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

Question

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

Answer 1

Since this question is the top result in search engines, here's a one-liner based on https://serverfault.com/a/547331 that uses a subshell instead of sponge (which often isn't part of a vanilla install like OS X):

echo "$(grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' file_name)" > file_name

The general case is:

echo "$(cat file_name)" > file_name

Edit, the above solution has some caveats:

• printf '%s' should be used instead of echo so that files containing -n don't cause undesired behavior.
• Command substitution strips trailing newlines (this is a bug/feature of shells like bash) so we should append a postfix character like x to the output and remove it on the outside via parameter expansion of a temporary variable like ${v%x}.
• Using a temporary variable $v stomps the value of any existing variable $v in the current shell environment, so we should nest the entire expression in parentheses to preserve the previous value.
• Another bug/feature of shells like bash is that command substitution strips unprintable characters like null from the output. I verified this by calling dd if=/dev/zero bs=1 count=1 >> file_name and viewing it in hex with cat file_name | xxd -p. But echo $(cat file_name) | xxd -p is stripped. So this answer should not be used on binary files or anything using unprintable characters, as Lynch pointed out.

The general solution (albiet slightly slower, more memory intensive and still stripping unprintable characters) is:

(v=$(cat file_name; printf x); printf '%s' ${v%x} > file_name)

Test from https://askubuntu.com/a/752451:

printf "hello\nworld\n" > file_uniquely_named.txt && for ((i=0; i<1000; i++)); do (v=$(cat file_uniquely_named.txt; printf x); printf '%s' ${v%x} > file_uniquely_named.txt); done; cat file_uniquely_named.txt; rm file_uniquely_named.txt

Should print:

hello
world

Whereas calling cat file_uniquely_named.txt > file_uniquely_named.txt in the current shell:

printf "hello\nworld\n" > file_uniquely_named.txt && for ((i=0; i<1000; i++)); do cat file_uniquely_named.txt > file_uniquely_named.txt; done; cat file_uniquely_named.txt; rm file_uniquely_named.txt

Prints an empty string.

I haven't tested this on large files (probably over 2 or 4 GB).

I have borrowed this answer from Hart Simha and kos.