git with --git-dir= results in 'not a git repository'

Apart of --git-dir, also setting --work-tree may help. For the script, it's good to use relevant environments (it's easier), e.g.

#!/bin/bash -ex
export GIT_WORK_TREE=/path/to/repo
export GIT_DIR=/path/to/repo/.git
git show -s --pretty=format:%h

When using --git-dir, you need to point at the .git folder of your repository. Try:

git --git-dir=projectDirPath/.git show -s --pretty=format:%h

The doc on --git-dir says that:

--git-dir=

Set the path to the repository (".git" directory). This can also be controlled by setting the GIT_DIR environment variable. It can be an absolute path or relative path to current working directory.

I use to have an issue remembering this myself. To help me remember what to do, I try to remember that the option is asking for exactly what it wants: the path to the .git directory (git-dir).

You are looking for the git -C command line option.

-C <path>

Run as if git was started in instead of the current working directory. When multiple -C options are given, each subsequent non-absolute -C <path> is interpreted relative to the preceding -C <path>. If <path> is present but empty, e.g. -C "", then the current working directory is left unchanged.

This option affects options that expect path name like --git-dir and --work-tree in that their interpretations of the path names would be made relative to the working directory caused by the -C option. For example the following invocations are equivalent:

git --git-dir=a.git --work-tree=b -C c status
git --git-dir=c/a.git --work-tree=c/b status