git update-index
wants the file names on its command line, not on its standard input.
Step 1:
cd
into the folder you want to assume is unchanged
Step 2:
You can do either this:
git update-index --assume-unchanged $(git ls-files | tr '\n' ' ')
or
git ls-files | tr '\n' ' ' | xargs git update-index --assume-unchanged
Although, with either case, file names with spaces will be problematic. If you have those, you can use this:
git ls-files -z | xargs -0 git update-index --assume-unchanged
Edit: incorporated input from @MatthewScharley regarding git ls-files -z
.
Windows Commands
Note: If you’re on windows, use Git Bash to run these commands