Bash script that edits itself

Question

I'm trying to write a bash script that can edit itself. The line of code I'm trying to use is:

sed "s/$STRING_TO_REPLACE/$NEW_STRING/" < $0 > $0

But after running the script I end up with an empty file. Any suggestions?

I am creating a script that has two variables that contain default values. The only problem is, these default values will be different for each of the many stations where it will be used. I would rather none of the people running these stations had to open the script and edit the defaults themselves, so I want to include the ability to change the defaults in the script. One of the requirements I have is that the script must be self contained; no other files are to accompany the script and there are to be no environment variables. This would require the script to be able to edit itself.

Answer 1

If you want to do an in-place substitution with sed, use the -i switch.

I wouldn't recommend re-writing the currently executing script on the fly. I'm not sure whether it's possible or not, but if it is, it would certainly be interesting (read: difficult) code to maintain :-)

Answer 2

Firstly, this is a very bad idea and I can't imagine any use case for it that can't be covered by something not this.

The reason this is happening is because when you do >file, it opens the file in read mode and truncates it to 0 length, and the input has nothing to read. You need to use a temp file. You can see more info here.

Answer 3

Perl handles in place edits. Try this:

perl -npi -e "s/$STRING_TO_REPLACE/$NEW_STRING/g" < $0

Answer 4

You're trying to read and write to the same stream, this is a bad programming practice and usually results in errors.

This is what you should do:

1. Write to a temp file
2. Delete the original file

3. Rename temp to the original file name

   sed "s/$STRING_TO_REPLACE/$NEW_STRING/" < $0 > temp

   rm $0

   mv temp $1

I haven't done bash scripting in while, so verify that I have the right commands.