Sed

References

• The SED Homepage on SourceForge
• The SED FAQ
• The SED man page
• Sed, a stream editor
• The sed one-liners

Installation

It is recommended to add the following alias in your ~/.bashrc:

alias sed="sed -r"

Of course, this alias has no effect on shell script. There you'll have to specify the option explicitly at each invokation.

Usage

Some basic usage:

sed [OPTION]... {script-only-if-no-other-script} [input-file]...
sed -n                              # Silent - suppress automatic printing of pattern space
sed -r                              # Use extended regular expression
sed -i "s/foo/bar/" *.txt           # In-place file modification

Use of address commands a\text, i\text, c\text. The command is terminated by a *newline*. To insert a newline character, use \n:

$ cat mytext
First line
Second line
$ cat mysedscript
1 {i\inserted text
s/$/ (not anymore)/g}
$ sed -f mysedscript mytext
inserted text
First line (not anymore)
Second line

# All on one line: use echo -e to generate the newline that terminates the command i\
$ echo -e "1 {i\\inserted text\ns/$/ (not anymore)/g}"| sed -f - mytext
inserted text
First line (not anymore)
Second line

#Same result without command \i:
$ sed "1 {s/^/inserted text\n/; s/$/ (not anymore)/}" mytext

Regular expressions

See Regular Expressions.

Script Examples

Remove <script>...</script> HTML tag

s!<script[>\x20\t].*</script>!!g
/<script[>\x20\t]/{
    s!<script[>\x20\t].*!!g
    :NEXTCYCLE
    n
    /<\/script>/!{
        s!.*!!g
        b NEXTCYCLE
    }
    s!.*</script>!!g
}

Remove newlines

Newline characters are added to the pattern space when using the append command N. The script below removes all newlines from standard input:

:a N
s/\n/ /g
b a

Recursive patterns

For instance, to transform a path like /usr/local/share/bin/../../../bin/foo into /usr/bin/foo:

s!^([^./])!\./\1!                  # Prefix with './' unless starts with '.' or '/'
s!/./!/!g                          # Remove any './' in middle
:a s!/[^/]*[^/.]/\.\.!!g           # Remove /foo/.. (1st letter must not be '/', last letter must not be '.')
t a                                # ... and repeat until no more substitutions

echo "/usr/local/share/bin/../../../bin/foo" | sed -r 's!^([^./])!\./\1!; s!/a./!/!g; :a s!/[^/]*[^/.]/\.\.!!g; t a'

Test paths:

/usr/local/share/../../../bin/foo     # /bin/foo
/usr/local/./share/../../../bin/foo   # /bin/foo
./usr/../bin/foo                      # ./bin/foo 
usr/../bin/foo                        # ./bin/foo
usr/../bin                            # ./bin
usr/../bin/..                         # .
usr/../bin/../..                      # ./..