The CLI isn't as scary as you think

a title screen of someone learning the cli in a classic 90s hacking style

VanJS

April 16th, 2025

Gavin Mogan

Senior Software Engineer at Digital Ocean. Digital Ocean Mascot, Sammy the Shark, holding a heart ballon

https://www.gavinmogan.com

Upfront notes

Talking about linux. Rest are similar
$ is usually a shell prompt
Generally, Linux doesn't care what order CLI parameters are in.
Linux tools, like npm packages, like to do one thing and do them well.

Okay, a few notes before we get started. While I know most developers these days, at least front end ones, will end up using an apple laptop, I'm going to cover linux as I know it the best, and but the apple cli, and windows with wsl, and even to some degrees powershell all have overlaps so it shouldn't be hard to adapt things. Firstly, whenever you see a dollar sign followed by a command, its usually to indicate a command prompt. I've stuck with that convention here. Upfront linux usually doesn't care about order of arguments, though some older apple versions do. So if something I cover doesn't work, try switching the order of the arguments. Classicly linux like node packages, like to try and do one thing well, so you can combine the best tools for the job.

Getting Help

Offline version

Most commands have a man(ual) pages
```
$ man find
```
Shell functions only have help
```
$ help for
```
Sometimes --help will return things
```
$ ls --help
```

Keyboard Shortcuts

Ctrl+r - Reverse search through your bash history (My fav)
Up / Down - Prev/next in history
Ctrl+w - Delete previous word
Ctrl+k - Delete rest of line
Ctrl+u - Delete entire line
Ctrl+a - Move to start of line
Ctrl+e - Move to end of line

Keyboard Shortcuts

Ctrl+r - Reverse search through your bash history

Where am I?


              $ pwd
              /home/gavinm

              $ echo $PWD
              /home/gavinm

How to Change Directory

Most OSes uses forward slash - / as folder separator


            $ cd git
            $ pwd
            /home/gavinm/git
            $ cd /home/gavinm/git2
            $ pwd
            /home/gavinm/git2

Spaces and other characters should be escaped or quoted


              $ cd /home/gavinm/Project\ Dir
              $ cd "/home/gavinm/Project Dir"

Get back there


        $ pwd
        /home/gavinm

        $ cd /home/gavinm/Develop
        $ pwd
        /home/gavinm/Develop

        $ cd -
        $ pwd
        /home/gavinm

What is in this directory?

Simple Directory Listing

$ ls

Long Listing

$ ls -l

Descending(r) by Time(t)

$ ls -ltr

Hidden files

$ ls -a

When am I?


              $ TZ=America/Vancouver date
              Sun Apr 20 12:00:00 PDT 2025

Note: time command times a command, not time of day


              $ time sleep 30s
              sleep 30s  0.00s user 0.00s system 0% cpu 28.108 total

Testing HTTP

Curl and wget work kinda the same, but under different designs.

Wget is great at downloading files.

$ wget http://i.imgur.com/Ia48QDR.jpg

Curl is better at retrieving content.

$ curl https://httpcodes-8pkfp.ondigitalocean.app/json/404

Note: probably less true these days

What's happened recently?

Head will give you the first n lines

$ head -n 2 /var/log/nginx/presentations.gavinmogan.com.access.log
  presentations.gavinmogan.com:80 108.172.217.87 - - [24/Jun/2015:07:02:03 +0000] "GET /stats/ HTTP/1.1" 200 5566 "http://odin.kodekoan.com:4080/halkeye/gavinmogan.com/" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/43.0.2357.124 Safari/537.36" "108.172.217.87"
  presentations.gavinmogan.com:80 108.172.217.87 - - [24/Jun/2015:07:02:03 +0000] "GET /stats/css/reveal.css HTTP/1.1" 200 48591 "http://presentations.gavinmogan.com/stats/" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/43.0.2357.124 Safari/537.36" "108.172.217.87"

Tail will give you the last n lines

$ tail -n 2 /var/log/nginx/presentations.gavinmogan.com.access.log
  presentations.gavinmogan.com:80 162.158.64.218 - - [27/Jun/2015:03:35:46 +0000] "HEAD / HTTP/1.1" 403 166 "-" "Mozilla/5.0 (compatible; CloudFlare-AlwaysOnline/1.0; +http://www.cloudflare.com/always-online)" "162.158.64.218"
  presentations.gavinmogan.com:80 162.158.64.218 - - [27/Jun/2015:03:35:46 +0000] "GET / HTTP/1.1" 403 345 "-" "Mozilla/5.0 (compatible; CloudFlare-AlwaysOnline/1.0; +http://www.cloudflare.com/always-online) AppleWebKit/534.34" "162.158.64.218"

Common Options

-f will keep tailing
-F will restart tailing if file is truncated
-n #num# will only print out #num# number of lines

Redirection output

Output to file

$ echo "hi" > file.txt

Errors to file

$ curl http://fake.server 2> errors.txt

Output to file and console

$ echo "hi" | tee file.txt

Redirection input

Input from file

$ mysql < import.sql

Wildcards (glob)

Will look find all directories that have a logs directory underneath it.

