Ed

ed(1) is a non-visual text editor. It is a powerful tool for scripting in-place file manipulation.

Most system also offer ex(1), which is the extended editor.

Input Mode

ed(1) enters input mode after one the following commands has been given:

• append (a)

• change (c)

• insert (i)

Exit input mode by giving a period (.) alone.

Examples

Insert a Line into a File

Shell scripts make it trivial to append to a file. There are excellent tools such as sed(1) for search/replacing text. But there isn't a trivial way to, for example, insert a new line of text into the Nth line of a file.

This script demonstrates how such a task could be automated in part.

# Inserts a new third line into the file,
# which sets `someYamlProperty` to `true` in a Markdown header of some file.
ed "$1" <<EOF
3i
someYamlProperty: true
.
w
EOF

Line Editing

This script will rewrite itself to echo a new message.

message=

#This function uses a heredoc to pass commands into `ed`.
#It works like this:
#  - `1c` targets the first line for changing. Everything entered in input mode will replace this line.
#  - `message="$1"` is being inserted. Note that shell variables are evaluated in a heredoc.
#  - `.` exits input mode
#  - `w` writes the edited file to disk
edScript() {
  ed -s "$(realpath -s $0)" <<EOF
1c
message="$1"
.
w
EOF
}

#If this script is given an arguments, the first argument is passed to the above function.
#Otherwise, the script just echoes "$message".
if [ $# -ge 1 ]; then
  edScript "$1"
else
  echo "$message"
fi

CategoryRicottone