Introduction to the Terminal

Aka console, CLI, command line, shell, etc.

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What we’ll cover

• Terminal Terminology
• Getting Familiar with the Prompt
• Navigating the file system
  • Inspecting files and directories
    • Creating, copying, moving, renaming, deleting files and directories
    • Editing files
• The art of Redirection

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Terminal Terminology

• Command Line Interface (CLI) - the text-based interface of a computer
• Console - See Command Line Interface
• Terminal - See Console

• Shell - See Terminal*

• A shell is the interactive program that runs on the terminal, but the word is often used interchangeably to refer to both.

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More Terminal Terminology

*nix - any Unix or Linux operating system. Examples:

• Solaris (Unix)
• Ubuntu (Linux)
• Red Hat (Linux)
• BSD (Unix)
• Apple OS X (Unix)
• Android (Linux)
• Chrome OS (Linux)

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Getting to Terminal on a Mac

• Press command + space
• Type ‘terminal’ and hit enter

OR

• Open Applications
• Go to the Utilities folder
• Double-click Terminal

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Getting Familiar with the Prompt

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What the prompt looks like

Simple examples: $, >, >>> or ->
Highly customizable.
Default OS X prompt:
Computername:directory username$ eg: Valhalla:HallOfOdin thor$
Idioms: /= root; ~=home

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Executing commands

Commands are executed when you hit enter
Can chain commands with semicolons
eg: echo "hello"; echo "goodbye"
Obviously incomplete commands will provide intermediate prompt (>)

$ echo "hello
> world"
hello
world

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Navigating the file system

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File system structure

*nix file systems are a tree with a root named /
Everything is within / or a descendant directory
directories are delimited with / character
This means we can list the exact position of a dir using its name (called an absolute path)
eg: /usr/bin/, /dev/null, /Users/loki

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Getting your bearings (basic commands)

• pwd - Print Working Directory aka. “Where am I?”
• ls - list directory contents
• cd - change directory (explained on next slide)

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Moving around

• cd - change directory
  Must provide a destination as relative or absolute path
  From home directory (aka ~ or /Users/thor)
  change to Desktop (~/Desktop) using one of these commands:
  cd Desktop
cd /Users/Thor/Desktop
cd ~/Desktop

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Notable directory names and idioms:

• / root directory
• ~ home directory
• . this dir
• .. parent directory
• - used with cd to mean “go to where I just was” (cd - twice and you haven’t moved)
• Go up a directory with cd ..
• Use these idioms to save time.
  eg: from Desktop use cd ../Documents or cd /etc/lib rather than multiple ../ to get to root first

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Class Exercise

• In terminal, navigate to the highest level directory you can reach.
• Write down the name of that directory
• Navigate back to your home directory, list each directory along the way and how many items it contains

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Class Exercise 2

• Find the bin directory and list its contents
• Identify at least one item you recognize in bin
• Now find the Applications directory
• List the contents of the directory
• Open the directory in finder with open . to see the same list of files in finder

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Inspecting files and directories

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File contents

• View file contents with cat or less
• use grep find specific pattern/text
  Patterns are defined by regular expressions (more on that in later lessons)

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Permissions

ls -l shows file permissions

-rw-r--r--  1 thor  staff  0 Jan 10 16:04 file
-rw-r--r--  1 root  staff  0 Jan 10 16:04 rootFile

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Running commands as superuser (aka root)

sudo command - Do something as Superuser (requires password)

$ cat file.txt
cat: file.txt: Permission denied
$ sudo cat file.txt
this is a file

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Creating, copying, moving, renaming, deleting files and directories

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Three ways to create a file:

• touch command ( touch newfile )
• Using a text editor
• Redirect output to a file eg: echo "hello" >file.txt

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Create directories with mkdir

• mkdir dirname
• Can use paths, but will not crate other dirs implicitly
  eg: mkdir dir1/dir2 fails if dir1 doesn’t exist.

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Editing files

Many terminal-based editors out there. We recommend VIM (explained in console lab 2).
Other options include:

• emacs - Popular and comparable to VIM in power
• nano - Very simple, easy for beginners but limited functionality

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The art of Redirection

Every command has access to at least three “file descriptors” (input and output sources). By default these are all routed to the command line

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Standard file descriptors

• STDIN (fd 0) - Whatever you type
• STDOUT (fd 1) - Normal command output
• STDERR (fd 2) - Error output

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Redirecting File Descriptors

File descriptors can be rerouted using these commands:

• < - Redirect STDIN to specified file.
• > - Redirects STDOUT to the specified file (if the file exists its contents are erased first)
• 2> Same as > but for STDERR
• &> - Redirects STDERR and STDOUT to the same place
• >> or 2>> - Appends output to the end of the specified file instead of overwriting it
• | - redirects the output from the previous command to be the input of the next command.
  Eg: ps aux | grep bash

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Backgrounding

Long running tasks can prevent use of the command prompt.
Run a command in the background with the & character eg: find / -name foo &
Send an already running foreground command to the background with ^z bg (^z suspends the current job, bg resumes it in the background)
It’s often wise to redirect the output when you background something

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Useful examples

• python -m SimpleHTTPServer &>http.log & - tiny local web server