03 bash intro

8/8/2019 03 Bash Intro

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Advanced UNIX (BASH)

Objectivesexplain how to write Bourne and BashShell scripts


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Overview

1. Making a File Executable2 . Combining Commands3 . Redirecting Output4 . Executing Scripts5 . Variables

6 . Control Flow

continued


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7 . Functions8 . Other Commands9 . H ere Documents10. Debugging11. More Information


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1.Making a File Executable

$ cat whosondateecho Users currently logged on

who

Wrong:$ whoson

whoson: Permission denied


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Right:$ ls -lg whoson

-rw-r--r-- 1 ad pubs 42 Jun 1 7 10 : 55 whoson$ chmod u+x whoson$ ls -lg whoson

-rwxr--r-- 1 ad pubs 42 Jun 1 7 10 : 55 whoson

$ whosonTue Nov 7 1 3: 21 : 34 ICT 2000Users currently logged inad consol Nov 7 08: 26jenny tty 0 2 Nov 7 10 :0 4


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Possible

PATH

P

r o

blem$ whosonwhoson: Command not found

Due to PATH shell variable (see later)

Quick fixes:

$ ./whosonor $ sh whoson


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2.C

ombining C

ommand

s

Sequencing:$ a ; b ; c

same as:$ a$ b$ c


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P r o cess Creatio n (&)$ a & b & c1 4271 (PID for a)1 4272 (PID for b)

$ a & b & c &1 429 01 429 11 4292$

$ a | b | c &1 43 0 2 (PID for piped commands)


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Pr o

cess

es

in Actio

n$ cat aecho -n aaaaaaaaaaaaaaaaaaaaaecho -n aaaaaaaaaaaaaaaaaaaaasleep 2echo -n aaaaaaaaaaaaaaaaaaaaaecho -n aaaaaaaaaaaaaaaaaaaaa

Similarly for b and c

Try the following a few times:$ a & b & c &

On the lab

machines thereisn't muchvariation unlessthe machine isloaded.


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3 . Redirecting Output

1> redirect standard output (stdout)2> redirect standard error (stderr)

$ cat a b 1 > out 2> err

cat a b

out

err

stdout

stderr

Files


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>&redirect one stream into another:

2>&1 redirect stderr intostdout

$ cat a b 1 > theLot 2>&1

cat a b

theLot

stderr

stdout


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4.Executing Script

s

Make sure that a script is executed by theBourne Shell:

$ sh whoson (no need for chmod )

or:$ cat boss#!/bin/shecho Definitely Bourne Shell Script

continued


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On Linux machines (e .g . calvin ), the Bourneshell has been replaced by Bash

sh means the Bash shell


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5.Variable

s

5 .1. User-defined Variables

5 .2 . Environment Variables

5 .3 . Readonly Shell Variables


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5.1

.U

ser-defined Variable

s

$ person=alex$ echo personperson$ echo $personalex

$var returns the value stored in var

called substitution

No spacesaround the =


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5 .1 .1 . Switch o ff Subs titutio n

Swich off substitution with 'var ' or \var

$ echo '$person'$person$ echo \$person$person


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5 .1 .2 . Switch o ff Special Char s (")"" switches off the special meaning of charactersused in filename generation (e .g. *, ?)

$ ls // directory contentsad.reportad.summary$ memo=ad*$ echo "$memo"ad*$ echo $memoad.report ad.summary

* means only *

* means any number of characters


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5 .1 .3 . Expo rting Variable s

Normally a variable is local to the runningscript (the process) .

It is sometimes useful if running scripts(processes) can access another scriptsvariables .

e .g.extest subtest

cheese=english

callswe want touse cheesein subtest


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No Expo rting: extest 1 &subtest

$ cat extest 1cheese=englishecho "extest 1 1 : $cheese"

subtestecho "extest 1 2: $cheese"

$cat subtestecho "subtest 1 : $cheese"cheese=swissecho "subtest 2: $cheese"

continued

Using " isa good habit,see later


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$ extest 1extest 1 1 : englishsubtest 1 :

subtest 2: swissextest 1 2: english

subtest does notsee extest 1 'scheese value

extest 1 is not affected

by subtest 's setting of cheese


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5 .1 .4 . Reading$ cat readlnecho -n Type: read ln

echo You entered: $ln

$ readlnType: The Wizard of Oz You entered: The Wizard of Oz

read inputs everythingup to the newline


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No Quo tes

$ cat readlnnqecho -n Type: read ln

echo You entered: $ln

$ lsad.report summary 1$ readlnnq

Type:*

You entered: ad.report summary 1

directorycontents


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5 .1 .5 . Executing Co mmand s

