modules

Command Line 1

Introduction

What is the command line?

The Command Line Interface (CLI) is a way of interacting with your computer using text-based commands. This is also referred to as a Text User Interface (TUI) which is different from the way most people interact with their computers, using their mouse and a Graphical User Interface (GUI).

Why should I use it?

Once you become comfortable with the basics, it can be a more powerful way to use your computer. You’re able to do many things more quickly and programatically. It is especially useful if you need to do something repetitively or in bulk. Even if you never use the command line again after this class, it can help you understand how software works at a more fundamental level.

Example Use Cases

find all files in a folder that contain a phrase
rename several files at the same time
resize or crop several images or pdfs
download a list of urls

Anatomy of a Command

<command> -<options> <arguments>

<command> is the action we want the computer to take. This is the program that you are running.
<options> (or “flags”) modify the behavior of the command
<arguments> (or positional arguments) are the things we want the command to act on. They are the “inputs” to the program that you are running.

You can view the manual for a command by typing man <command>. Sometimes, you can view a help page by typing <command> --help.

`df`

As an example, let’s take disect the df (disk free) command. This command shows how much disk space is used and remaining on your computer.

If you run

df

by itself, it will show you a result that looks similar to:

Filesystem    512-blocks      Used Available Capacity iused               ifree %iused  Mounted on
/dev/disk1s1   488555536 438995376  33577784    93% 3186543 9223372036851589264    0%   /
devfs                411       411         0   100%     712                   0  100%   /dev
/dev/disk1s4   488555536  14681200  33577784    31%       7 9223372036854775800    0%   /private/var/vm
map -hosts             0         0         0   100%       0                   0  100%   /net
map auto_home          0         0         0   100%       0                   0  100%   /home

The output isn’t all that readable. I don’t know how to translate how “512-blocks” to gigabytes in my head.

We can use the -h option/flag to make the output human readable text.

df -h

The output now looks like this:

Filesystem      Size   Used  Avail Capacity iused               ifree %iused  Mounted on
/dev/disk1s1   233Gi  209Gi   16Gi    93% 3186562 9223372036851589245    0%   /
devfs          206Ki  206Ki    0Bi   100%     712                   0  100%   /dev
/dev/disk1s4   233Gi  7.0Gi   16Gi    31%       7 9223372036854775800    0%   /private/var/vm
map -hosts       0Bi    0Bi    0Bi   100%       0                   0  100%   /net
map auto_home    0Bi    0Bi    0Bi   100%       0                   0  100%   /home

We can see above that I only have 16 gigabytes remaining on my main hard drive. Yikes!

`ping`

Let’s also disect the ping command. You might hear people in normal converation saying “Can you ping me when you’re back in the office?”. The word originates from this old unix command. ping sends packets of data to an ip address every few seconds in order to check for a response. One use case for this this command can be used to check if a website is down or not or see how quickly it responds.

In this command, we’ll start by using a single position argument.

ping google.com

You can see a response that looks like:

PING google.com (172.217.12.142): 56 data bytes
bytes from 172.217.12.142: icmp_seq=0 ttl=55 time=14.140 ms
bytes from 172.217.12.142: icmp_seq=1 ttl=55 time=18.354 ms
bytes from 172.217.12.142: icmp_seq=2 ttl=55 time=12.728 ms
bytes from 172.217.12.142: icmp_seq=3 ttl=55 time=18.537 ms
bytes from 172.217.12.142: icmp_seq=4 ttl=55 time=12.509 ms
bytes from 172.217.12.142: icmp_seq=5 ttl=55 time=14.982 ms
bytes from 172.217.12.142: icmp_seq=6 ttl=55 time=11.185 ms
bytes from 172.217.12.142: icmp_seq=7 ttl=55 time=26.755 ms
bytes from 172.217.12.142: icmp_seq=8 ttl=55 time=11.612 ms
bytes from 172.217.12.142: icmp_seq=9 ttl=55 time=10.857 ms
bytes from 172.217.12.142: icmp_seq=10 ttl=55 time=13.876 ms
...

This command will keep going on forever sending packets of data to google.com and getting back a response each time. If we want the command to terminate after some number of iterations, we can use the -c count option/flag.

ping -c 5 google.com

Another option on ping is -a which makes each ping audible.

ping -a google.com

You can also have multiple options at the same time.

ping -a -c 5 google.com

This command can also be written as ping -ac 5 google.com as a shorthand, but make sure the 5 comes after the -c.

Another option that can be used with ping is the -i (interval) option. This will cause ping to wait some number of seconds between each iteration.

ping -ai 2 -c 5 google.com

You can use the man command to understand all the different options that can be used with ping.

man ping

Use your arrow keys to move up and down and press q to exit the man page.

Now that you have an understanding of option/flags vs. positional arguments, let’s check out some other basic commands you can use on the command line.

Basic Commands

`whoami`

prints your username to the terminal

`hostname`

prints your computer’s name to the terminal

`echo`

prints a text string (denoted by quotes) to the terminal
usage: echo "Hello, World"

`man`

shows you the manual page - get help on how to use any command
press q to quit
usage: man <name_of_command>

`uname`

shows you the operating system name
usage: uname -a

`curl`

downloads and shows you the source code of a website
usage 1: curl 'https://api.ipify.org'
usage 2: curl 'http://example.org'
usage 3: curl 'http://dhrumilmehta.com'

`clear`

clears all output from your console
on macOS, you can also press Ctrl+L to clear your console while retaining history or Cmd-K to clear console while deleting the history. Ctrl+L should work on Ubuntu as well.

:large_orange_diamond: Example

print your username to the command line using whoami
print your computer’s name to the command line using hostname
print the name of your operating system using uname
take a look at the manual page for the uname command. Then, use uname with options to:
- print the operating system name
- print the operating system release
- print the operating system name and release together
- print the machine hardware name and the processor architecture together
- print as much information as you can all at once using uname.
print ‘I did it. I’m so cool.’ to the terminal using echo

Bonus

print “I did it! I’m so cool!” (hint: google “escaping characters in bash”)

Protip: Use the ↑ and ↓ arrow keys to navigate previously entered commands.

Figuring out New Commands

You can always check the manual page for complete documentation

for a given command, its usually either

man <command>
<command> --help
or <command> -h

if its not easy to figure out from there, try Google

https://www.google.com/

…StackOverflow is also your friend

http://stackoverflow.com/

also, know when to reach out to someone…

don’t go down rabbit holes trying to figure something out if there is someone who knows better nearby and you’ve already tried the three things above, they might be able to save you a lot of stress. I’m always happy to help out.

:white_check_mark: Try It

Let’s see if you can learn how to use a new command! Experiment working with cowsay in the command line by completing the following assignment: https://classroom.github.com/a/4yAnW2NV

Dangers

The terminal is a powerful tool (especially in linux) http://www.howtogeek.com/125157/8-deadly-commands-you-should-never-run-on-linux/