NETW-240 Shells Last Update 2013.04.09 1.4.0 Copyright 2000-2012 Kenneth M. Chipps Ph.D. 1

NETW-240Shells

Last Update 2013.04.091.4.0

Copyright 2000-2012 Kenneth M. Chipps Ph.D. www.chipps.com

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Objectives of This Section

• Learn– What a shell is– How to use a shell


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What is a Shell

• It is a command line interpreter• In other words if you type something on

the command line and press enter, the shell will either do something with that command or tell you it cannot


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Common Linux Shells

• The most common shells on Linux and UNIX systems are the– sh - Bourne Shell– csh - C Shell– ksh - Korn Shell– bash - Bourne Again Shell

• bash is an advanced version of the sh shell first written for Unix systems

• Bash is the one most often used in LinuxCopyright 2000-2012 Kenneth M. Chipps Ph.D.

www.chipps.com4

Common Linux Shells

• To leave a shell– Ctrl D or exit


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bash

• When Linux is started after the load finishes the shell presents a prompt such as


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bash


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bash

• A nice feature of bash is the command history

• It will keep track of a certain number of previously issued commands in the– ~/.bash_history

• file• The number of commands in the history is

likewise customizable


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bash

• When bash is first started, the history list is initialized by the defined history file -~/.bash_history

• The history list is kept in memory until you exit the shell, at which point the list is written to the history file, overwriting its current contents

• Even when you log in initially, you have a history of what you did during your last login session


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bash

• You can access the commands that are stored in the history file by using the up and down arrow keys to move to a previously issued command

• Once you've selected a command, you can edit it by inserting or deleting characters


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bash

• Another method of accessing the command history is to use the history command

• The history command, by default, lists the lines that are contained in the history file

• For example, issuing the command– history 10

• displays the last 10 lines in the history file


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bash

• The command-line options for history are quite simple– -r

• Tells history to read the contents of the history file and use them as the current history list

– -w• Tells history to write the current history list to the

history file and overwrite its current contents


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bash

– -a• Tells history to append the current history list to the

history file

– -n• Causes the lines that are in the history file to be

read into the current history list


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bash

• Pipes are a way to string a number of commands together

• The output from the first command is piped or sent into the second command as input, the output from the second command is piped into the third command as input, and so on

• The output from the last command is printed to the screen or redirected to a file


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bash

• In order to pipe commands, you must use the pipe - vertical bar, which is the | character

• It tells bash where one command ends and the next begins

• There's no limit to the number of pipes that can be used

• bash, like every other shell, supports redirection for commands


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bash

• You can redirect the output from a command to a file or device other than the local console

• Likewise, you can use a file or device to provide input to a command

• To redirect the output from a command to a new file, use the > symbol


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bash

• Another feature of bash is its ability to suspend currently running processes, known as Job Control

• Pressing– CTRL Z

• suspends the currently running process


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bash

• Then these two commands move the process to the background or back to the foreground– bg– fg

• To start a command in the background use the &, as in– commandname &


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bash

• Like everything in Linux bash uses several startup files

• One of these is a common file• The other three are user specific• The common file is

– /etc/profile


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bash

• The user specific files are– ~/.bash.profile

• Contains information about the user’s shell environment

– ~/.bash.login• Executed at each login to the shell

– ~/.profile


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bash

– ~/bashrc• A configuration file that is executed each time the

bash shell is entered• And also each time a subshell is generated

– ~/bash_logout• Executed on user logout of the shell


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bash

• .bash_profile is executed at user login• To reexecute it after a change to the file

precede it with– . – a period

• As in– . .bash_profile

• or– source .bash_profile


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bash

• The ~ means from the user’s home directory

• bash does it this make just to make it harder on you as an administrator or user

• /etc/profile– This is the main startup file for bash– It is used for the commands that all users

need


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Common Commands

• pwd– Shows the current directory

• cd– Change directories

• cp– Copy

• mv– Rename


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Common Commands

• rm– Remove

• mkmir– Make a directory

• rmdir– Delete empty directories


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Common Commands

• ls– Show a list of whatever is in the directory– Switches

• -a– Show everything

• -F– List by type

• -l– Long listing

• -t– List by date


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Common Commands

• To show one screen full of filenames at a time– ls –l | less


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Common Commands

• ln– Create a link

• file– To find out what type of file the file is

• cat– Display what is inside a file


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Common Commands

• head– Show the first 10 lines of a file– As in

• head filename

– Or specify the number of lines with• head –n20 filename


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Common Commands

• tail– Show the last 10 lines of a file– As in

• tail filename

– Or specify the number of lines with• tail –n20 filename


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Common Commands

• more– Shows a file one page at a time

• less– Shows a file one page at a time– But allows scrolling through the file– q to quit it


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Common Commands

• grep– To search through a file– For example

• grep whattosearchfor filetolookin• or• grep ‘word word’ filetolookin

• find– Looks through directories for a file


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Getting Help

• locate– The same as find, but faster

• help– Basic information– Such as

• cat --help


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Getting Help

• man– More information– As in

• man apache

– Switches• -k

– Shows commands


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Getting Help

• info– Provides more structure to the information

than plain man does– But does not cover all man pages


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Documents

NETW-240 Shells Last Update 2013.04.09 1.4.0 Copyright 2000-2012 Kenneth M. Chipps Ph.D. 1