$ cat proc_cmdecho -n Enter a command: read command

$c ommand echo Thanks

$ proc_cmdEnter a command: echo Display this

Display thisThanks

A very simple shell


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5 .1 .6 . Splitting Input$ cat split 3echo -n Enter something: read word 1 word 2 word 3echo Word 1 is: $word 1 echo Word 2 is: $word 2echo Word 3 is: $word 3

$ split 3Enter something: this is something

Word 1 is: thisWord 2 is: isWord 3 is: something

Text is split based on whitespace.


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$ split 3Enter something: this is something else, x Word 1 is: thisWord 2 is: isWord 3 is: something else, x

The last variable getseverything that is leftin the input.


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5 .1 .7 . Co mmandSubs itutio n

$ cat mydirthis_dir=pwdecho Using the $this_dir directory.

this_date=$(date)echo "Today's date: $this_date"

$ mydirUsing /home/ad/teach/adv-unix/bourne directoryToday's date: Tue Nov 7 1 3: 52 : 46 ICT 2000$

A Bashaddition

Must use


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5.2

.Envir

onment Variable

s

Most environment variables get their values fromthe shell at login .

The values of some of the variables can be setby editing the .profile file in your home directory

Bash uses .bash_profile and .bashrc


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5 .2 .1 . Examples

HOME pathname of your home directory

$ pwd/home/ad/planning$ echo $HOME/home/ad$ cd$ pwd/home/ad

continued

cd uses HOME

to return to your home directory


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PATH

directories where executable can be foundrepresented as a string of pathnames separated by:s

$ echo $PATH/usr/local/bin:/usr/bin:/bin:$ PATH=SPATH":/home/ad/bin:."$ echo $PATH/usr/local/bin:/usr/bin:/bin:/home/ad/bin:.

Extend thedefault PATH


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No te f o r Sys Admins

If you are the system administrator (superuser, root) for your machine,

do not extend your path with "."it opens you to potential attack byhackerse .g . 'fake' U NIX utilities placed inthe current directory


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5 .2 .2 . Typical .pr o file$ cat .profileTERM=vt 100PATH=$PATH":/home/ad/bin:."PS 1 =ad: CDPATH=:$HOMEexport TERM PATH PS 1 CDPATH

stty kill ^u

$ . .profile export neededin the Bourneshell


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Typical .bas hrcPS 1 ="\u@\h$ "# PS 1 ="\w[\#]$ "PATH=$PATH":."

alias ls='/bin/ls -F'alias dir='ls -ba'alias cls="clear"

::

psgrep(){

ps aux | grep $1 | grep -v grep}

These featureswill be explained

later.

No export needed


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5 .2 .4 . s etThe current settings for the environmentvariables can be listed with set :

$ set | moreBASH=/bin/bash

:PATH=/usr/local/bin:/usr/bin:/bin:.

:

PS1

='\u@\h$ ':


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5.3

. R

eado

nly Shell Variables

These are environmentvariables that ca nnot have their values changed .

Most of them relate to the

arguments supplied to a scriptwhen it is called .


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5.3

.1

.Script Name ($0 )

$ cat abcecho The name of this script is $0

$ abcThe name of this script is ab c


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5 .3 .2 . Script Argument s ($1, $2,..., $9)

$ cat display_ 5argsecho The first five command lineecho arguments are $1 $2 $3 $4 $5

$ display_ 5args jenny alex helenThe first five command linearguments are j enny alex helen

If the variable has no value,then nothing is printed.


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5 .3 .3 . All Arguments ($*)

$ cat display_allecho $*

$ display_all a b c de fg hi jk mno pqrstu w x y z

a b c de fg hi jk mno pqr stu w x y z

$@ is like $* but puts ... around each printedargument


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5 .3 .4 . Number o f Arguments

($#)$ cat num_argsecho This script has $# arguments.

num_args helen alex jennyThis script has 3 arguments


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5 .3 .5 . The shift Co mmnd

shift moves argument values onthe command line one $to the left .

