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.txtcore worm seed jewelalpha2: cat apple.txt | wc 2 4 21alpha2:

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.txtcore worm seed jewelalpha2: grep jewel apple.txtseed 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