How can I use a file in a command and redirect output to the same file without truncating it?

Basically I want to take as input text from a file, remove a line from that file, and send the output back to the same file. Something along these lines if that makes it any clearer.

grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' file_name > file_name

however, when I do this I end up with a blank file. Any thoughts?

Use sponge for this kind of tasks. Its part of moreutils.

Try this command:

 grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' file_name | sponge file_name

You cannot do that because bash processes the redirections first, then executes the command. So by the time grep looks at file_name, it is already empty. You can use a temporary file though.

#!/bin/sh
tmpfile=$(mktemp)
grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' file_name > ${tmpfile}
cat ${tmpfile} > file_name
rm -f ${tmpfile}

like that, consider using mktemp to create the tmpfile but note that it's not POSIX.

Use sed instead:

sed -i '/seg[0-9]\{1,\}\.[0-9]\{1\}/d' file_name

try this simple one

grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' file_name | tee file_name

Your file will not be blank this time :) and your output is also printed to your terminal.