General Information

Stream editor tips and tricks.

Checklist

• Distro(s): Any
• Package: 'sed' package installed

Different ways to print specific lines.

Print only line 5 pattern space

sed -n '5p' /etc/passwd

• -n ⇒ silent; do not print the entire file

Line 5, don't delete it

sed '5!d' /etc/passwd

• !d ⇒ don't delete

Print up to line 5, quit and delete the rest of the output

sed '5q;d' /etc/passwd

• q ⇒ quit/stop processing more input
• d ⇒ delete the pattern space (removes all but line 5 from output)

Edit files by swapping pattern spaces.

• Before

  cat /etc/resolv.conf 
  # Generated by NetworkManager
  search local
  nameserver 208.67.222.222
  nameserver 208.67.220.220

• After

  sed -i 's/208.67.222.222/8.8.8.8/g' /etc/resolv.conf 
  # Generated by NetworkManager
  search local
  nameserver 8.8.8.8
  nameserver 208.67.220.220

  • -i ⇒ In-line editing
  • s ⇒ swap
  • g ⇒ global swap (swap every match, not just the first one)

Insert lines before a found matched pattern.

Given the contents of /etc/resolv.conf:

# Generated by NetworkManager
nameserver 192.168.1.254

Insert a new DNS entry that will be checked first:

sed -i '/nameserver/ i\nameserver 8.8.8.8' /etc/resolv.conf

• /nameserver/ ⇒ pattern to match
• -i ⇒ in-line editing (changes the file in place)
• i\ ⇒ insert

Results in:

# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 192.168.1.254

Insert also works against line numbers.

Insert a new comment at line 1:

sed -i '1 i\# My first line comment' /etc/resolv.conf

Results in:

# My first line comment
# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 192.168.1.254

Append lines after a found matched pattern.

Given the contents of /etc/resolv.conf:

# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 192.168.1.254

Append a new DNS entry that will be checked 2nd:

sed -i '/nameserver 8.8.8.8/ a\nameserver 8.8.4.4' /etc/resolv.conf

Results in:

# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 8.8.4.4
nameserver 192.168.1.254

Remove lines that match a pattern.

Given /etc/resolv.conf:

# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 8.8.4.4
nameserver 192.168.1.254

Remove the 8.8.4.4 entry:

sed -i '/8.8.4.4/d' /etc/resolv.conf

Results in:

# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 192.168.1.254