Here I use sed to replace the entire 2nd line in a lot of files with a new 2nd line.
The shell script:
for fle in `ls *.msg` do # copy file first so as can keep permissions and ownership identical cp -p $fle test_$fle # replace entire 2nd line with a new 2nd line sed -f t.sed $fle > test_$fle done
Which calls this sed command file, named t.sed:
2c\ \To: firstname.lastname@example.org
The result is that the 2nd line in each file reads: