sed

Edit text in a scriptable manner. See also: awk, ed. More information: https://www.gnu.org/software/sed/manual/sed.html.

• Replace all apple (basic regex) occurrences with mango (basic regex) in all input lines and print the result to stdout:

command | sed 's/apple/mango/g'

• Replace all apple (extended regex) occurrences with APPLE (extended regex) in all input lines and print the result to stdout:

command | sed -E 's/(apple)/\U\1/g'

• Replace all apple (basic regex) occurrences with mango (basic regex) in a specific file and overwrite the original file in place:

sed -i 's/apple/mango/g' path/to/file

• Execute a specific script [f]ile and print the result to stdout:

command | sed -f path/to/script.sed

• Print just the first line to stdout:

command | sed -n '1p'

• [d]elete the first line of a file:

sed -i 1d path/to/file

• [i]nsert a new line at the first line of a file:

sed -i '1i\your new line text\' path/to/file