fIm trying to find a string betwen two match patterns and then add that string before a pattern using sed

Question

Say I have the below line in file named "logs_test":

Sample input:

"at 10947 usecs after Tue Feb 23 18:29:46 2021 [119] init: Event=populatedonRestart"

I wanted to find a string between "at" and "usecs" and add the string before "2021" in the above line

sample output:

"at 10947 usecs after Tue Feb 23 18:29:46 10947 2021 [119] init: Event=populatedonRestart"

sed command to find a string between two matching patterns:

sed "s/at//;s/usecs.*//“ <file_name>

sed command to add a string before a pattern:

sed 's/2021/string &/g' <file_name>

How can I accomplish two tasks using one sed command? Is there were to use the sed command inside sed to do this?

Answer 1

This will do it for your example (with GNU sed):

sed 's/^\(at \)\(.* \)\(usecs.*\)\(2021.*\)/\1\2\3\2\4/' your_file

The ways it's working is as follows:

• I remember the stuff in between \( and \) (these are called capture groups)
• I break the string into 4 capture groups, the 2nd capture group is the number you care about. And I put the groups back together and use the 2nd capture group twice: /\1\2\3\2\4/

Once you've confirmed it does what you want, you could add the -i to do the replacement in-place:

sed -i 's/^\(at \)\(.* \)\(usecs.*\)\(2021.*\)/\1\2\3\2\4/' your_file