Insert a new line at the beginning of a file [duplicate]

Question

Is it any way to find all files like *.js and insert some specific line at the beginning of the file? looking for something like:

find . -name '*.js' -exec sh -c '[insert this 'string'] "${0%.js}"' {} \;

Answer 1

Here's a way to add a line to the beginning of a file:

sed -i '1s/^/line_to_be_added\n/' file

Then, you could use the code above with find to achieve your ultimate goal:

find . -type f -name '*.js' -exec sed -i '1s/^/line_to_be_added\n/' {} \;

Note: this answer was tested and works with GNU sed.

Edit: the code above would not work properly on a .js file that is empty, as the sed command above does not work as expected on empty files. A workaround to that would be to test if the file is empty, and if it is, add the desired line via echo, otherwise add the desired line via sed. This all can be done in a (long) one-liner, as follows:

find . -type f -name '*.js' -exec bash -c 'if [ ! -s "{}" ]; then echo "line_to_be_added" >> {}; else sed -i "1s/^/line_to_be_added\n/" {}; fi' \;

Edit 2: As user Sarkis Arutiunian pointed out, we need to add '' before the expression and \'$' before \n to make this work properly in MacOS sed. Here an example

sed -i '' '1s/^/line_to_be_added\'$'\n/' filename.js

Edit 3: This also works, and editors will know how to syntax highlight it:

sed -i '' $'1s/^/line_to_be_added\\\n/' filename.js

Answer 2

Portability: Use Sed's Hold Space

If you want a solution that works portably with both GNU and BSD sed, use the following to prepend text to the first line, and then pass the rest through unchanged:

sed '1 { x; s/^.*/foo/; G; }' example.txt

Assuming a corpus of:

bar
baz

this will print:

foo
bar
baz

You can save the transformation in a script, and then use find or xargs and the sed -i flag for in-place edits once you're sure everything is working the way you expect.

Caveat

Using sed has one major limitation: if the file is empty, it won't work as you might expect. That's not something that seems likely from your original question, but if that's a possibility then you need to use something like cat <(echo foo) example.txt | sponge example.txt to edit the file in-place, or use temporary files and move them after concatenating them.

NB: Sponge is non-standard. It can be found in the moreutils package.