README.md

UNIX COMMANDS CHEATSHEET

This document lists useful commands to be used while working on a UNIX shell: from basic file and folder navigation, to running processes.

Note: all the commands listed on this document will only work on a UNIX terminal.

1 - Basic commands

1.0 Get help

<command> --help

• Will output syntax guidelines to run any command

1.1 File and folder navigation

Move through folders

cd <path>

• cd stands for "change directory"; this command moves the folder the terminal is looking to into to the specified absolute or relative path. Example: cd /folder will move to folder (which should be inside the folder we are currently looking to).
• cd .. moves to the upwards (or 'preceding') directory

Note: in some terminals, typing cd (with a space), and pressing Tab allows the terminal to jump between available sub-folders possible options.

pwd

• stands for "Print Working Directory": returns the absolute path of the current directory

Show folder contents

ls

• lists current directory contents (prints the filenames) Options:
• use ls -l to output the file details. -l stands for 'long' listing format.
• use ls -a to print all files (normal files and hidden ones). -a stands for 'all'
• use ls -la to combine above

Create, Update, Delete files and folders

• rm <directory>/<file>: deletes (rm stands for 'remove') <file> in <directory>
• rm -r <directory> deletes <directory> and its contents. Note: this command needs -r (Recursive), since the OS will need to recursively enter every folder and file in the directory to completely erase it.
• mv <old_directory>/<file> <new_directory>/<file> moves <file> from a directory to another
• mv <old-file-name> <new-file-name> renames file
• mkdir <new-directory-name> creates a new directory in current path

1.2 - Check file contents

The cat command

The command cat is one of the most useful commands to quickly check on the terminal the contents of a file. cat stands for 'concatenate': the contents of the file will be 'concatenated' and shown in the terminal.

cat <filename>

• Prints on the terminal the contents of <filename>

Additional arguments we can use for cat:

• cat -n <filename> will print the contents of the file with a number showing the line number

1.3 - Display on the terminal

The command echo displays a string directly on the terminal. It can be used to display some statement if something specific happens such as ('Launching application'), or accessing environment variables (env. variables explained here). echo "This will be displayed on the terminal

• displays the text above directly on the terminal

2 - Advanced commands

2.1 - Downloading and updating packages

the apt-get command

apt-get (Advanced Packaging tool) is a command which handles packages in Linux. It retrieves information about packages from the authenticated sources; it allows to install, upgrade and remove packages along with their dependencies (We will use apt-get to download Python inside our VM, for example).

apt-get install <package-name>

• Downloads and installs the package added from the authenticated source

Note: we can add -y for 'Yes' to let the apt-get reply 'Yes' for all [yes/no] queries (such as 'are you sure you want to install?').

apt-get update

• Checks if there are any updates for the packages installed

2.2 - Modifying permissions

How do Unix permissions work?

Unix systems have three kinds of permissions for each file: r for Read permissions, w for Write permissions, and x for Execute permissions (permissions can be checked using the ls -l command, which will show - when a permission is not available). These permissions need to be set for each one of the three types of users defined in a Unix system.

Note: Unix systems have three kinds of users: 'user' (the owner of the file), 'group' (which is useful when many computers have access to one file), and 'others' (which is any user not on the first two groups).

The chmod command

The chmod (or 'change mode') command allows us to modify the permissions of a file for the three types of users. The easiest way of doing so is by using the three-digit code, which uses a each digit to define the permissions for each type of user. Each digit is converted into a binary 'mapping' (1 for 'permission granted' and 0 for 'permission disabled') of the permissions using the rwx (read-write-execute) format: for example, 0 is 000 in binary, so it stands for 'none of the permissions granted'; 4 is 100 (or r-- so 'Only read permission enabled', 7 is 111 so 'All permissions granted'.

That way, 400, for example, will enable reading permissions for the first type of user (which is the 'user' of the file itself), and disable any permissions for rest ('groups' and 'others'); 777, as another example, would enable all permissions by all users.

chmod <three-digit-code> <filename>

• Modifies the permissions of <filename> and sets them to the ones specified on the three digit code.

For example, chmod 400 file.txt will only enable reading of file.txt by the main user, and will not allow groups or others to read it, write it or execute it.

3 - Environment variables

'Environment variables' are variables stored in special folders, in order to only reveal its contents locally (on the terminal), and if requested.

Environment variables very useful to store confidential data such as passwords and API keys, which we do not want to reveal in our source code or uploaded in GitHub, for example. They can also be used to run code in different machines, where the value of HOME is different, for example.

Note: in Unix systems, global environment variables are stored in the /etc/environment folder and user level variables in .bashrc and .profile files of the user's Home folder.

Environment variables to know

• PATH is the list of folder paths (separated by :) that our terminal will look into to understand the commands we run in the terminal.
• HOME is the absolute location of the user's home directory

Get list of environment variables

printenv

• prints in the terminal the list of currently set environment variables (alternatively, we can use the command env)

set

• displays the entire list of set or unset values of shell options (environment variables). Gives a much more complete list than env, with all predefined evironment variables (even those that have no value assigned to it)

View contents of environment variables

echo $<environment-variable>

• Displays the value of the environment variable. Example: echo $PATH

Note: the $ sign is used by Linux to access the value of environment variables

Update environment variables

export <env-variable>=<variable-content>

• Sets up the contents of an environment variable. Example: export PATH=$PATH:opt/bin adds an address to PATH.