Command Line Interface¶

Command Line Interface or CLI is the fastest way of communicating with a computer. CLI can retrive data from the current system or run a task on a remote server. Also, CLI provides a way to run periodic commands and runs long running commands in the background, Daemons,.

In this chapter, we will discover the import commands, and we will teach you to use them. After opening up a terminal in Ubuntu, type in the following command ls. As you can see in the terminal, a list of files and directories showed up. ls is the command to list the contents of the current directory.

Important commands¶

As you might expect, anything that you can do in the GUI you can do in the command line. CLI has many Shells e.g. Bourne Shell, Korn SHell, Bourne_Again_SHell ...etc. Shells are program that help you execute commands on the computer. They have a special language, and they provide basic programming capabilities. Below are a list of important commands that you should be comfortable with.

pwd

Stands for Print Working Directory:

pwd prints the current working directory from / or root directory
mkdir

Make Directory:

mkdir foo creates a directory called foo

mkdir -p foo/bar/baz creates baz and all the missing directories in the path to it

Exercise:
- Make a directory called temp
- Make a directory at the path temp/stuff
- Make a directory at the path temp/stuff/things
- Can you create a directory at temp/stuff/things/frank/joe/alex/john using a single command?
cd

Change Directory.

Once invoked it will change your working directory to a new one:

cd temp to change to the temp directory you created in the last exercise

Exercise:
- Change to the temp directory. Check where you are using pwd.
- Change to the stuff directory. Check where you are.
- Change directly to the john directory in one command.
- Use the command cd ... Where did you end up?
- Where does cd ../../.. take you?
- What about cd .?
- Just cd?
ls

Lists the contents of the current directory:

ls -a to list all files and directory including hidden directory or dotfiles

ls -l to list the files and directory with more information about their permissions, owner, group that owns it, disk size and creation date

Exercise:
- Navigate back to your home directory ~.
- Go to temp and use ls to see what is in it.
- While in temp try ls -lR. What did it do?
- Use a combination of cd, ls and pwd to explore the files on your machine.
touch

Creates an empty file in the current directory:

touch CaptainAwesomesauce.txt creates a blank text file called CaptainAwesomesauce.txt
cp

Copy file or directory from one location to another:

cp file1 file2 copies the contents of file1 into file2

cp file1 Documents/ copies file1 into the Documents directory

cp -r /tmp Documents/tmp copies the contents of /tmp into Documents/tmp

Exercise:
- Inside temp create a file called iamcool.txt.
- Make a copy of it called awesome.txt.
- Make a directory called stuff and copy awesome.txt into it.
- Without leaving temp check the contents of stuff
- Copy stuff and all its contents into a new directory called things.
- Without leaving temp check the contents of things.
mv

Moves files or directories to different location (path). Also it can be used

to rename files or directories:

mv file file.txt renames file to file.txt

mv Downloads/file.zip Documents/ moves file.zip from Downloads/ to Documents/

Exercise:
- Change the name of the file awesome.txt to notawesome.txt
- Change the name of the directory stuff to foo
- Move the file iamcool.txt from temp into foo

Warning

Be careful when passing paths!

Paths can be:

Absolute Paths relative to root

e.g. /etc/init/, ~/Desktop/bar.py
Realtive Paths from your current working directory

e.g. ../foo/bar/, temp/stuff/awesome.txt

nano

Nano is an easy to use terminal text editor:

nano file1 opens file1 for editing
less

Less is a file viewer, and it has search features. The name came from the

Unix philosophy “Less is more, more is less”[1]:

less foo.txt page through foo.txt

cat
Concatenate files and prints them to stdout:
cat file1 spits the content of file1 to stdout
cat file1 file2 concatenates file1 to file2 then spits the contents to stdout
Exercise:

Create a file called zen.txt with the following content:

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

View it using less and cat. What’s the difference?

rm

Removes a file or directory:

rm /path/to/file1 to delete file1

rm -r /path/to/dir1 to recursively delete dir1 and all its contents

Exercise:
- Go to the temp directory
- Remove the file notawesome.txt.
- Remove the directory things and all its content.
echo

Takes a string of text and prints it to stdout

echo Hello world
|

The Pipe character which takes the output of the left command and inputs it to the right command

ls | grep ""
>

Redirect to character; it redirect the output of the command to a file

echo Hello > foo.txt
>>

Append character; it appends the output to a file

echo Hello >> foo.txt
<

Input in character; it inputs the text of a file to the command

cat < foo.txt
man

Return the help manual for any command in the system:

man shell-command
find

Find is a powerful command. Take a look at the manual of find to see all

the options that you can use with it:

find . -type f -name foo looks for a file that’s named foo
diff

Differences between two files. The command diff prints out the difference

between two files:

diff v1/foo1 v2/foo1
comm

Common is a command that compares two files and print the common bytes

between them:

comm v1/foo1 v2/foo1
head

Head prints out first lines of a file:

head foo.txt
tail

Tail is simliar to head but it prints out the last lines of a file:

tail foo.txt
sort

Sort sorts text:

sort foo
* - The Wildcard

* is known as the wildcard because it matches everything.

It’s great when you want to do a command on a set of files all at once:

ls *.py lists all the files in the current directory ending in .py

rm -r h* removes all files and directories beginning with h

rm h*.* removes only files beginning with h

Exercise:
- Create the following files in temp:
  
  ex12.txt
  
  ex13.txt
  
  ex14.py
  
  stupid.vb
  
  useless.vb
  
  wasteoftime.vb
- List all the .txt files in temp.
- List all the files that begin with ex.
- Delete all the vb files!
- Use find and less to see all the .txt files under your home directory.

Hint

You will need a | pipe for that last exercise

grep

Grep is a pattern search that uses regular expressions to look for a pattern

in text. It’s powerful if you know regular expressions:

grep this words.txt looks for the word this inside a file named words.txt

Exercise:
- Create a file in temp called newfile.txt with the following text:
  This is a new file. This is a new file. This is a new file.
- Create another file called oldfile.txt but with:
  This is an old file. This is an old file. This is an old file.
- Search for all occurences of the word new in all the .txt files in temp.
- Search for all occurences of old.
- Search for all occurences of file.
- How would you search for the words This is?

Hint

You can quickly type text into a file using cat > file.txt

This will overwrite file.txt with whatever you type until you close the file using CTRL-c.

Dot files/directories¶

Dot files and directories play a big role in the Unix/Linux operating system. Once a file or directory starts with a . it will be hidden from the regular ls command, and to display it you need to run ls with -a flag. The flag display the all the files in the current directory. There are many special dotfiles that you need to be aware of. The list below lists couple of them.

~/.bashrc
~/.vimrc
~/.emacs
~/.bash_history

Exercises¶

run ls -a ~ to display the files and try to get familiar with them

[1]	Less history