Sed replace pattern with line number

Question

I need to replace the pattern ### with the current line number.

I managed to Print in the next line with both AWK and SED.

sed -n \"/###/{

Answer 1

As noted by Lev Levitsky this isn't possible with one invocation of sed, because the line number is sent directly to standard out.

You could have sed write a sed-script for you, and do the replacement in two passes:

infile

a
b
c
d
e
###
###
###
a
b
###
c
d
e
###

Find the lines that contain the pattern:

sed -n '/###/=' infile

Output:

6
7
8
11
15

Pipe that into a sed-script writing a new sed-script:

sed 's:.*:&s/###/&/:'

Output:

6s/###/6/
7s/###/7/
8s/###/8/
11s/###/11/
15s/###/15/

Execute:

sed -n '/###/=' infile | sed 's:.*:&s/^/& \&/:' | sed -f - infile

Output:

a
b
c
d
e
6
7
8
a
b
11
c
d
e
15