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WEEK 2

4/5/2005

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TOPICS

• Functions (contd.)• pushd, popd, dirs• Debugging Shell scripts• Scheduling Unix jobs• Job Management

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Sample Files

• Download states.txt and statecap.txt from acelinux.tripod.com

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cut• Used to extract fields from a line• By defaults, fields are delimited by tab• Syntax: cut –d <delimiter> -f <fields>• e.g. cut –d ‘:’ –f1 /etc/passwd displays 1st

column of file /etc/passwd cut –d ‘:’ –f2 /etc/passwd displays 2nd

column of file /etc/passwd

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cut (contd.)more /etc/passwdcut –d’:’ -f1 /etc/passwd > usernamesmore usernames

cut –d’:’ –f1-3 /etc/passwdcut –d’ ‘ –f2 statecap.txtcut –d’ ‘ –f2 statecap.txt > capcities

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Examples of cut

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Exercise• Write a shell script that displays the

current date and time on separate lines`

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dt=`date`day=`echo $dt | cut –d’ ‘ –f1`echo Day is $daymnth=`echo $dt | cut –d’ ‘ –f2`echo Month is $mnthdte=`echo $dt | cut –d’ ‘ –f3`echo date is $dtetme=`echo $dt | cut –d’ ‘ –f4`echo time is $tmeyr=`echo $dt | cut –d’ ‘ –f6’echo year is $yr

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day=`date | cut –d’ ‘ –f1`echo Day is $daymnth=`date | cut –d’ ‘ –f2`echo Month is $mnthdte=`date | cut –d’ ‘ –f3`echo date is $dtetme=`date | cut –d’ ‘ –f4`echo time is $tmeyr=`date | cut –d’ ‘ –f6’echo year is $yr

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paste

• Command to combine files.• By default, adds a space between fields• e.g. paste /etc/passwd /etc/passwd paste –d “X” /etc/passwd /etc/passwd

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paste - examples

more states.txt paste states.txt states.txt > 2states.txtmore 2states.txt

paste states.txt /etc/passwd > junkmore junk

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wc• wc – word count• Displays the number of lines, words and

characters in the file(s)• Syntax: wc filenamee.g wc states.txt

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wc Options

• wc –l states.txtdisplays # of lines

• wc -m /etc/passwd displays # of characters

• wc –w states.txtdisplays # of words

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grep bash /etc/passwd | wc –l # displays # of

users with bash as default shell

grep csh /etc/passwd | wc –l # displays # of

users with csh as default shell

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export• Used to make variables available to all

child shell processes• By default, shell variables are available

only the shell that they are defined in• To make shell variables available in

child shell processes, they have to be “exported”

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mainscriptA in childscript is

echo A in childscript is $A

A=apple # define a variable Achildscript # call another script. This script

# executes in a child shell process

mainscript

childscript

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mainscriptA in childscript is apple

echo A in childscript is $A

export A=apple # define a variable Achildscript # call another script. This script

# executes in a child shell process

mainscript

childscript

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source, . (dot command)

• source, . are used to execute scripts in the same shell

• . (dot) is more portable than source i.e. available on almost all versions of all shells

• Can be used from command line or from inside a script

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. (cmd-line)

• When invoked on command line, executes the script

. myscript Executes myscript in the current shell

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county=fairfaxmyscript

myscript

. myscript or source myscript

echo $county

fairfaxecho $county

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export city=centrevillemyscript2

myscript2

./myscript2

echo $city

echo $city

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echo Setting county variablecounty=fairfax

myscript

myscript

. myscript or source myscript

echo $county

fairfaxecho $countySetting county variable

Setting county variable

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. (within script)

• When used within a script, . executes the called script in the same shell as the main script

• Same effect as if “sourced” script was imported into the main script

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echo Setting county variablecounty=montgomery

myscript

echo executing myscriptmyscriptecho Value of county is $county

mymainscript

mymainscriptexecuting myscriptSetting county variable

Value of county is

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echo Setting county variablecounty=montgomery

myscript

echo executing myscript. myscriptecho Value of county is $county

mymainscript

mymainscriptexecuting myscriptSetting county variable

Value of county is montgomery

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Importing functions

• Functions can be imported into a script using the . command

• Once imported, functions can be used in script as if they were defined in the script.

• Need to import the complete function file• Functions are NOT executed when imported

– only when “called”

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sum_fn(){ let result=$1+$2 return $result}diff_fn(){ let result=$1-$2 return $result}

funfile

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. funfilesum_fn $1 $2echo sum of $1 and $2 is $?diff_fn $1 $2echo difference of $1 and $2 is $?

mathscript

mathscript 10 30sum of 10 and 30 is 40difference of 10 and 30 is -20

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Exercise

• Rewrite the calculator script by writing all the functions in a single file and “importing” that file into the main script

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pushd, popd, dirs• Set of commands to “bookmark” working

directories• Traverse directories in order• First In Last Out• pushd dir-name pushes dir-name to

directory stack and changes current directory to dir-name

• popdremoves top directory in stack and changes current dir to the dir popped from stack

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pushd, popd, dirs

• dirs lists contents of directory stack• pushd, popd commands also list

contents of directory stack• Used in scripts that require changes to

current-working directory without hard-coding directory name

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pushd - examplespushd /tmppwdpushd /usrpwddirspushd /binpwddirs

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popd - examplespwdpopddirspwdpopdpwddirspopdpwd