Overcomes limit of 9 argument

variables($1 , $2, ... , $9 )

continued


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$ cat demo_shift

echo arg 1 = $ 1 arg 2= $ 2 arg 3= $ 3shiftecho arg 1 = $ 1 arg 2= $ 2 arg 3= $ 3shiftecho arg 1 = $ 1 arg 2= $ 2 arg 3= $ 3shift

$ demo_shift alice helen jenny junearg 1 = alice arg 2= helen arg 3= jennyarg 1 = helen arg 2= jenny arg 3= junearg 1 = jenny arg 2= june arg 3=

jenny "moves"to the left.


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5 .3 .6 . The set Co mmand

(Again)set cmd (mustuse )

evaluates cmd and assigns itsvalues to the script command linearguments ($ 1 , $2, ... , $9, $*)the values in cmd output areseparated by whitespace

continued


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$ dateFri Jun 1 7 23 : 0 4: 0 9 GMT+7 1 996$ cat datesetset dateecho $*

echoecho Argument 1 : $ 1 echo Argument 2: $ 2echo Argument 3: $ 3echo $ 2 $3, $6

continued

The date valuesare assigned to $ 1 ,$2, etc.


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$ datasetFri Jun 1 7 23 : 0 4: 1 3 GMT+7 1996

Argument1

: FriArgument 2: JunArgument 3: 1 7Jun 1 7 , 1 996


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6 . Co mmand Flo w

6 .1. Branching6 .2 . Test Forms

6 .3 . Looping6 .4 . break , continue , :6 .5 . trap


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6 .1 . Branching

$ cat same_wordecho -n word 1 : read word 1

echo -n word 2: read word 2if test " $ word 1" = " $ word 2"then

e c ho Mat c hfiecho End of Program

Use " to stopfilename

expansion

continued


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$ same_wordword 1 : peachword 2: peach

MatchEnd of Program


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$ cat chkargsif [ $# = 0 ]then

e c ho Usage: c hkargs argument... 1>&2exit 1

fiecho Program runningexit 0

$ chkargs

Usage: chkargs argument...$ chkargs abcProgram running

R edirectstdout to stderr

6 .1 .1 . Seco nd Fo rmat


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6 .1 .2 . if-then-els e$ cat showif [ $# = 0 ]; then

echo Usage: show [-v] filenames 1 >&2exit 1

fiif [ $ 1 != -v ]then

cat $@else

shiftmore $@

fi

C programmers prefer this format

Use:$ show -v f 1 .txt f 2.txt


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6 .1 .3 . if-then-elif

$ cat same 3echo -n word 1 : read word 1echo -n word 2:

read word 2echo -n word 3: read word 3if [ $word 1 = $word 2 - a \

$word 2 = $word 3 ]then

echo Match: words 1 , 2, and 3

continued

For multiway branches

-a means "and"

\ means "continuedon next line"


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elif [ $word 1 = $word 2 ]then

echo Match: words 1 and 2elif [ $word 1 = $word 3 ]then

echo Match: words 1 and 3

elif [ $word 2 = $word 3 ]thenecho Match: words 2 and 3

elseecho No match

fi


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6 .1 .4 . cas e$cat command_menu#!/bin/sh# menu interface to simple commands

echo \n COMMAND MENU\necho a. Current date and timeecho b. Users currently logged inecho c. Name of working directoryecho d. Contents of working directoryecho -n Enter a, b, c, or d: read answerecho

continued

Better style: specifythe shell, and commentthe code.


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c ase $answer ina) date

;;b) who

;;c) pwd

;;d) ls -C;;

*) echo $answer not legal;;

esa c

echo

* is the default;always include itat the end


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$ command_menuCOMMAND MENU

a. Current date and timeb. Users currently logged inc. Name of working directoryd. Contents of working directory

Enter a, b, c, or d: a

Fri Jun 1 7 1 4: 11 : 57 GMT 1 996


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O ther ca s e P attern s? matches a single character

[... ] any character in the brackets .Use - to specify a range (e .g . a-z )

| or (e .g . a|A )

* it can be used to match "anynumber of characters "


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$cat timeDay

#!/bin/shecho Is it morning? Answer yes or noread timeofday

case "$timeofday" in"yes" | "y" | "Yes" | "YES" )

echo "Good Morning";;

[nN]* )echo "Good Afternoon";;

*) echo "Uhh??";;esac


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6 .2 . tes t Fo rms

Format:test expr

[ expr ]e .g .

test $word 1 = $word 2[ $ 1 != -v ]

[ $word 2 = $word 3 ][ $word 1 = $word 2 -a \

$word 2 = $word 3]

Used in the conditions of if's and loops.


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6 .2 .1 . String

Express io nsstring 1 = string 2

string 1 != string 2

-n string (true if string is not )

-z string (true if string is )


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6 .2 .2 . Numerical

Express io nsnumber 1 -eq number 2 (equality)number 1 -ne number 2

(inequality)

number 1 -lt number 2 ( =)


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6 .2 .4 . File Tes tExpress io ns

-f file (file exists)-d file (file exists but is a directory)

-r file (file is readable)-w file (file is writable)-x file (file is executable)


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6 .2 .5 . Co mbiningExpress io ns

! expr (not expr)

expr 1 -a expr 2 (and)Bash allows && as well

expr 1 -o expr 2 (or)Bash allows || as well

( expr )


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$ cat fruitfor fruit in apples oranges pearsdo

echo $fruitdone

echo Task completed

$ fruitapplesorangespearsTask completed


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Loo king f o r f o r in files

First version of script:

$ cat file-fors 1for f in lsdo

echo $fdone


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$ cat file-fors 2for f in lsdo

grep -i for $f > /dev/nullif [ $? == 0 ]then

echo $ffi

done

Ignore output


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6 .3 .1 . f o r $ cat whos# Give user details from /etc/passwdif [ $# = 0 ]then

echo Usage: whos id... 1 >&2exit 1

fi

for i # read as for i in $@do

awk -F: {print $ 1 , $5 } /etc/passwd |grep -i $i

done awk variableswith


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6 .3 .2 . while$ cat countnumber= 0while [ $number -lt 10 ]do

echo -n "$number"number=expr $number + 1 doneecho

$ count01 23456 789$


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Watching f o r Mary to lo g in

while sleep 60do

who | grep mary

done

Disadvantages:if Mary is already logged in, then

we mustwait 6 0 secs to find outwe keep being told that Mary islogged in


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6 .3 .4 . ExtendingWatching

Generalise so can watch for anyone:

$ watchfor ad

Watch for everyone logging in/out:$ watchwho


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watchf o r #!/bin/sh# watchfor# watch for person supplied as argument

if [ $# = 0 ]

thenecho Usage: watchfor personexit 1

fiuntil who | grep $ 1 do

sleep 60done


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watchwhoOnce a minute, run who and compare itsoutput to that from a minute ago . Report anydifferences .

Keep the who output in files in /tmp

G ive the files unique names by adding theshell variable $$ .$$ is the PID of the users shell


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#!/bin/sh# watchwho: watch who logs in and out

new=/tmp/wwho 1 .$$old=/tmp/wwho 2.$$> $old # create an empty file

while truedo

who > $newdiff $old $newmv $new $old

sleep 60done | awk />/ {$ 1 = in: ; print}/


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Us e$ watchwhoin: root tty 1 Nov 6 09: 32in: ad pts/ 3 Nov 8 0 8: 49 (myrrh.coe.psu.ac.th)in: s 4010 441 pts/ 5 Nov 8 10: 11 ( 1 92 . 1 68 . 0 . 1 34 )in: ad pts/ 4 Nov 8 10 : 1 2 (myrrh.coe.psu.ac.th)in: s 40101 43 pts/ 1 7 Nov 7 23: 57 ( 1 92 . 1 68 . 0 . 20 4)out: ad pts/ 4 Nov 8 10 : 1 2 (myrrh.coe.psu.ac.th)in: ad pts/ 4 Nov 8 10 : 1 6 (myrrh.coe.psu.ac.th)out: ad pts/ 4 Nov 8 10 : 1 8 (myrrh.coe.psu.ac.th)

:

initialusers


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No tesdiff uses to distinguish data from$old and $new

$ diff old new< ad> mary

john

ad

old john

mary

new

continued


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The output from the while loop is piped intoawk

only one call to awk is requiredthe "pipe " programming style

A calvin problem:

awk had to be called with the -W interactiveoption so that its output was not bufferedotherwise nothing would appear

whileloop

awk


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6 .3 .5 . Checking mail

H ave a script watch your mailboxperiodically, and report whenever

the mailbox changes by printingYou have mail .

