Examples

Invocation of ed

bash$ ed textfile.txt < commandfile

bash$ ed textfile.txt <<EOF
a
This is now the last line.
.
wq
EOF

Inserts a line at the end of a file (the initial position is the last line).
A single period exits from insert mode
The notation "<<string\n...\nstring": here-document of the shell

Print all lines matching a RE

bash$ ed textfile.txt <<EOF
g/re/p
q
EOF