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pushd, popd in scripts# scriptpushd somedirectory# we are now in somedirectory…popd# back in original directory…

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Shell Debugging

• No debugger nor any debugging commands/constructs

• Cryptic error messages• Use bash options for primitive

debugging

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Bash debugging options

-v echoes each command before executing it (before evaluating)

-n checks for syntax errors without actually executing the script (weak check)

-x print each command before executing (after evaluating)

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Debugging method-1

• Add option to bash definition in 1st line of script

#!/bin/bash –v…

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#!/bin/bash –x

echo Enter passwordread passif [ “$pass” != “magic” ]then echo Wrong passwordelse echo Correct passwordfi

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#!/bin/bash –v

echo Enter passwordread passif [ “$pass” != “magic” ]then echo Wrong passwordelse echo Correct passwordfi

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#!/bin/bash –n

echo Enter passwordread passif[ “$pass” != “magic” ]then echo Wrong passwordelse echo Correct passwordfi

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Debugging Method - 2

• Execute script in cmd-line bash with required option

• Don’t need to hard-code option inside script

$ bash –v myscript

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#!/bin/bashecho Enter passwordread passif [ “$pass” != “magic” ]then echo Wrong passwordelse echo Correct passwordfi

$ bash –v myscript

myscript

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#!/bin/bashecho Enter passwordread passif [ “$pass” != “magic” ]then echo Wrong passwordelse echo Correct passwordfi

$ bash –x myscript

myscript

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#!/bin/bashecho Enter passwordread passif[ “$pass” != “magic” ]then echo Wrong passwordelse echo Correct passwordfi

$ bash –n myscript

myscript

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Downloading bash 3.0

Create a temporary directory /tmp/bash mkdir /tmp/bashGo to: http://ftp.gnu.org/gnu/bash/File: bash-3.0-tar.gzDownload file to /tmp/bash

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Downloading bash-3.0 (contd.)

cd /tmp/bash cd to directorygunzip bash-3.0.tar.gz Unzip filetar –xvf bash-3.0.tar Un-tar the filecd bash-3.0./configure Run configure scriptmake Run make utility to Build bash binary

ls –l bash Verify bash was built successfullycp bash ~ Copy bash binary to home dir

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read (re-visited v3.0+)

• read command can be used to read multiple variables

• -s do not echo input• -n N accept only N chars of input• -p string echo string as prompt before

reading input

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read –s –n 6 –p “Enter password” pass accepts 6 char input, does not echo

keystrokes and prompt is “Enter password”

read –p “Enter 2 numbers” x y

accepts 2 values into x and y. If less values are passed, remaining variables are not set

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#!/home/acestudent7/bash

read –s –n 6 –p “Enter password” pass

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Exercise• Write a script that prompts the user to create

a new 6 character password. • The user must be prompted to reconfirm the

new password. • A success message must be displayed if the

user re-enters the password correctly else a failure message must be displayed.

• The password must not be echoed on the screen.

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Enter 6 char passwordRe-enter passwordPassword entered successfully

Enter 6 char passwordRe-enter passwordPasswords do not match

Sample execution of script

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dt=`date`day=`echo $dt | cut –d’ ‘ –f1`mnth=`echo $dt | cut –d’ ‘ –f2`dte=`echo $dt | cut –d’ ‘ –f3`tme=`echo $dt | cut –d’ ‘ –f4`yr=`echo $dt | cut –d’ ‘ –f6’bkupdir=/tmp/backup_$day_$month_$tme_yrmkdir $bkupdircp –R ~ $bkupdir

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Exercise

• Write a shell script that does the following:

• Creates a directory called backup_<timestamp> in the /tmp/directory

• Copies all files and directories in your home directory to the new directory

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Scheduling Jobs - cron

• cron – utility used to execute commands at specified times/dates

• Output/error of commands is e-mailed to user

• Commands are executed with privileges of user who setup the cron-job

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crontab

• Command to schedule jobs• Each input line corresponds to a job• Each input line must contain 6 space

separated fields• 1-5 date/time• 6 – command to be executed

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crontab options• -l list user’s crontab tasks• -r remove all of user’s crontab tasks• -e edit user’s crontab file

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crontab time/date fields1. Minute (0 – 59)2. Hour (0 – 23)3. Day-of-month (1 – 31)4. Month-of-year (1 – 12)5. Day-of-week (0 – 6) (0=Sunday) * all valid valuesComma-separated values Specify range using hyphen6. Command to be executed

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crontab - examples00 05 * * 1,2,3,4,5 script1 arg1 5am M-

F45 06 * * 5 somescript 6:45am every Fri00 01 1 * * myscript 1am 1st of every

month

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crontab – setting up jobs

crontab* * * * * /home/acestudent7/mybackupscriptCtrl-d

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crontab – setting up jobs (contd.)

• crontab – e allows to setup jobs in a editor

• EDITOR variable must be set to a valid editor

• export EDITOR=gedit

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crontab – viewing jobs

• crontab –l displays current job list

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jobs

• List jobs running in the background

sleep 1000 &jobs

kill %1 stops the first jobkill 1932 stops job with id 1932

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wait• Stops script execution until all jobs

running in background have terminated• Or until job with number specified

terminates

…wait 200 # wait till job with process id 200 terminates

…

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$!

• Process id of last back-ground process

find / -name core –print 2> /tmp/err > out &pid=$!…

wait $pidmail user < out

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Next Week

• Application Development Procedures • Compilers and GNU development tools• Version control demo