with sed, how can I replace word within a matching line?

Question

I am trying to write a sed expression to detect lines similer to this:

s0:12345:respawn:/sbin/agetty -8 -s 115200 ttyS0 linux

and replace the "ttys0" with "something_else"

I do not want to detect commented out lines (beginning with #)

I can replace the whole line, but how to I just replace the substring, leaving the rest of the line intact ?

Answer 1

One way to do this is as follows:

echo 's0:12345:respawn:/sbin/agetty -8 -s 115200 ttyS0 linux' | 
    sed -e '/^[^#]/ s/ttyS0/foo/'

Answer 2

Match the part before ttyS0 and those after ttyS0, also check if line doesn't begin with #:

sed 's/^[^#]\(.*\)ttyS0\(.*\)/\1something\2/'

Result:

%> echo "s0:12345:respawn:/sbin/agetty -8 -s 115200 ttyS0 linux" | sed 's/^[^#]\(.*\)ttyS0\(.*\)/\1something\2/'
0:12345:respawn:/sbin/agetty -8 -s 115200 something linux

Answer 3

If it always starts with s[number]colon you could do (with GNU sed):

sed -r '/^s[0-9]+:/s/ttyS0/something_else/' infile

Which changes any line that starts with the regex, adapt to suit your needs.