The main topics of this cheatsheet include an intro to the shell, navigating around the shell, common commands, environment variables, connectors, piping, I/O redirection, permissions, and keyboard shortcuts.

The shell is a program, in our case, called "bash" which stands for Bourne Again Shell. How the shell works is it takes your commands and gives them to the operating system to perform. In order to interact with the shell, we use "terminal emulators" such as the gnome-terminal, eterm, nxterm, etc.

On a Linux system, files are organized in a hierarchical directory structure. This means there is a starting directory called the root directory. This directory contains files and subdirectories that lead into other subdirectories.

The pwd command, short for print working directory, displays your current location in the directory structure.

The cd command allows you to enter a new directory.

The mkdir command makes a new directory in your current directory.

The man command directs you to the command manuals.
For example, the following command gives us all the information we need about the command cat.

  $ man cat

The cat command reads a file passed as a parameter and by default print its contents to standard output.
Passing multiply files as parameters concatenates the files and then prints to standard output.

The echo command prints its arguments to standard output.

  $ echo Hello World
  Hello World

If you call echo without any parameters, the command prints a new line.

The head command reads the first 10 lines of any passed in text and prints its contents to standard output. You can change the default 10 lines to any number by manually passing in the desired size. For example, the following prints all 50 lines of the file.

$ head -50 test.txt

The tail command reads the last 10 lines of any passed in text and prints its contents to standard output. You can change the default 10 lines to any number by manually passing in the desired size. For example, the following prints all 50 lines of the file.

  $ tail -50 test.txt

You can also view in real time any text appended to the file with the -f flag.

$ tail -f test.txt

The less command gives you a way to navigate through a passed file or block of text. Unlike the more command, less allows you to move backward through the file as well.

$ less test.txt
$ ps aux | less

The true command always returns the exit status zero to indicate success.

The false command always returns the exit status non-zero to indicate failure. ###$? $? is a variable that will return the exit code of the last command you ran.

$ true
$ echo $?
  0
$ false
$ echo $?
  1

The grep command is a search function.
Passing a string and a file searches the file for the given string and prints the occurrences to standard output.

  $ cat users.txt
  user:student password:123
  user:teacher password:321
  $ grep 'student` file1.txt
  user:student password:123

grep can take multiple files as parameters and regular expressions to specify a pattern in text.

The sed command is a stream editor that performs text transformations on an input.
Common use of this command is to replace expressions which takes the form s/regexp/replacement/g For example, the following replaces all occurrences of the phrase "Hello" with "Hi".

  $ cat test.txt
  Hello World
  $ sed 's/Hello/Hi/g' test.txt
  Hi World

See sed tutorial

The history command prints out an incremented command line history.
It is common to use the grep command with the history command in order to search for a particular command. For example, the following searches your history for all occurrences of the string g++.

  $ history | grep g++
  155  g++ file1.txt
  159  g++ file2.txt

The export command sets an environment variable to be passed to child processes in the environment.
For example, the following exports the variable "name" with the value "student".

  $ export name=student

The ps command, short for process status, prints out information about the processes running.

  $ ps
  PID TTY          TIME CMD
  35346 pts/2    00:00:00 bash

There are four items displayed:

• process identification number (PID)
• terminal type (TTY),
• how long process has been running (TIME)
• name of command that launched the process (CMD)

The awk command finds and replaces text by searching through files for lines that have a pattern.
Syntax: awk 'pattern {action}' test.txt

The wget command downloads files from the web and stores it in the current working directory.

  $ wget https://github.com/mikeizbicki/ucr-cs100

The nc command, short for netcat, is a utility used to debug and investigate the network.
See nc tutorial

The ping command tests a network connection.

  $ ping google.com
  PING google.com (74.125.224.34) 56(84) bytes of data.
  64 bytes from lax17s01-in-f2.1e100.net (74.125.224.34): icmp_req=1 ttl=57 time=7.82 ms
  --- google.com ping statistics ---
  1 packets transmitted, 1 received, 0% packet loss, time 8ms
  rtt min/avg/max/mdev = 7.794/8.422/10.792/0.699 ms

The statistics at the end show an overview of how many connections went through before we called ^C and how long it took.

Git is a version control system that is commonly used in the industry and in open source projects.
See git tutorial

Environment variables are named variables that contain values used by one or more applications.
The PATH variable contains a list of directories where systems look for executable files.
The HOME variable contains the path to the home directory of the current user.
The PS1 variable is the default prompt to control appearances of the command prompt.

Connectors allow you to run multiple commands at once.

Connector | Description --- | --- | --- && | first command always executes and the next command will only execute if the one before it succeeds || | first command always executes and the next command will only execute if the one before it fails ; | first command and the following commands always execute

  $ true && echo Hello
  Hello
  $ false || echo Hello
  Hello
  $ echo Hello ; ls
  Hello
  test.txt file1.txt file2.txt

Pipes connect multiple commands together by sending the stdout of the first command to the stdin of the next command. For example, the following sends the ls output to head so that only the top 10 items get printed.

  $ ls -l | head

Standard output redirection uses the symbols > and >>.
For example, the following sends the output of ls into the file instead of printing to the screen.

  ls > files.txt
  $ cat files.txt
  file1.cpp sample.txt

If the file isn't already in your working directory, the file gets created. If the file already exists, then the contents of the command overwrites what is already in the file.
To avoid overwriting a file, the >> command appends to the end of the file instead.

Standard input redirection uses the symbol <.
For example, the following causes sort to access its input from the file instead of the keyboard.

  $ cat files.txt
  c
  b
  $ sort < files.txt
  b
  c

The sort command prints the contents of the file and prints to the screen because we haven't redirected its output. But we can combine I/O redirection into one command line, such as:

  $ sort < files.txt > files_sorted.txt

Adding a & with the > symbol results in redirecting both standard out and standard error. For example, the test.cpp file prints the string "stdout" with cout and the string "stderr" with cerr.

  $ g++ test.cpp
  $ ./a.out >& test.txt
  $ cat test.txt
  stdout
  stderr

The > symbol alone only redirects standard output.
If you only want to redirect a specific file descriptor you can attach the file descriptor number to >.

  $ g++ test.cpp
  $ ./a.out 2> test.txt
  stdout
  $ cat test.txt
  stderr

The command ls -l prints out a lot of information about each file that is informative about the permissions.

  $ ls -l test.txt
  -rw-rw-r--  1  user  group  1097374 January 26 2:48 test.txt

• | File type:
  -= regular file
  d= directory rw- | Permissions for owner of file rw- | Permissions for members of the group owning the file r-- | Permissions for all other users user | name of user owning the file group | name of group owning the file

The chmod command, short for change mode, changes the permissions of a file.
There is a combination of letters that need to be known in order to change specific users' permission.

  $ chmod ug+rw test.txt
  $ ls -l test.txt
  -rw-rw----  1  user  group  1097374 January 26 2:48 test.txt

Alternatively, we can use chmod with hex numbers. You can think of each permission setting as a bit where it is a 1 if there is permission for the file and 0 otherwise.

  rwx = 111 = 7
  rw- = 110 = 6
  r-x = 101 = 5
  r-- = 100 = 4

Each set of permissions represents a single digit so the following commands have the same outcome as above.

  $ chmod 660 test.txt

See permissions tutorial