How can I replace text after a specific word using sed?

Question

I have a file named .ignore. In need to replace the projdir. For example:

ignore \..*
ignore README
projdir Snake

I need to replace Snake with, for example, "PacMan". I read the man page, but I have no idea what to do.

Answer 1

Search for a line that starts with projdir, and replace the whole line with a new one:

sed -i 's/^projdir .*$/projdir PacMan/' .ignore

^ and $ are beginning/end-of-line markers, so the pattern will match the whole line; .* matches anything. The -i tells sed to write the changes directly to .ignore, instead of just outputting them

Answer 2

One approach is to rewrite the entire line, using backreferences for the parts you want to keep:

sed -e 's/^\( *projdir  *\)[^ ]*\(.*\)*$/\1PacMan\2/'

Another approach is to rewrite that part of the line, but only if some other part matches:

sed -e '/^ *projdir / s/ [^# ]/ PacMan/'

Both examples rewrite the second whitespace-delimited word on lines where the first word is projdir.