Complex Git branch name broke all Git commands

Problem

Can anyone explain what happened? [...] I'd love to be able to delete that branch, but Git won't work for me.

By running

git branch SSLOC-201_Implement___str__()_of_ProductSearchQuery

in zsh, you did not create any branch. Instead, you accidentally defined three shell functions, called git, branch, and SSLOC-201_Implement___str__, which ignore their parameters (if any) and whose body is _of_ProductSearchQuery. You can check for yourself that this is indeed what happened, by invoking the builtin zsh command called functions, which lists all existing shell functions:

$ functions                                                     
SSLOC-201_Implement___str__ () {
    _of_ProductSearchQuery
}
branch () {
    _of_ProductSearchQuery
}
git () {
    _of_ProductSearchQuery
}

Unfortunately, although the other two shell functions are not problematic, the shell function called "git" now shadows the bona fide git command!

$ which git
git () {
    _of_ProductSearchQuery
}
# but the real "git" is a binary file that lives in /usr/local/bin/git (or some similar path)

Therefore, you will subsequently get the error

command not found: _of_ProductSearchQuery

whenever you attempt to run a Git command, e.g. git log, git status, etc. (assuming, of course, that no command called _of_ProductSearchQuery exists).

Side note

[...] I get the same error: git:176: command not found: _of_ProductSearchQuery (with the number after git increasing every time I type a command)

That number simply corresponds to the value of HISTCMD, an environment variable that holds

[t]he current history event number in an interactive shell, in other words the event number for the command that caused $HISTCMD to be read.

See the zsh manual for more details.

Solution

And how do I get back to normal?

Simply delete the problematic shell function (and the other two you created by accident, while you're at it):

unset -f git
unset -f branch SSLOC-201_Implement___str__

Then everything should be fine.

What if unset is shadowed also?!

Good question! I refer you to Wumpus W. Wumbley's excellent comment below.

Branch-naming tips

Avoid any special shell characters

Yes, as pointed out in the comments, parentheses are valid characters in Git branch names; you just need to quote the name appropriately, e.g.

$ git branch 'foo()bar'
$ git branch
  foo()bar
* master
$ git checkout 'foo()bar'
Switched to branch 'foo()bar'

However, the need for quoting such names every single time when used as command-line arguments should convince you to eschew parentheses in reference names. More generally, you should (as much as possible) avoid characters that have a special meaning in shells, to prevent surprises like this one.

Use simple branch names

You should keep your branch names short and sweet anyway. Long descriptions like

SSLOC-201_Implement___str__()_of_ProductSearchQuery

belong in commit messages, not in branch names.

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