Linux: rename files in loop while only targeting the first instance of a specific character

Question

This thread (https://superuser.com/questions/659876/how-to-rename-files-and-replace-characters) has proven bountiful and does what I need it to do, except, I need to replace just the first instance of a character in a filename.

How can I make it so that this:

for f in *:*; do mv -v "$f" $(echo "$f" | tr '.' '_'); done

Only replaces the first instance of . in a filename, with a filename such as:

2022-10-07T071101.8495077Z_QueryHistory.txt

Answer 1

Unfortunately, the method you tried is more complex than it needs to be, and fragile (it breaks if file names contain certain special characters). Here's a simpler method relying on parameter expansion to transform the file name:

for f in *; do mv -v -- "$f" "${f/./_}"; done   # replace the first .
for f in *; do mv -v -- "$f" "${f//./_}"; done  # replace every .

This requires bash, ksh or zsh as the shell: other shells such as dash (which is Ubuntu's /bin/sh, so commonly used for scripting, but hardly ever used interactively) don't have the ${VARIABLE/PATTERN/REPLACEMENT} form of parameter expansion.

Alternatively, you can use prename (apt install rename):

rename 's/\./_/' *     # replace the first .
rename 's/\./_/g' *    # replace every .

Alternatively, you can use zsh's zmv:

autoload -U zmv   # put this in your .zshrc
zmv '*' '${f/./_}'        # replace the first .
zmv -W '*.*.*' '*_*.*'    # replace the next-to-last .
zmv '*' '${f//./_}'       # replace every .

All the snippets in my answer skip files whose name begins with a dot.

Answer 2

Using Perl's rename commandline:

    rename -n 's/\./_/' ./2022-10-07T071101.8495077Z_QueryHistory.txt

Remove -n (aka dry-run) when the output looks satisfactory.