Useful Bash commands for DevOps work :toolbox:

So Bash is no longer the hottest ticket on the market. Happens. I may have seen the actual point where this peaked and let me tell you, that’s something. Still, there’s some cool stuff to be done with Bash.

For example, if you are writing a quick and dirty Terraform configuration, you can get really far with TF templates, HCL interpolation and well placed Bash commands:

[Terraform Bash template for Jitsi installation](https://github. com/edamsoft-sre/jitsi-azure-terraform/blob/trunk/environments/prod/templates /setup.tpl)

1. iterm2 or csshX :tv:

The best terminal software will practically do your job for you. Just install, sit back and cash in. If you don’t know what either is, try iterm2 first. If you can find csshX and install it, then all your problems will be solved.
You will have new ones, for sure. :crocodile: but the old ones, gone. Using multiple windows efficiently is almost better than actually automating anything. It can be lot’s of fun, and also very dangerous.

iterm Shortcuts :cactus:
Ctrl-shift-i - Send output to all windows AND tabs (watch out!)
Ctrl-D - Open a window to the right
Ctrl-Shift-D - Open a window below
Ctrl-Shift-Return - toggle current window full screen (fill iterm2 window)

If you need or WANT to have dozens of servers open and send commands to different sub groups, and, and, and….

Get csshX !!! Then, relax and eat some ramen :ramen:

2. top :crystal_ball:

The Linux top command is pretty well documented.

I like to type “zbx” when I start it, and then capital I and 1 to check the number of cpus/cores.

This doesn’t do much, but it looks nice.

options
-  *x*:    enable bold for current sort_by column
-  *z*:     enable color
-  *Z*:     edit colors
-  *I*:     Irix mode - detects single cpu
-  *b*:     toggle solid column/row highlight background
-  *k*:     kill pid
-  *m*:     memory ( I think, check the man page)
-  *l*:     *load* haha but really, just type *uptime*

3. find

Find the largest directories or files in a tree

find . -type d -print0 | xargs -0 du -s | sort -n | tail -15 | cut -f2 | xargs -I{} du -sh {}

Xargs allows us to pass multiple arguments to a command like Python map, so the disk usage du command receives each subsequent file result from find and gets its size. Then we do some table transformation, basically, and re-run the results through xargs and du to generate a final listing that is sorted across all results.

Run a command on the findings

Another way to find files or directories and execute on command with them is the explicit -exec switch.

find . -mtime 0 -exec stat '{}' \;

4. capture whole packets

tcpdump -n -A -q -w /tmp/packet.cap <some filter>

Capturing packets is a lot of fun, if you’re an evil hacker. Otherwise it’s a huge pain in the ass. Most likely, you kids will be looking at packet statistics aggregated in a nice GUI like Grafana, Kibana, or not at all thanks to advances in the software industry. You’re welcome.

nc -v -u google.org 5060

read captures in CLI

tcpdump -r | less

5. ssh and scp

scp -o 'ProxyJump user@bastion' -i key user@host:/path/to/remote/file /local/path/file

You should always be using secure shell with strong RSA keys and a good key management system.

Other than that, use your ~/.ssh/config file to set some sensible defaults

Host *
   UseKeyChain yes
   AddKeysToAgent yes
   IdentityFile ~/.ssh/id_rsa_private_keyfile

Add ssh keys to the macOS agent, if that’s your OS. If not, it won’t work. Well, it should but who knows what the Ubuntu or RedHat options are. Who cares?

 ssh-add -l
 ssh-add ~/path/to/my/key
 ssh-add -l

:bulb: Use a passphrase on your keys. Why not.

6. Awk and sed and cut

Learn them well, you can really go down a rabbithole with these tools. Through probably the late 90’s they were best in class. But you still have thousands of websites and production platforms relying on a well placed Awk or Sed, trust me.

:bulb: Use sed to replace text with regex

Sed allows simple regular expression matching which is what you’re going to have to do to fix your mistakes. All of them.

sed -i "s/component_secret =.*/component_secret = \"$JICOFO_SECRET\"/" $PROSODY_HOST_CONF

sed -i "s#some_text/next_one#next_one#g" $INPUT_FILE

On Ubuntu Hirsute 21.04 get a list of only IPv4 addresses. Can be done in better ways, probably, but you get the jist.

   ip -4 addr | awk '{ print $2 }' | sed 's#/.*##' | sed "s#^[^0-9].*##" | sed '/^$/d' | sed 1d
  • Awk is a full programming language, but we can use it to get text fields or csv columns from text.
  • sed regular expression replace with the ’s/old/new/g’ command and global switches
  • Regex ‘[^0-9].*’ match starts with a non-digit
  • sed Delete the first line, which is localhost, and Delete empty lines
  • Use a pound “#” separator if you need to match a “/” forward slash.,

7. While and For Loops

These are also useful. You can make a list of numbers like {1..100}. This is where you realize Bash really IS a programming language. Imagine that! :unlock:

for i in {1..100};
   do echo "Some interesting thing one hundred times with an index: $i";
done

content = "content coming soon"

for i in More
    do 
        if [ $RANDOM -le 7000 ]
            then echo "$i groovy $content"
        else echo "$i questionable $content" 
        fi
    done

Ultimately, you will not use very much Bash scripting, unless you want to. But you can get really far, accessing Databases and stuff. Thought you should know.

8. Let us know your favorite commands!

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  echo $feedback > /dev/null
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