Tool for doing stuff with lines, or perhaps stream of characters..

By default, everything that is not deleted (by a delete command) is printed to stdout. To inhibit this, pass '-n' flag.

Forward dash '/' is a separator character. It is used to separate different commands and their arguments.

sed takes a string as argument, which can contain commands, that in turn contain matching strings.

The string that is passed to sed must contain a valid command. A valid command is fucked up. It can be either the first character, or the trailing character.

For example the command to print a line that matches something would be

echo testfil.txt | sed -n 'something/p'

While the command to substitute comes before a string. To substitute "dude" with "shit":

echo testfil.txt | sed 's/dude/shit/'

But you could also combine them, and print only the lines that matches the substitutions. Thus having commands both first and last in the string:

echo testfil.txt | sed -n 's/dude/shit is fucked up/p'

sed '/actual pattern/command'
 
# ex
sed '/Matches this string/p' # command p is printed. But since -d flag is not provided, so will everything else too
 
sed -n '/Matches this string/p' # Only prints rows containing "Matches this string"