Prepend data from one file to another

Question

How do I prepend the data from file1.txt to file2.txt?

Answer 1

The following command will take the two files and merge them into one

cat file1.txt file2.txt > file3.txt; mv file3.txt file2.txt

Answer 2

If it's available on your system, then sponge from moreutils is designed for this. Here is an example:

cat file1.txt file2.txt | sponge file2.txt

If you don't have sponge, then the following script does the same job using a temporary file. It makes sure that the temporary file is not accessible by other users, and cleans it up at the end.

If your system, or the script crashes, you may need to clean up the temporary file manually. Tested on Bash 4.4.23, and Debian 10 (Buster) Gnu/Linux.

#!/bin/bash
#
# ---------------------------------------------------------------------------------------------------------------------- 
# usage [ from, to ]
#       [ from, to ]
# ---------------------------------------------------------------------------------------------------------------------- 
# Purpose:
# Prepend the contents of file [from], to file [to], leaving the result in file [to].
# ---------------------------------------------------------------------------------------------------------------------- 

# check 
[[ $# -ne 2 ]] && echo "[exit]: two filenames are required" >&2 && exit 1

# init
from="$1"
to="$2"
tmp_fn=$( mktemp -t TEMP_FILE_prepend.XXXXXXXX )
chmod 600 "$tmp_fn"

# prepend
cat "$from" "$to" > "$tmp_fn"
mv "$tmp_fn" "$to"

# cleanup
rm -f "$tmp_fn"

# [End]