a new keyword defer that defers the execution of a given command.

global shell

defer echo defered

echo foobar

Output:

foobar
defered

functions

it works in functions too:

function foo(){
    defer echo defered

    echo foobar
}

foo

Output:

foobar
defered

sub-shells

works on subshells and command substitution as well:

(
    defer echo defered

    echo foobar
)

Output:

foobar
defered

command substitution

echo $(
    defer echo defered

    echo foobar
)

Output:

foobar
defered

This feature is inspired by the Go programming language.

Thoughts: @simonw @ianwalter @pohart @tangowithfoxtrot @vanodevium

[vanodevium]

@yassinebenaid As for me, defer is mostly golang based word. What about finally?

Anyway, idea is cool.

LGTM

[tangowithfoxtrot]

Agree. I think this is a cool idea. In the case of functions would something like the following work?

my_func() {
  defer rm -rf "$TMP_DIR"
  TMP_DIR="$(mktemp -d)"
  
  # do stuff in $TMP_DIR...
}

# $TMP_DIR was deleted when my_func() is out-of-scope

If so, I could see it being very nice to have. Not sure how challenging it would be to implement it in such a way that if you had something like set -u in your script, it could handle TMP_DIR being set later, but ideally that would work too.