$ tail -F ~/Develop/*/logs/development.log

Will find all log files under all directories that have a log directory (one level deep)

$ tail -F ~/Develop/*/logs/*.log

Will find all log files under all directories that have a log directory (infinite levels deep)

$ tail -F ~/Develop/**/logs/*.log

Chaining

true && echo true

false || echo false

false; echo always

if and while statements

if true; then echo "its-a-me-truethy"; fi

while true; do echo "its-a-me-truethy"; done

for i in a b c; do echo $i; done

Serious wildcards

Find all directories

$ find -type d

Find all files

$ find -type f

Find all files ending in log


          $ find -name '*.log' # find all files ending in .log
          $ find -name '*.log' -exec ls {} \; # executes a command for each file
          $ find -name '*.log' -exec ls {} + # appends all files to one command

Run something in a directory with logs

$ find /var/log -name '*.log' -execdir pwd \;

Create Edit Update Destroy


          cat <file> # outputs contents
          tac <file> # outputs contents in reverse order
          less <file> # outputs content (controlled)


          rm <file> # Remove a file
          rmdir <dir> # Remove an empty directory
          mkdir <dir> # Create a directory (mkdir -p as bonus)
          # Create all the directories required to make the full path
          # (and doesn't error if already exists)
          mkdir -p <dir/subdir/subdir2>


          touch <file> # update timestamp/create empty file

Power of pipes

With a few combos you can do anything

Grep and Awk Can Do anything


          $ grep /favicon.ico access.log
          # long output


          $ grep /favicon.ico access.log | awk '{print $17}'
          AppleWebKit/537.36


          $ grep /favicon.ico access.log | awk -F'"' '{print $6}'
          Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/40.0.2214.111 Safari/537.36

For Loops


          $ for i in gavin likes pie; do mkdir $i; done
          $ ls -l
          drwxr-xr-x 2 1000 1000 4096 Apr 15 04:23 gavin
          drwxr-xr-x 2 1000 1000 4096 Apr 15 04:23 likes
          drwxr-xr-x 2 1000 1000 4096 Apr 15 04:23 pie

For Loops - wildcards


          $ for i in *; do mv $i $i.bak; done
          $ ls -l
          drwxr-xr-x 2 1000 1000 4096 Apr 15 04:23 gavin.bak
          drwxr-xr-x 2 1000 1000 4096 Apr 15 04:23 likes.bak
          drwxr-xr-x 2 1000 1000 4096 Apr 15 04:23 pie.bak

For Loops - subshells


          $ for i in $(seq 1 10); do echo $i; done
          1
          2
          3
          4
          5
          6
          7
          8
          9
          10

Search and Replace

Sed, perl, python, etc

I prefer perl pie


        $ echo "Gavin likes pie" > file.txt
        $ cat file.txt
        Gavin likes pie
        $ perl -pi -e 's/Gavin/Bibi/' file.txt
        $ cat file.txt
        Bibi likes pie

Real world example


          for i in $(git ls-files -m); do
            jsonlint $i && git add $i;
          done

Real World Example - Find and Replace multiple files


          grep -r onedr0p . | \
            awk -F: '{print $1}' | \
            xargs perl -pi -e 's#ghcr.io/onedr0p#ghcr.io/home-operations#g'

Non standard tools - JQ

https://jqlang.org/


        $ curl -qs 'https://jsonplaceholder.typicode.com/todos/1' | jq '.'
        {
          "userId": 1,
          "id": 1,
          "title": "delectus aut autem",
          "completed": false
        }
        $ curl -qs 'https://jsonplaceholder.typicode.com/todos/1' | jq '.id'
        1
        $ curl -qs 'https://jsonplaceholder.typicode.com/todos/1' | jq '.completed'
        false

Non standard tools - gron

https://github.com/tomnomnom/gron


        $ curl -qs 'https://jsonplaceholder.typicode.com/todos/1' | gron
        json = {};
        json.completed = false;
        json.id = 1;
        json.title = "delectus aut autem";
        json.userId = 1;

Eg curl + jq + gron


        $ curl -qs 'https://jsonplaceholder.typicode.com/todos' | jq '[.[] | select(.completed == true)][0]' | gron
        json = {};
        json.completed = true;
        json.id = 4;
        json.title = "et porro tempora";
        json.userId = 1;

The End

https://m.do.co/c/7d6859326b6a

Extra Stuffsssss

Slides that were already done but decided against bringing them up

Debugging

Run a script in debug mode

$ bash -x script.sh

Enable debugging right now

$ set -x

SSH

What are ssh keys?

Why would you want them?

How do you use them?

man ssh_config

Whats going on?

ps xf -A (My Favourite)
ps aux (Very portable)
- More w's with ps = more wide
- ps auxwww
pstree
top

Home Directories


          $ cd
          /home/gavinm


          $ cd ~
          /home/gavinm


          $ cd $HOME
          /home/gavinm

Another users home dir


          $ cd ~halkeye
          /home/halkeye

How to get there? - Apple

Terminal.app
Iterm2