Why is this Bash function within a git alias executing twice, and why does adding `exit` fix it?

Question

If I fail to explicitly call exit for certain function-based Bash scripts then there are additional unexpected executions for some functions. What is causing this? The behavior

Answer 1

That's because of the way git handles aliases:

Given an alias

[alias]
    myalias = !string

where string is any string that represents some code, when calling git myalias args where args is a (possibly empty) list of arguments, git will execute:

    sh -c 'string "$@"' 'string' args

For example:

[alias]
    banana = !echo "$1,$2,SNIP "

and calling

git banana one 'two two' three

git will execute:

sh -c 'echo "$1,$2,SNIP " "$@"' 'echo "$1,$2,SNIP "' one 'two two' three

and so the output will be:

one,two two,SNIP one two two three

In your case,

[alias]
    encrypt-for = "!g(){ echo \"once\";};$1;"

and calling

git encrypt-for g

git will execute:

sh -c 'g(){ echo "once";};$1;"$@"' 'g(){ echo "once";};$1;' g

For clarity, let me rewrite this in an equivalent form:

sh -c 'g(){ echo "once";};$1;"$@"' - g

I only replaced the 'g(){ echo "once";};$1;' part (that will be sh's $0's positional parameter and will not play any role here) by a dummy argument -. It should be clear that it's like executing:

g(){ echo "once";};g;g

so you'll see:

once
once

To remedy this: don't use parameters! just use:

[alias]
    encrypt-for = "!g(){ echo "once";};"

Now, if you really want to use parameters, make sure that the trailing parameters given are not executed at all. One possibility is to add a trailing comment character like so:

[alias]
    encrypt-for = "!g(){ echo "once";};$1 #"

For your full example, a cleaner way could also be to wrap everything in a function:

[alias]
    encrypt-for = "!main() {\
        case $1 in \
            (github) echo github;; \
            (twitter) echo twitter;; \
            (facebook) echo facebook;; \
            (*) echo >&2 \"error, unknown $1"\; exit 1;; \
        esac \
    }; main"

---

Hopefully you understood what git is doing under the hood with aliases! it really appends "$@" to the alias string and calls sh -c with this string and the given arguments.