grep a file and pipe the output to sed and store sed output in a file

Contents of source.txt:

gold green white black blue yellow magenta brown tram kilo charlie tango 

Hi everyone! I need to solve a mystery.

I'm trying to run a small script to grep a file source.txt, pipe grep output to sed replace a string and store that line in a new file pol.txt

grep -l "gold" source.txt | xargs sed 's/green/red/' > pol.txt 

Instead of having the only that line stored in pol.txt:

gold red white black blue 

I have the entire file itself with the string I replaced

gold red white black blue yellow magenta brown tram kilo charlie tango 

When I remove the option -l from grep command I have this and of course nothing in pol.txt

sed: can't read gold: No such file or directory sed: can't read green: No such file or directory sed: can't read white: No such file or directory sed: can't read black: No such file or directory sed: can't read blue: No such file or directory 

grep is needed as a tester and unfortunately " if " is not an option.

2 Answers

To select any line containing gold from source.txt and replace the first occurrence of green with red:

$ sed -n '/gold/{s/green/red/; p}' source.txt gold red white black blue 

To save that in a file:

sed -n '/gold/{s/green/red/; p}' source.txt >pol.txt 

How it works

• -n tells sed not to print lines unless we explicitly ask it to.

• /gold/ selects lines that match the regex gold.

• s/green/red/ performs the substitution

• p prints.

Using awk

With the same logic:

$ awk '/gold/{gsub(/green/, "red"); print}' source.txt gold red white black blue 

Using grep

If we are forced, for reasons not yet explained, to use a grep pipeline, then try:

$ grep -l --null "gold" source.txt | xargs -0 sed -n '/gold/s/green/red/p' gold red white black blue 

grep -l "gold" source.txt will output source.txt if the file contains the word gold xargs sed 's/green/red/' will run sed 's/green/red/' source.txt and the final redirect saves the result in your output.

If I understand your intent correctly, you want the following command:

sed -n '/gold/s/green/red/p' source.txt > pol.txt 

The /gold/ selects lines matching gold and the s command does the replacement you want.