====== Sed ======

**General Information**

Stream editor tips and tricks. 

**Checklist**
  * Distro(s): Any
  * Package: 'sed' package installed

----

====== Print Lines ======

Different ways to print specific lines.

Print only line 5 pattern space<code bash>sed -n '5p' /etc/passwd</code>
  * -n => silent; do not print the entire file 

Line 5, don't delete it<code bash>sed '5!d' /etc/passwd</code>
  * !d => don't delete

Print up to line 5, quit and delete the rest of the output<code bash>sed '5q;d' /etc/passwd</code>
  * q => quit/stop processing more input
  * d => delete the pattern space (removes all but line 5 from output)

----

====== Swap Patterns ======

Edit files by swapping pattern spaces.

  * Before<code bash>cat /etc/resolv.conf 
# Generated by NetworkManager
search local
nameserver 208.67.222.222
nameserver 208.67.220.220</code>
  * After<code bash>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</code>
    * -i => In-line editing
    * s => swap 
    * g => global swap (swap every match, not just the first one)

----

====== Insert Lines ======

Insert lines before a found matched pattern.

Given the contents of /etc/resolv.conf:
<code bash>
# Generated by NetworkManager
nameserver 192.168.1.254
</code>

Insert a new DNS entry that will be checked first:
<code bash>
sed -i '/nameserver/ i\nameserver 8.8.8.8' /etc/resolv.conf
</code>
  * /nameserver/ => pattern to match
  * -i => in-line editing (changes the file in place)
  * i\ => insert

Results in:
<code bash>
# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 192.168.1.254
</code>

===== Insert at first line =====

Insert also works against line numbers.

Insert a new comment at line 1:
<code bash>
sed -i '1 i\# My first line comment' /etc/resolv.conf
</code>

Results in:
<code bash>
# My first line comment
# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 192.168.1.254
</code>

----

====== Append Lines ======

Append lines after a found matched pattern.

Given the contents of /etc/resolv.conf:
<code bash>
# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 192.168.1.254
</code>

Append a new DNS entry that will be checked 2nd:
<code bash>
sed -i '/nameserver 8.8.8.8/ a\nameserver 8.8.4.4' /etc/resolv.conf
</code>

Results in:
<code bash>
# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 8.8.4.4
nameserver 192.168.1.254
</code>

----

====== Remove Lines ======

Remove lines that match a pattern.

Given /etc/resolv.conf:
<code bash>
# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 8.8.4.4
nameserver 192.168.1.254
</code>

Remove the 8.8.4.4 entry:
<code bash>
sed -i '/8.8.4.4/d' /etc/resolv.conf
</code>

Results in:
<code bash>
# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 192.168.1.254
</code>

----