A Brief Overview of UnixA Brief Overview of UnixBrandon Bohrer

TopicsTopicsWhat is Unix? – Quick introductionDocumentation – Where to get it, how to use

itText Editors – Know the dominant Unix text

editors, and their relative strengths and weaknesses.

Filesystem – Learn the differences between Unix filesystems and those on other operating systems

Using the Unix shell – Basic commands and shell syntax

What is Unix?What is Unix?Operating system first developed at Bell

Labs in 1969Based on design philosophy of simplicity and

modularityGave birth to a large family of operating

systems.The most common of these (and the one

used at Drexel) is GNU/LinuxFree and open-source

Accessing DocumentationAccessing Documentation man command

◦ Access online documentation for programs, functions, and more.

info command◦ More advanced documentation format, but not as widely

used. apropos command

◦ Searches man pages for a command; helpful when you don’t know the exact name

Course materials – Probably best source for basic commands Software manuals – More detailed and readable than man

pages Books - Unix in a Nutshell available from Drexel Unix standards

◦ POSIX, Single Unix Specification◦ Poor choice for everyday use, but may be helpful for

portability questions.

Text EditorsText EditorsVi (Visual Editor)

◦ Simple◦ Available almost everywhere (part of Single Unix

Specification)◦ Mode-based UI takes some getting used to

Emacs (Editor Macros)◦ Lots of features (even has a vi mode)◦ Highly extensible◦ Uses multi-key keyboard shortcuts instead of modes

Other editors:◦ vim◦ nano◦ Kate◦ And many more

Filesystem OverviewFilesystem OverviewFile paths are delimited with forward

slashes(/)All directories are descendants of a root

directory, called /Parent directory is denoted with two periods

(..)Current directory is denoted with one period

(.)Files are assigned permissions, which

determine how they can be accessed and by whom.

Filesystem - Related Filesystem - Related CommandsCommandsDirectory manipulation

◦ cd change directory◦ ls list files in directory◦ mkdir make directory◦ rmdir remove directory◦ pwd print working directory

File manipulation◦ mv move file◦ cp copy file◦ rm remove file◦ touch make file / update file

timestamp

More common commandsMore common commands Search

◦ grep search text (General Regular Expression Parser)◦ find search for file by name

Display◦ cat concatenate file to output◦ echo output a given string

Misc◦ wc word/line count◦ tail return the end of a file◦ cmp compare files, show first difference◦ diff compare files, show all differences◦ ps show running processes◦ less/more make large outputs easy to read

And many more

Important DirectoriesImportant Directories/ Root directory/home Personal files/usr User software files/bin Executables/sbin Restricted Executables/dev Devices/usr/man Manuals/usr/src Source code/lib Important libraries/tmp Temporary, deleted on boot For full list, see Filesystem Hierarchy Standard:

http://www.pathname.com/fhs/pub/fhs-2.3.html

File permissionsFile permissions Three sets of permissions

◦ Owner: Generally the file’s creator, but can be changed with chown

◦ Group: A group of users with a special set of permissions Defined in /etc/passwd and /etc/group File’s group is changed with chgrp

◦ Other: All other usersThree flags in each set

◦ Read, write, execute◦ Set with chmod

Often encoded with each set represented by an octal digit, and each permission flag stored in a single bit◦ E.g “Read and execute” encoded as 5 ( 2^2 + 2^0)◦ E.g All permissions for all users encoded as 777

Shell Pipes and Shell Pipes and MetacharactersMetacharacters Pipes help programs work together. Use | to send output from one program to input for another.

◦ Example: cat file.txt | grep “hello” prints lines of file.txt containing the string “hello”

Use > to write output to a file◦ Example: ls > file.txt writes a directory listing to file.txt

Use < to send a file to a program’s input◦ Example: cat < file.txt prints file.txt

* expands to all filenames in the current directory◦ Example: rm ./* deletes all files in current directory.◦ Use carefully:

Easy to type rm /* and destroy the filesystem. Use rm –i to avoid accidental deletions.

See course notes or sh manual for more shell syntax.

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