A One-Liner `sed` Command to Format SSH Config File

SED Command

sed -i -e '/^\s*#/!{/^\s*Host /!s/^\s*/\t/}' \
    -e '/^\s*Host /s/^\s*//' \
    ~/.ssh/config

With backup of old config file having extension .old

sed -i.old -e '/^\s*#/!{/^\s*Host /!s/^\s*/\t/}' \
    -e '/^\s*Host /s/^\s*//' \
    ~/.ssh/config

Breakdown

• Skip Commented Lines:
  • Ignores lines starting with #, possibly preceded by whitespace.

  /^\s*#/!{}

• Indent Non-Host and Non-Comment Lines:

  {/^\s*Host /!s/^\s*/\t/}

• Matches lines that do not start with Host.

  /^\s*Host /!

• Replaces leading spaces or tabs with a single tab.

  s/^\s*/\t/

• Remove Leading Spaces from Host Lines:

  -e '/^\s*Host /s/^\s*//'

• Matches lines that start with Host.

  /^\s*Host /

• Removes all leading spaces or tabs from those lines.

  s/^\s*//

Example

Input:

Host myserver
HostName ssh.example.com
User myuser
 Port 22
 IdentityFile ~/.ssh/mykey

   Host another-server
    HostName another.example.com
   User anotheruser
Port 2022
IdentityFile ~/.ssh/anotherkey

Output:

Host myserver
    HostName ssh.example.com
    User myuser
    Port 22
    IdentityFile ~/.ssh/mykey

Host another-server
    HostName another.example.com
    User anotheruser
    Port 2022
    IdentityFile ~/.ssh/anotherkey

This formats the file in place (-i flag), ensuring consistent indentation and removing leading spaces.