The Unix Shell

OperatingSystem

shell The shell is a command interpreterIt forms the interface between auser and the operating system

When you log in to a Unix system,a shell starts running. You interactwith the shell

this is the shell prompt

the SHELL environment variable tellswhich shell is being used

this is where the shellprogram is located/bin/bash

Unix Shells

Shell name Program (Command) namerc rcBourne Shell shC Shell cshBourne Again Shell bashZ shell zshKorn Shell kshTC tcsh

Unix Shells

Shell name descriptionrc a very simple shell - similar to shBourne Shell developed at Bell labs - popularC Shell bsd unix - based on C languageBourne Again Shell open source version of sh - linux/osX Z shell extended sh - very large shellKorn Shell like sh but extensive scripting languageTC c shell with emacs-like command line

you can change shells by typing the shell command

return to the default shellby typing “exit”

The shell command line

prompt showscurrent directory.~ is your home directory

commandlist of arguments

command options (usually preceded with a hyphen)

What happens when you type a command?

The shell parses the command lineIt looks for a program that matches the commandIt starts a new process and runs that programWhile the program executes, the shell sleepsWhen the program is done, the shell wakes upIt displays its promptIt waits for the user to type another command

Commandstdin stdout

Commands generally get their input fromstdin and send their output to stdout

if you run the who command, the system tells you who is logged in and at what terminal.

but . . . this is really a file in the Unix file system thatrepresents a real device, in this case a terminal

commands read from and write to this file!

the cat command is a good example. It takes its input from a fileand outputs to stdout.

if you type the command with no parameters, it takes its inputfrom stdin. It will do this until you type ctrl-D (end of file).

Redirection

You can cause the shell to get its input from some placeother than stdin or send its output to some place other than stdout by using redirection.

Redirecting standard output

command [arguments] > filename

redirect output to newduh.txt

Concatenating Files with cat command

Redirecting standard input

command [arguments] < filename

cat takes its input from the file suppliesand writes to standard output

Appending standard ouputto a file

command [arguments] >> filename

Pipes

The shell uses a pipe to connect the output of onecommand to the input of another command.

Using a pipe

command_a [arguments] | command_b [arguments]

The tr command translates each character in itsinput stream to the corresponding character in itsoutput stream.

Running a command in the background

command_a [arguments] &

the & tells the shell to run the command inthe background. This means that the shellprompt will appear immediately and you cantype in new commands without waiting for the background command to finish.

Some useful Unix Commands

cal

cal month yearcal yearcal

displays a monthly calendar

cat

cat [options] file-list

concatenates files end to end

-e marks end of each line with a $-n displays line numbers

cd

cd [directory]

change to the specified directory

cd with no arguments changes to your home directory

chmod

chmod [options] mode file-list

symbolic u user + add permission g group - remove permission o other a all

changes permissions

chmod

chmod [options] mode file-list

absolute xxx - a binary encoded value

777 - everyone can read, write, execute 755 - owner can read, write, execute, others can read, execute

changes permissions

cp

cp [options] source-file destination-file

-i interactive, prompt if this will overwrite an existing file-r recursive, for directories, copies everything

copies files

diff

diff [options] file-1 file-2

compares files

find

find directory-list criteria

recursively searches in a given directory

-name file-name-type file-type-user user-name

finger

finger [options] user-list

displays information for logged in users

-l detailed listing-s brief listing

grep

grep [options] pattern [file-list]

searches files for a given pattern

-c display the number of lines that match-i ignore case-l display the filenames where a match is found-n displays each line with its line number in the file

grep uses regular expressions in its pattern matching

Consider the file testregex that contains the lines

ringringingbringingtalktalkingwalking

Simple strings

Consider the file testregex that contains the lines

ringringingbringingtalktalkingwalking

> grep ring testregexringringingbringing

Period - represents any character

Consider the file testregex that contains the lines

ringringingbringingtalktalkingwalking

> grep .ing testregexringringingbringingtalkingwalking

[ ] - represents a set of characters

Consider the file testregex that contains the lines

ringringingbringingtalktalkingwalking

> grep [tw] testregextalktalkingwalking

^ - matches a string at the beginning of a line

Consider the file testregex that contains the lines

ringringingbringingtalktalkingwalking

> grep ^ri testregexringringing

$ - matches a string at the end of a line

Consider the file testregex that contains the lines

ringringingbringingtalktalkingwalking

> grep ing$ testregexringringingbringingwalkingtalking

head

head [number] file-list

displays the first number lines of a file

kill

kill [signal-number] PID-list

kills a process

ln

ln [option] existing-file link-name

create a link to a file

by default ln creates a hard link. Hard links must bein the same directory as the file being linked to.

the -s option creates a symbolic link. Symbolic links canbe across file systems and directories.

ls

ls [options] file-list

list information about one or more files

-a list all entries, including invisible files-l show all file information-r shows all subdirectories

t r w x r w x r w x links owner group size date_last_modified filename

type of filed directory- regular fileb block devicec character devicel symbolic linkp pipes socket

file

ow

ner

gro

up

oth

er

mkdir

mkdir [option] directory-list

make a new directory

-p if the parent directory does not already exist, the create it.

more

more [options] [file-list]

display a file, one screenful at a time

-n number output lines

mv

mv [options] existing-name new-name

move (rename) a file

-f moves regardless of file permissions-i prompts if move would overwrite an existing file

ps

ps [options]

displays the status of existing processes

-a display all processes associated with a terminal-e displays some environment information-l long display-x displays daemon processes

rm

rm [options] file-list

remove a file

-f removes files for which you do not have write permission-i prompts before removing each file-r recursive

rmdir

rmdir directory-list

remove an empty directory

tail

tail [number] [file]

display the last number lines of a file

who

who

display logged in users