How to “hide” an executable from a bash script?

Question

I want to test the output of a bash script when one of the executables it depends on is missing, so I want to run that script with the dependency "hidden" but no others. PATH= ./script isn't an option because the script needs to run other executables before it reaches the statement I want to test. Is there a way of "hiding" an executable from a script without altering the filesystem?

For a concrete example, I want to run this script but hide the git executable (which is its main dependency) from it so that I can test its output under these conditions.

Answer 1

You can use the builtin command, hash :

hash [-r] [-p filename] [-dt] [name]

Each time hash is invoked, it remembers the full pathnames of the commands specified as name arguments, so they need not be searched for on subsequent invocations. ... The -p option inhibits the path search, and filename is used as the location of name. ... The -d option causes the shell to forget the remembered location of each name.

By passing a non-existent file to the -p option, it will be as if the command can't be found (although it can still be accessed by the full path). Passing -d undoes the effect.

$ hash -p /dev/null/git git
$ git --version
bash: /dev/null/git: command not found
$ /usr/bin/git --version
git version 1.9.5
$ hash -d git
$ git --version
git version 1.9.5

Answer 2

Add a function named git

git() { false; }

That will "hide" the git command

To copy @npostavs's idea, you can still get to the "real" git with the command builtin:

command git --version

Answer 3

Since we know the program is running in bash, one solution is to - instead of "hiding" the program - emulate the behaviour of bash in this circumstance. We can find out what bash does when a command isn't found quite easily:

$ bash
$ not-a-command > stdout 2> stderr
$ echo $?
127
$ cat stdout
$ cat stderr
bash: not-a-command: command not found

We can then write this behaviour to a script with the executable name, such as git in the question's example:

$ echo 'echo >&2 "bash: git: command not found" && exit 127' > git
$ chmod +x git
$ PATH="$PWD:$PATH" git
$ echo $?
127
$ cat stdout
$ cat stderr
bash: git: command not found