W402 - Writing UNIX Scripts: The Basics...Writing shell scripts can be an important part of UNIX...

Summit 2007: Get Connected

W402 - Writing UNIX Scripts: The Basics

Al FreemanTechnical Services

Writing shell scripts can be an important part of UNIX system administration, relieving administrators and users from manually performing repetitive tasks.This session will cover the basics of writing shell scripts as well as some basic UNIX commands with parallels between AIX, Linux and SCO UNIX.

Pre-requisites: None

Wednesday, 1:15 p.m.

© 2007 Activant Solutions Inc. All Rights Reserved

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Session Objectives

At the end of this course, you will be able toUse basic UNIX commands Combine commands using pipesWrite simple shell scripts to automate proceduresKnow where to find more information on commands and their options and arguments



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Return on Investment Opportunity

Avoid unplanned downtimeWork more efficiently, giving more time to

Assist sales and management by implementing high-level tools which turn data into useable information, such as graphs and reports highlighting opportunities and potential problems Provide longer hours of system availability and increase remote accessibility


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Case Study

Activant SDI customer, APCO, Inc., wrote a shell script to import price updates into their SDI system saving them hours of manual labor each month They saved additionally by not having a custom program written to do these updates



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Basic UNIX Commands

Most UNIX commands can be broken down into groups based on their functionThis class covers commands for

File / Directory administrationPrinter administrationProcess administration


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File / Directory Administration

UNIX has a hierarchical, single-tree file structureFiles / directories can be

Created (touch, mkdir)Copied (cp)Moved (mv)Deleted (rm, rmdir)



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File / Directory Creation Commands

Files can be created by programs or by redirection

Example: To create a file called Summit under the /usr/p21 directory

touch /usr/p21/Summit >/usr/p21/Summit

Directories can be created with the “mkdir” command

Example: Create a directory under /usr called p21mkdir /usr/p21


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Copying Files / Directories

Use the “cp” command to copy filesExample: To copy /usr/bin/ls to /tmp/lsuse the command:

cp /usr/bin/ls /tmp/ls

To copy directories use “cp –r”Example: To copy the directory /etc/rc2.dand everything underneath it to /tmp/rc2.duse the command:

cp -r /etc/rc2.d /tmp



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Moving Files

Moving a file is the same as renaming itUse the “mv” command to move files to another directory or to rename them

Example: rename /usr/adm/syslog as /tmp/syslog.oldmv /usr/adm/syslog /tmp/syslog.old


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Moving Directories

Can change the directory’s name, its location or bothUse the “mv” or “mv -r” command to move directories

Example: Renaming a directorymv /usr/p21 /usr/activant

Example: Changing a directory’s locationmv /yardley/headoffice /livermore/headoffice

Example: Changing a directory’s name and locationmv /yardley/p21 /livermore/activant



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Deleting Files

Use the “rm” (remove) command to delete filesThe “rm -i” command will ask before deleting filesA successful remove will have no outputExample:

rm myfile


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Deleting Directories

Use the “rmdir” or “rm -r” to delete directoriesThe rmdir command will only delete empty directoriesThe “rm -r” command can be very dangerous and should be avoided if possibleExample:

rmdir /users/myemptydirectory



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Printer Administration

Printer commands belong to one of the following categories

Printer status commandsPrint job creation commandsPrint job deletion commandsPrinter control commands


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Printer Status Commands

Use the “lpstat” command to check the status of printers and print jobsOptions to the “lpstat” command

“lpstat -t” gives a complete status of the printer subsystem“lpstat -o” gives a listing of outstanding print jobs(non-AIX)“lpstat -v” gives a listing of printer devices (non-AIX)



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Print Job Creation Commands

Use the “lp” command to create print jobs“lp -dqueuename” creates a print job for a specific print queueUse the “qprt” command to create a print job with printer control options (AIX only)


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Print Job Deletion Commands

Use the “cancel” command to delete print jobsExamples

cancel job# (AIX)cancel queuename-job# (UNIX/Linux)



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Printer Control Commands

Printer can be either UP (enabled) or DOWN (disabled)Enable / disable commands

To enable a printer, use the “enable printqueuename” commandTo disable a printer, use the “disable printqueuename” command


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Printer Control Commands

Non-AIX systems also allow a print queue to accept or reject print job requestsAccept / Reject commands

Use the “accept printqueuename” command to set a print queue to receive print jobsUse the “reject printqueuename” command to set a print queue so it will not accept print jobs

Print jobs sent to a rejecting print queue will failThese errors can cause application programs to end abnormally



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Process Administration Commands

Process Administration commands can be divided into:

Process Status commandsProcess Priority Control commandsProcess Termination commands


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Process Status commandsUse the “ps” command to view status of running processes“ps -e” to view status of all running processes on the system“ps -uusername” to view status of processes for the user username“fuser -u filename” to show the process IDs of all processes using filename



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Priority Control commands“nice” and “renice”

Priority ranges are from -20 (low) to +20 (high)Only “root” user can increase the priority of a processExamples:

nice -10 myprogram (lower priority)nice --10 myprogram (higher priority)


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Priority Control Commands

Use the “renice” command to change the priority of a process while the command is running

Examplerenice --20 processID# (runs at highest priority)renice -20 processID#(runs at lowest priority)



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Process Termination Commands

