The following shell history shows two attempts to rename files, the second attempt worked. But the first attempt did not and I have no idea why...
❯ ls
step-10.txt step-12.txt step-14.txt step-16.txt step-18.txt step-1.txt step-3.txt step-5.txt step-7.txt step-9.txt
step-11.txt step-13.txt step-15.txt step-17.txt step-19.txt step-2.txt step-4.txt step-6.txt step-8.txt
❯ for f in *.txt; do
newname=$(echo $f | sed 's/txt/md/')\
mv $f $newname
done
zsh: command not found: step-10.txt
zsh: command not found: step-11.txt
zsh: command not found: step-12.txt
zsh: command not found: step-13.txt
zsh: command not found: step-14.txt
zsh: command not found: step-15.txt
zsh: command not found: step-16.txt
zsh: command not found: step-17.txt
zsh: command not found: step-18.txt
zsh: command not found: step-19.txt
zsh: command not found: step-1.txt
zsh: command not found: step-2.txt
zsh: command not found: step-3.txt
zsh: command not found: step-4.txt
zsh: command not found: step-5.txt
zsh: command not found: step-6.txt
zsh: command not found: step-7.txt
zsh: command not found: step-8.txt
zsh: command not found: step-9.txt
↵ Error. Exit status 127.
❯ for f in *.txt; do
mv $f $(echo $f | sed 's/txt/md/')
done
❯ ls
step-10.md step-12.md step-14.md step-16.md step-18.md step-1.md step-3.md step-5.md step-7.md step-9.md
step-11.md step-13.md step-15.md step-17.md step-19.md step-2.md step-4.md step-6.md step-8.md
Why does it work when I place $(echo $f | sed 's/txt/md/') on the line with mv but not when it's assigned to the variable? Why does it say command not found: $f when my loop is providing $f as an argument to the mv command?
That backslash means it's evaluating
newname=$(echo $f | sed 's/txt/md/')mv $f $newname
which of course treats the filename as the name of a command to run (The mv is part of the newname variable)
You don't need sed at all here; just use parameter expansion:
for f in *.txt; do
mv "$f" "${f/%txt/md}"
done
There are also several different programs named rename (prename, rename.ul, etc. depending on OS) that can mass rename files; check man pages for which ones (if any) you have installed and usage.