Question

I want to replace for example all instances of “123” with “321” contained within all .txt files in a folder (recursively).
I thought of doing this

sed -i 's/123/321/g' | find . -name \*.txt

but before possibly screwing all my files I would like to ask if it will work.

Answer 1

You have the sed and the find back to front. With GNU sed and the -i option, you could use:

find . -name '*.txt' -type f -exec sed -i s/123/321/g {} +

The find finds files with extension .txt and runs the sed -i command on groups of them (that’s the + at the end; it’s standard in POSIX 2008, but not all versions of find necessarily support it). In this example substitution, there’s no danger of misinterpretation of the s/123/321/g command so I’ve not enclosed it in quotes. However, for simplicity and general safety, it is probably better to enclose the sed script in single quotes whenever possible.

You could also use xargs (and again using GNU extensions -print0 to find and -0 and -r to xargs):

find . -name '*.txt' -type f -print0 | xargs -0 -r sed -i 's/123/321/g'

The -r means ‘do not run if there are no arguments’ (so the find doesn’t find anything). The -print0 and -0 work in tandem, generating file names ending with the C null byte '\0' instead of a newline, and avoiding misinterpretation of file names containing newlines, blanks and so on.

Note that before running the script on the real data, you can and should test it. Make a dummy directory (I usually call it junk), copy some sample files into the junk directory, change directory into the junk directory, and test your script on those files. Since they’re copies, there’s no harm done if something goes wrong. And you can simply remove everything in the directory afterwards: rm -fr junk should never cause you anguish.