To terminate a process, you need to know its “process id #”Use the “kill -signal# processid#” to terminate a processCommon signals are

15 SIGTERM (default) terminate1 SIGHUP (the terminal was turned off)9 SIGKILL (terminate immediately without cleanup)

Examplekill -1 123456


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Combining Commands with Pipes

The vertical bar or “pipe” character can be used to combine commands to produce different outputCommand lines containing pipes are often referred to as “pipelines”Data in a pipeline flows from left to right

In the line “command1 | command2”the output of command1 becomes the input of command2



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Frequently used pipe receivers“grep object” displays only lines containing “object”

ps -e|grep p21 displays lines from “ps -e” containing “p21”

“grep -v object” displays all lines except those containing “object”

ps -e|grep -v p21displays lines from “ps -e” not containing “p21”


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Frequently used pipe receivers“pg” displays output on terminals one page at a time

ps -e|pgdisplays output of “ps -e” on a terminal, one page at a time

“lp” prints output on a printerps -e|lpprints the output of the “ps -e” command on the default printer

“cut” narrows output by column or delimited fieldps -e|cut -c1-6 # displays only the first 6 charactersps -e|cut -f2 -d" " # displays the second space delimited field



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Pipelines can be very long and are limited only by the length of a UNIX command line (normally 255 characters)Pipelines can combine numerous commands to narrow data to exactly what's needed, for example:

ps -e|grep p21|grep -v ralph|grep -v dave|pgdisplays the output of “ps -e” that contains “p21” and doesn’t contain “ralph” or “dave”, one page at a time


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Shell Structures

VariablesPositional parametersCommand substitutionConditional execution



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Shell Structures

VariablesVariables are used to store information during the execution of a shell scriptVariable names should be unique within the first eight charactersBraces { } can be used to delineate the start and end of variable names for clarity

Example:today=${month}${day}${year}


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Shell Structures

Positional parametersAre special variables created by the shellEvery word on the command line is assigned a number, the first word is 0, the second is 1, etcUse $# to get the value of the parameter

$0 is the first word (the command), $1 is the first argument to the command, etc

Use $* to get the values of all parametersThe "shift" command will move all positional parameters one number lower, discarding the first



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Shell Structures

Command substitutionCommands put in backwards quotes (`) are executed and their output is substituted in place of the commandThis is often used to get information into variables in shell scripts

Example:today=`date +%m%d%y`puts the current date in mmddyy format into the variable called “today”


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Shell Structures

Conditional executiontestifcase

Looping structuresforwhile



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Conditional Execution

The “test” command can be used to determine if a condition is true or not“test” has two formsExamples of two equivalent “test” statements:

test "$color" = "white"[ "$color" = "white" ]

“test” is often used with “if” and “while” statements to control execution


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The “if” statement is used to control execution of statements when some condition is metAt the end of an “if” statement is a “fi”“else” and “elif” are allowed to control what happens when the condition for the if is not true



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The “if” Statement Structure

if [ "$color" = "white" ]thenwater=hot

elsewater=cold

fi


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The “case” statement is used when execution depends upon multiple values of a single variableAt the end of a “case” statement is an “esac”Value possibilities should be listed from most specific to least specific



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A “case” Statement Example

case $dayofweek in"Mon") echo "Today is Monday";;"Tue") echo "Today is Tuesday";;"Wed") echo "Today is Wednesday";;"Thu" | "Fri" ) echo "Almost the Weekend";;* ) echo "The weekend is here!";;

esac


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Looping Structures

The “for” statement can be used to loop with automatic parameter substitutionThe structure of a for loop isfor variable in list of valuesdo

Statements to be executeddone



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A “for” Statement Example

i=1for file in `ls /p21`

doecho “File number ${i} is ${file}"i=`expr $i + 1` # increments i

done


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Looping Structures

The “while” statement is used to loop while a condition is trueThe structure for a while loop iswhile conditional statement

doStatements to be executed

done



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A “while” Statement Example

# This example will “sleep” until 11:00 PM

now=`date +%H%M` #Gets current timewhile [ "$now" –lt "2300" ]

dosleep 60now=`date +%H%M` #updates now

done


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Shell Script Examples

Stopping and restarting the print spoolerstopsrc –sqdaemonsleep 120startsrc –sqdaemonsleep 300if [ ! "`ps –e|grep qdaemon`" ]; then

echo "qdaemon did not start"|mail rootfi



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Notify administrator of permanent hardware errors

errpt –a –dH –TPERM | mail root


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E-mail administrator about more than three failed login attempts

lsuser -a unsuccessful_login_count ALL|awk -F= '{if ($2 > 3) print}' -|mail root



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Additional Resources

Exploring the UNIX System by Stephen G. Kochan and Patrick H. WoodEssential System Administration (Third Edition)by Æleen FrischUNIX “man” command, to get more information about any UNIX command, type “man command” at the $ or # prompt


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Get to a $ prompt and experiment with non-destructive commands (no rm or mv commands)When writing shells, test each command individually before adding it to the larger shell scriptTry writing at least one shell script to do something that you frequently are required to do

Suggested Action Plan



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Summary

UNIX commands can be used to administer files, processes and printersCommand output can be “customized” with pipesShell scripts can be used to automate procedures


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Thank You for Attending

W402 – Writing Unix Scripts: The BasicsAl FreemanPlease submit the Session Feedback Form

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