Replacing string based on line number

You can specify line number in sed or NR (number of record) in awk.

awk 'NR==34 { sub("AAA", "BBB") }'

or use FNR (file number record) if you want to specify more than one file on the command line.

awk 'FNR==34 { sub("AAA", "BBB") }'

sed '34s/AAA/BBB/'

to do in-place replacement with sed

sed -i '34s/AAA/BBB/' file_name

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