4.1 Introduction to UNIX

What is Unix?

Unix is a family of operating systems officially trademarked as UNIX®. These operating systesms are computing environments that are optimized for multi-tasking across multiple users. The original system was developed by AT&T in 1969 as a text only system. There are many Unix variants or Unix-like systems (e.g. GNU/Linux, Sun Solaris, IBM AIX, and Mac OS X). ON Windows, Cygwin is a program that provides a Unix-like environment.

The main components of a Unix operating system include:

• Kernel – bridge between hardware (i.e. silicon) and application (i.e. software)

• Shell – command line interface to enable user interaction with the system

• File System – the organization structure for how files are stored

Unix File System

The Unix file system organizes files and directories into a hierarchical structure like root system of a tree.

• The "root" directory (e.g. "/") is the top of the hierarchy

  • Standard directories within the root directory

  • /bin and /usr contain commands needed by system administrators and users

  • /etc contains system-wide configuration files and system databases

  • /home contains the home directory (~) for each user (in some systems, the location of home directories may be in a different location such as /users or /Users)

• When traversing directories

  • working directory (.) is the directory that a user currently is in

  • parent directory (..) is the directory above the working directory

  • path or pathname specifies where a user is in the file system

  • full path or absolute path points to the same location regardless of the working directory (i.e., it is written in reference to the root directory)

  • relative path is the path relative to the working directory

If the working directory is the home directory for bcbi, the full path for the course directory is /home/bcbi/course while the relative path is just course. A schematic of this is below:

If code then becomes the working directory, the full path for the data directory from there is /home/bcbi/course/data while the relative path is ../data. A schematic of this is below:

Unix Shell

The Unix shell provides a command line interface for interacting with the operating system and is where commands are entered. An example below is a Mac OS X Terminal Shell logged into a RedHat Linux Server as user_name.

• The prompt may look different depending on your shell (e.g., Bourne shell [sh], C shell [csh], or Bourne-Again shell [bash])

• Default prompts include $ and %

• The prompt # typically appears when logged in as the superuser or root user who can do anything on the system, so should be restricted to trusted users, used only when necessary and with caution. While you may be able to do this on a system you control, you are unlikely to ever have root priviledges on a shared computing resource (e.g. Oscar or Stronghold at Brown University)

• The prompt can be configured to include additional information such as hostname, username, and pathname (e.g., computer:/home/bcbi/course bcbi $).

Unix Commands

There are many Unix commands. Some commands will display output and then return to the shell prompt while others will just return to the shell prompt to indicate that it has executed the last command. Unix command syntax:

• Case-sensitive (pwd ≠ PWD)

• May involve one or more arguments

• Argument may be an option (or flag or switch) for that command

• Argument may be a file or directory

To get to a Unix shell on your computer:

• For Mac, launch the Terminal application (under Applications → Utilities → Terminal)

• For Linux, launch the Terminal application

• For Windows, launch the PowerShell application

#Run applications like cal:
$ cal
    January 2020
Su Mo Tu We Th Fr Sa
          1  2  3  4
 5  6  7  8  9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30 31

Get help from manual (man) pages on commands: (Use spacebar or up and down arrows to scroll through pages and then press q to quit)

$ man cal

Determine what directory you are currently in with pwd (present working directory):

$ pwd
/home/bcbi

Get a listing of current directory contents using ls:

$ ls

Create course directory using mkdir: (Replace course with class name - e.g., methods2020 or biol6535)

$ mkdir course

Get a listing of current directory contents with details using ls:

$ ls
course
$ ls -l
total 4
drwxrwxr-x. 2 bcbi bcbi 4096 Jan 30 12:53 course

Change into course directory using cd: (Replace course with class name - e.g., methods2020 or biol6535)

$ cd course
$ pwd
/home/bcbi/course

Working with Data

Using the Unix commands: more, less, head, tail, wc, cut, sort, uniq, grep, sed, and awk

• Adult (Income Census) Data Set

• MIMIC-III Demo Data Set

File and Directory Navigation

Command	Action (with sftp specific notes)
ls	directory listing (remotely in sftp)
lls	local directory listing (sftp only)
ls -al	formatted listing with hidden files
cd dir	change directory to dir (remotely in sftp)
lcd dir	change local directory to dir (sftp only)
cd	change to home (remotely in sftp)
pwd	show current directory (remote directory in sftp)
lpwd	show current local directory
mkdir dir	create a directory dir
rm file	delete file
rm -r dir	delete directory dir
rm -f file	force remove file
rm -rf dir	force remove directory dir *
cp file1 file2	copy file1 to file2
cp -r dir1 dir2	copy dir1 to dir2; create dir2 if it doesn't exist
mv file1 file2	rename or move file1 to file2 if file2 is an existing n directory, moves file1 into directory file2
put file	copy local file to current remote directory (sftp only)
get file	copy remote file to current local directory (sftp only)

System Information

Command	Action
date	show the current date and time
cal	show this month's calendar
uptime	show current uptime
who	display who is online
whoami	who you are logged in as

Analyzing and Searching Files

Command	Action
wc	counts the number of lines, words, bytes in file
wc -l	counts the number of lines in file
cut -f1 file	cut out selected portions (first head ) of each line of a tab-delimited file
cut -d\| -f1,2,3 file	cut out columns 1,2, and 3 from a pipe-delimited file
sort file	sort lines of text file file
uniq file	report or filter out repeated lines in a file
grep pattern files	search for pattern in files
grep -v pattern files	search for lines that do not contain pattern in files
awk pattern file	manipulate data and generate reports
sed pattern file	text stream editor

Common Shortcuts

Keys	Action
Ctrl+A	go to beginning of current command
Ctrl+E	go to end of current command
Ctrl+C	halts the current command
Ctrl+Z	stops the current command, resume with fg in the foreground or bg in the background
Ctrl+D	log out of current session, similar to exit
Ctrl+W	erases one word in the current line
Ctrl+U	erases the whole line
Ctrl+R	type to bring up a recent command
!!	repeats the last command
exit	log out of current session

File Content Browsing

Command	Action
less file	displays file contents one screen at a time (similar to more but enables mouse scrolling because less is more)
head file	displays the first few lines of a file.
tail file	displays the last few lines of a file.

File Permissions

Command	Action
chmod octal file	change the permissions (in either a ssh or sftp session) of file to octal, which can be found separately for user, group, and world by adding:
4	read (r)
2	write (w)
1	execute (x)