Usage:$ checkmail # checks every 6 0 secs$ checkmail 1 20 # checksevery 1 20 secs


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checkmail#!/bin/sh# checkmail: watch mailbox for growth

time=${ 1 - 60 }

oldls="ls -l $MAIL"while truedo

newls="ls -l $MAIL"echo $oldls $newlsoldls="$newls"sleep $time

done | awk $ 5 < $ 1 4 {print You have mail}

uses the pipe technique


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No tes

$MAIL is a builtin shell variable, withthe value /var/spool/mail/$USER

t=${ 1 -60 } sets t to $1 or, if noargument is provided, to 60

General form: ${var-thing}

returns value of var if defined;otherwise thing

continued


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Use awk to print a message only when themailbox gets bigger:

awk $ 5 $ 1 4 compares the size fields of the two ls -lcalls output at the end of each iteration

e .g. $ ls -l foo-rw-r--r-- 1 ad ad 345 1 2 Nov 1 3 1 996 foo$ ls -l foo

-rw-r--r-- 1 ad ad 345 1 2 Nov 1 3 1 996 foo

5th value

1 4th value


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6 .4 . break, c o ntinue, :

break and continue are used as in C:break escapes from a loop

for file in fred*do

if [ -d "$file" ]; then # deleted?break # finish loop

fi# do something#

done

continued


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continue goes to the top of a loop:

for file in fred*doif [ -d "$file" ]; then # deleted?

continue # go back to loop topfi

# do something#

done

continued


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The ':' command is the same astrue , but runs a tiny bit faster

often used to simplify control logicif [ -f fred ]; then # is freda file?

: # do nothingelse

# do something#

fi


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6 .5 . trapCapture user interrupts or system call failures .

Format (with ):trap commands signal-numbers-or-name

Some signal numbers (names):2 ( INT ) press delete or control-C3 (QUIT ) press control- | or control- \1 5 (TERM) kill command signal

continued


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A complete l ist of ava i l a b l e sign a l s is givenby typing trap -l at the command line .

To ignore a signal, set the trap commandto be empty ( )

To reset signal processing, set thecommand to -

continued


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$ cat intertrap echo PROGRAM INTERRUPTED; exit 1 INTwhile truedo

echo Program running.sleep 2

done


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7 . Functio ns

Bash allows shell scripts to usefunctions:

function_name() {statements}

Functions must be definedtextually before they are used .

continued


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Functions can return integer values

Function parameters are passed by modifying$*, $@ , $#, $1 -- $9 while the function isexecuting .

Local variables are defined with the localkeyword

the other variables in a script are global bydefault


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my_name#!/bin/sh

yes_or_no() { # a functionecho "Is your name $* ?"while truedo

echo -n "Enter yes or no: "read xcase "$x" in

y | yes ) return 0;;n | no ) return 1 ;;* ) echo "Answer yes or no"

esacdone

} # end of yes_or_no()

continued


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# the main part of the script

echo "Original parameters are $*"

if yes_or_no " $1"then

echo "Hi $1 , nice name"else

echo "Never mind"fi

exit 0


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Us e$ my_name andrew davisonOriginal parameters are andrew davisonIs your name andrew ?Enter yes or no: y

Hi andrew, nice name$


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8 . Us eful Scripting

Co

mmands

expr evaluates its argument as anexpression:

ans=expr $x + 1

The usually operators are available:+ - * / (integer divison) % (modulo)

> > = <


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Bash supports printf , as a more flexible echoprintf "format string" parameter 1 ...

Very similar to printf() in Cmain restriction is no support for floatsonly integers are supported in the shell

$ printf "%s %d\t%s\n" "hi there" 1 5 studentshi there 1 5 students$

printf


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9 . Here Do cument s

A here document allows input to bepassed into a command from

within a scriptthe command thinks the input iscoming from a file or input stream

A here document begins with


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$ cat here 1#!/bin/sh

cat $ here 1hello

this is a heredocument$

the heredocument


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10 . Debugging

Various options can be set wheninvoking a shell script to help with

debugging.

There are two ways to set theoptions:

from the c omm a nd l ine whencalling the scriptfrom within the script by using set

continued


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Command Line Setsh -n script set -n

do not execute the script, only parse it

sh -v script set -v

echoes commands after executing them

sh -x script set -u

warns when an undefined variable is used


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11 . Mo re Inf o rmatio n

The Bourne Shell:Ch . 10 , Sobell

Bourne/Bash:B eginning Linux Progr a mming Neil Matthew and Rick StonesChapter 2