How to modify a content of particular line in a file using Shell scripting, when the text in that line is repeated many time in other lines as well

Question

How to modify a content of particular line in a file using Shell scripting, when the text in that line is repeated many time in other lines as well. For example in the file i have :

##############################
[CC-COARSEGRID-PRECONDITIONER]
##############################
# Relaxation parameter
domega = 1.0
##############
[CC-PRECONDMK]
##############
# Relaxation parameter
domega = 0.9

I want to change the last line to: domega = 0.4

Answer 1

You could use sed/tac:

tac file | sed '0,/domega = .*/s/domega = .*/domega = 0.4/' | tac

• tac file reverses the file.
• 0,/domega = .*/ only do the replacement between the line 0 and the first occurence of domega (is now the last).
• s/domega = .*/domega = 0.4/ then do the relacement.
• tac reverses again.