Insert text at top of a file

If you want to insert text at the end of a file (“append”) you can simply use:

 $ echo "test" >> my_file.txt

which then adds “test” on a new line at the end of the “my_file.txt”. In my case I had to insert text at top of a file. If you start to google for that you discover quite fancy solution like using vi or some sed magic.

For me this was all too much. I have a file with quite static text in it. At the end of a shell script I want to insert the content of this file at the beginning of a logfile created by the script.

When using files it’s getting easier:

 $ cat my_static_file.txt logfile.txt > temp_logfile.txt 
 $ mv temp_logfile.txt logfile.txt

But my script can run parallel and therefore this approach is very dangerous. I was looking for a quick solution and came up with this:

 $ cat my_static_file.txt logfile.txt > $$ && mv $$ logfile.txt

This has the advantage of being on a single line and using “\$\$” as filename. \$\$ is the process ID (PID) of the current running script, and therefore cannot exist twice at the same time.

Because of the usage of && and calling everything on one line is, that the following command is only executed when the predecessor command finished successfully. Otherwise the whole command quits. In my case this means the logfile will be kept and not moved.