How to directly overwrite with 'unexpand' (spaces-to-tabs conversion)?

You can very seldom use output redirection to replace the input. Replacing works with commands that support it internally (since they then do the basic steps themselves). From the shell level, it's far better to work in two steps, like so:

1. Do the operation on foo, creating foo.tmp
2. Move (rename) foo.tmp to foo, overwriting the original

This will be fast. It will require a bit more disk space, but if you do both steps before continuing to the next file, you will only need as much extra space as the largest single file, this should not be a problem.

Sketch script:

for a in *.phpdo  unexpand -t 4 $a >$a-notab  mv $a-notab $adone

You could do better (error-checking, and so on), but that is the basic outline.

Here's the command I used:

for p in $(find . -iname "*.js")do    unexpand -t 4 $(dirname $p)/"$(basename $p)" > $(dirname $p)/"$(basename $p)-tab"    mv $(dirname $p)/"$(basename $p)-tab" $(dirname $p)/"$(basename $p)"done

This version changes all files within the directory hierarchy rooted at the current working directory.

In my case, I only wanted to make this change to .js files; you can omit the iname clause from find if you wish, or use different args to cast your net differently.

My version wraps filenames in quotes, but it doesn't use quotes around 'interesting' directory names that appear in the paths of matching files.

To get it all on one line, add a semi after lines 1, 3, & 4.

This is potentially dangerous, so make a backup or use git before running the command. If you're using git, you can verify that only whitespace was changed with git diff -w.