I basically want to do this:
cat file | grep '<expression>' | sed 's/<expression>/<replacement>/g'
without having to write the expression twice:
cat file | sed 's/<expression>/<replacement>/g'
Is there a way to tell sed not to print lines that does not match the regular expression in the substitute command?
Say you have a file which contains text you want to substitute.
$ cat new.text
A
B
If you want to change A to a then ideally we do the following -
$ sed 's/A/a/' new.text
a
B
But if you don't wish to get lines that are not affected with the substitution then you can use the combination of n and p like follows -
$ sed -n 's/A/a/p' new.text
a
This might work for you:
sed '/<expression>/!d;s//<replacement>/g' file
Or
sed 's/<expression>/<replacement>/gp;d' file
cat file | sed -n '/<expression>/{s//<replacement>/g;p;}'
...g;p} would only work on gnu sed, but the way I put in my answer is standard, to the best of my knowledge.; is standard: Command verbs other than {, a, b, c, i, r, t, w, :, and # can be followed by a semicolon, optional <blank>s, and another command verb. However, when the s command verb is used with the w flag, following it with another command in this manner produces undefined results., so those "other" sed implementations are broken :)How about:
cat file | sed 'd/<expression>/'
Will delete matching patterns from the input. Of course, this is opposite of what you want, but maybe you can make an opposite regular expression?
Please not that I'm not completely sure of the syntax, only used it a couple of times some time ago.
sed -n) to suppress the output and use p option (next to your g as in/g;pto print only those lines where changes takes place.