sed

sed is the superman of UNIX stream editing.

• Input Stream
• Pattern Space
• Hold Buffer
• Output Stream

Think about the spaces this way - sed reads the input stream and produces the output stream. Internally it has the pattern space and the hold buffer. Sed reads data from the input stream until it finds the newline character \n. Then it places the data read so far, without the newline, into the pattern space. Most of the sed commands operate on the data in the pattern space. The hold buffer is there for your convenience. Think about it as temporary buffer. You can copy or exchange data between the pattern space and the hold buffer. Once sed has executed all the commands, it outputs the pattern space and adds a \n at the end.

• source

Capture groups

$ sed -e 's/version=\(.+\)/\1/'

Match zero or more instances

$ sed 's/.*//g'

Insert on first line of file

$ sed "1 i my magical line"

Insert on specific line of file

sed -i '8i This is Line 8' <filename>

Delete first line of output

$ printf '1\n2\n3\n' | sed -e '1d'
# => 2
# => 3

Get file extension

$ echo abc.txt | sed 's/.*\.//'
# => txt