How can I convert tabs to spaces in every file of a directory?

Collecting the best comments from Gene's answer, the best solution by far, is by using sponge from moreutils.

sudo apt-get install moreutils
# The complete one-liner:
find ./ -iname '*.java' -type f -exec bash -c 'expand -t 4 "$0" | sponge "$0"' {} \;

Explanation:

• ./ is recursively searching from current directory
• -iname is a case insensitive match (for both *.java and *.JAVA likes)
• type -f finds only regular files (no directories, binaries or symlinks)
• -exec bash -c execute following commands in a subshell for each file name, {}
• expand -t 4 expands all TABs to 4 spaces
• sponge soak up standard input (from expand) and write to a file (the same one)*.

NOTE: * A simple file redirection (> "$0") won't work here because it would overwrite the file too soon.

Advantage: All original file permissions are retained and no intermediate tmp files are used.

Simple replacement with sed is okay but not the best possible solution. If there are "extra" spaces between the tabs they will still be there after substitution, so the margins will be ragged. Tabs expanded in the middle of lines will also not work correctly. In bash, we can say instead

find . -name '*.java' ! -type d -exec bash -c 'expand -t 4 "$0" > /tmp/e && mv /tmp/e "$0"' {} \;

to apply expand to every Java file in the current directory tree. Remove / replace the -name argument if you're targeting some other file types. As one of the comments mentions, be very careful when removing -name or using a weak, wildcard. You can easily clobber repository and other hidden files without intent. This is why the original answer included this:

You should always make a backup copy of the tree before trying something like this in case something goes wrong.

Try the command line tool expand.

expand -i -t 4 input | sponge output

where

• -i is used to expand only leading tabs on each line;
• -t 4 means that each tab will be converted to 4 whitespace chars (8 by default).
• sponge is from the moreutils package, and avoids clearing the input file. On macOS, the package moreutils is available via Homebrew (brew install moreutils) or MacPorts (sudo port install moreutils).

Finally, you can use gexpand on macOS, after installing coreutils with Homebrew (brew install coreutils) or MacPorts (sudo port install coreutils).

Warning: This will break your repo.

This will corrupt binary files, including those under svn, .git! Read the comments before using!

find . -iname '*.java' -type f -exec sed -i.orig 's/\t/ /g' {} +

The original file is saved as [filename].orig.

Replace '*.java' with the file ending of the file type you are looking for. This way you can prevent accidental corruption of binary files.

Downsides:

• Will replace tabs everywhere in a file.
• Will take a long time if you happen to have a 5GB SQL dump in this directory.