The Wiert Corner – irregular stream of stuff

Jeroen W. Pluimers on .NET, C#, Delphi, databases, and personal interests

  • My badges

  • Twitter Updates

  • My Flickr Stream

  • Pages

  • All categories

  • Enter your email address to subscribe to this blog and receive notifications of new posts by email.

    Join 1,512 other followers

sed in a bash script: backslash escape anything that looks suspicious

Posted by jpluimers on 2019/02/26

Did I ever tell I dislike regular expressions and old-skool shells?

They’re not good for anything but basic commands, so if you try any scripts in them, you’re basically lost.

If you disagree, please read [WayBack] Don’t write Shell scripts. I would recommend Python, but I tried “pip search mysql”…. – Kristian Köhntopp – Google+) and [WayBack] How did this shit ever work? by the same author.

On the other hand: on many system, the baseline isn’t much more than a shell and a very limited tool set.

With nx like systems that usually comes down to sed and a shell like bash.

Since I wanted to modify an openssh hardening script to cover more permutations that was using sed in a bash script, I had not much choice but to bite the bullet.

TL;DR:

When you use any of the below characters, prepend them with a backslash as they have a bash meaning in addition to a sed meaning.

  • ? becomes \?
  • ( becomes \(
  • ) becomes \)
  • | becomes \|

The script

Hopefully by now it’s [Archive.is] been merged into https://github.com/comotion/gone/blob/github/modules/ssh. If not, it’s at https://github.com/jpluimers/gone/blob/jpluimers-ssh-hardening-patch/modules/ssh.

The diff: [Archive.is] https://github.com/jpluimers/gone/commit/329bf12a320704080e68eee90f4c099e92d8388d?diff=unified

The relevant portion (which also uses backslashes as line continuation and wrap a command over multiple lines [WayBack]):

sed -i \
-e 's/#\?MaxAuthTries *[0-9]*.*/MaxAuthTries 2/' \
-e 's/#\?PermitRootLogin *\(yes\|no\).*/PermitRootLogin no/' \
-e 's/#\?UsePrivilegeSeparation *\(yes\|no\|sandbox\).*/UsePrivilegeSeparation sandbox/' \
-e 's/#\?StrictModes *\(yes\|no\).*/StrictModes yes/' \
-e 's/#\?IgnoreRhosts *\(yes\|no\).*/IgnoreRhosts yes/' \
-e 's/#\?PermitEmptyPasswords *\(yes\|no\).*/PermitEmptyPasswords no/' \
-e 's/#\?ChallengeResponseAuthentication *\(yes\|no\).*/ChallengeResponseAuthentication yes/' \
-e 's/#\?KerberosAuthentication *\(yes\|no\).*/KerberosAuthentication no/' \
-e 's/#\?GSSAPIAuthentication *\(yes\|no\).*/GSSAPIAuthentication no/' \
-e 's/#\?GatewayPorts *\(yes\|no\).*/GatewayPorts no/' \
-e 's/#\?X11Forwarding *\(yes\|no\).*/X11Forwarding no/' \
-e 's/#\?PrintMotd *\(yes\|no\).*/PrintMotd no/' \
-e 's/#\?PrintLastLog *\(yes\|no\).*/PrintLastLog yes/' \
-e 's/#\?TCPKeepAlive *\(yes\|no\).*/TCPKeepAlive no/' \
-e 's/#\?PermitUserEnvironment *\(yes\|no\).*/PermitUserEnvironment no/' \
-e 's/^\(HostKey .*ssh_host_dsa_key\)/#\1/' \
sshd_config

–jeroen

 

 

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

 
%d bloggers like this: