CIS 118 – Intro to UNIX

Shells

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Shells

What is a shell?

Bourne shell

– Developed by Steve Bourne at AT&T

Korn shell

– Developed by David Korn at AT&T

C-shell

– Developed by Bill Joy for Berkeley Unix

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How the shell works

Shell displays a prompt

You type in a command

You press the return key

The shell interprets the commands you typed

and tries to find the correct programs to run

The kernel runs the requested programs and

returns the results to the shell

The shell displays the command prompt

again

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Standard Input, Output and Error

Standard input

– stdin

– The place the program

normally looks for input.

– The keyboard.

Standard output

– stdout

– The place where the

program normally sends

its output.

– The screen.

Standard error

– stderr

– Used by programs to

display error messages.

– Also the screen.

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Redirection <, >, >>

<

– Redirects the standard input

[command] < [file name]

– The command will open the file and use its

content as its source of input

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Redirection <, >, >>

>

– Redirects the standard output

[command] > [file name]

– The results of the command will be sent to the

specified file

– Will create or overwrite the destination file

cat june july aug > summer2000

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Redirection <, >, >>

>>

Also redirects the standard output

– [command] >> [file name]

The results of the command will be sent to

the specified file

Will append the results of the command to

the existing file

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Wildcards

Typing in Unix can be tedious

Unix supports three wild-card characters:

– Asterisk (*): matches any string of characters

including blanks

– Question mark (?): matches single characters

– Square brackest ([]): Tells the shell to match any

characters that appear inside the brackets

Quoting special characters

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Grouping commands

Executing one command at at time can be

tedious

Unix allows for grouping of commands by

separating commands with a semi-colon (;)

– pwd; cal 1 2000; date

Though they are all on the same line, this is

still 3 commands

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Pipes & Filters

You can construct powerful Unix command

lines by combining several Unix commands

Unix commands alone are powerful, but

when you combine them together, you can

accomplish complex tasks with ease

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| (pipe)

Similar to redirection and grouping combined

Used to link commands together

– [command] | [command] etc.

The output of the first command is sent as

the input to the second command, and so on,

and so on …

– who | more

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Using a pipe

A pipe sends the standard output of the command to the left of the pipe to the standard input of the command to the right of the pipe – This is similar to the > symbol used to redirect the standard

output of a command to a file

– However, the pipe is different because it is used to pass the output of a command to another command, not a file

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Using a filter

A filter is a Unix command that does some

manipulation of the text of a file

Some simple filters include wc, sort & more

One of the most commonly used filters is

grep

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wc

word count

Used to display a word count of a file

– wc [-c l w] [file name(s)]

The output you will see will be a line showing

the number of lines, words and characters

Limit display with the flags

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sort

Sorts the contents of a file

– sort [-b f n r u] [file name(s)]

Takes the contents of a file and displays it in

sorted order

Flags: – -b: ignores blanks

– -f: folds upper- and lowercase letters together

– -n: numeric sort

– -r: reverse usual order

– -u: prints duplicate entries only once

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Here is an example:

alpha2: cat apple.txt core worm seed jewel alpha2: cat apple.txt | wc 2 4 21 alpha2:

After the first shell prompt, we see the contents of the file apple.txt

In the next shell prompt, the cat command displays the contents of the applex.txt file – The contents are displayed, not to the screen, but through a pipe to the wc

(word count) command

The wc command then does its job and counts the lines, words, and characters of its input

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grep

search for a string in a a file, display the line in

which it appears

alpha2: cat apple.txt

core worm

seed jewel

alpha2: grep jewel apple.txt

seed jewel

alpha2: cat apple.txt | grep jewel

seed jewel

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Job control

Unix works via jobs or processes

Every command or program is a separate

job/process executed by a user

Jobs are usually run in the foreground, but

can be made to run in the background

Jobs can be killed by the user who created

them

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Job control

ctrl-c: cancels a command/job

ctrl-z: suspends a command/job

jobs

– Lists the jobs (programs) that you currently have

running.

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bg

Forces a job to the background

First, type a ctrl-z to suspend the job

Then type bg and the job is forced to the

background

Use the jobs command to see it

You can force a job to the background

immediately with the &

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fg

Brings a job to the foreground

Use the jobs command to see the jobs you

have running

Type fg %[number] and that job will be

brought to the foreground

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kill

Kills a job that you have running

Use the jobs command to see what you have

running

Type kill %[number]

Not the most graceful way out, but it works