shell scripting

10/2/2007

Net212

SHELL PROGRAMMING WITH BASH

Intro 

The difference between tcsh and bash is not so noticeable



What is a shell script?



 

It is apparent when writing shell scripts Set of commands that are executed from a file Flow of execution can be controlled

Shell script anatomy 

Beginning



Body



 



#!/bin/bash #this is a remark or comment grep $1 /etc/passwd

File attributes 

‐rwxrwxr‐x 1 student student 31 Sept 21 18:15 printinfo.sh

1

10/2/2007

Making a script 1. Open new file 

vi printinfo.sh

2. Put in beginning (specify shell) 

#!/bin/bash

3. Put in body (commands and structures) 

grep $1 /etc/passwd

4. Save file 

Esc , :wq

5. Make it executable 

chmoda+x a+xprintinfo.sh

Executing it 

If it’s not in your path



If it’s in your path



If it’s not in your path and in your current folder







/path/to/script/printinfo.sh student printinfo.shstudent

./printinfo.shstudent

Script arguments 

grep $1 /etc/passwd   



Expand last example   



$1 stands for first argument $2 would be the second argument and so on… $0 is the script name grep $1 /etc/passwd grep $1 /etc/passwd| cut –f $2 –d “:” echo thank you for using script $0

Use it 

./printinfo.shstudent3 student:x:500:500:student:/home/student:/bin/bash

 

500 Thank you for using script ./printinfo.sh



2

10/2/2007

Another example  

Last example printed username from passwd file and its uid How about an account creation script   



#!/bin/sh useradd $1 passwd $1

Save it as mkuser.sh and run it  

./mkuserlinuxguy ./mkuser linuxguy Changing password for linuxguy

Another example 

Save process list 

#!/bin/bash



ps–U $USER > processes.txt chmod a+x a+xlistprocess.sh listprocess. sh ./listprocess.sh

 

Control the execution flow 

Use if statements  



If (condition is true) Statements

Use while loops  

While(conditionis true) Statements

3

10/2/2007

If statements 

Does a file exist? 

if [ ‐f /etc/inittab ] #if [condition]



then



fi #end if





ec o yes statements

Run it   

chmod chm od a+x a+xcheckinittab.sh checki nittab.sh ./checkinittab.sh yes

Variables 

Argument variables



Internal variables



 



$0, $1, $2,… myvar=“/bin/bash” avalue=`grep student /etc/passwd| cut –f 7 –d “:”`

Back quotes are for command results!!!  

cnt=`ls| wc –l`  same key as tilde! cnt=‘ls | wc –l’ <‐ Same key as double quote

Variables with if 

Check if anything in current folder



cnt=`ls| wc –l` if [ $cnt–gt 0] #is count greater than zero? then #yes it is echo there are files here else #no it isn’t echo no files here



fi

    

4

10/2/2007

Conditional operators 

Operators for numbers    



‐gt Greater than ‐eq Equal to ‐lt less than ‐ne Not equal to

Operators for words and sentences (strings)  

= equal != not equal

Another if example 

Check if .bash_profile exists 

bashfile=“.bash_profile”#a string variable



if [ `ls –a .bash_profile` = “$bashfile” ] then echo it exists

 



else echo it’s not there



fi



Sum up variable usage 

Numbers do not use double quotes    



Mynum=10 if [ $Mynum–lt 11 ] Echo M numis less than 11 Fi

Strings do use double quotes    

Mystr=“david” if [ “$Mystr” != “david” ] echo This is not david fi

5

10/2/2007

Sum up if statements 

For comparing values    



if [ value1 operator value2 ] then statements fi

For checking the exit code    

if some command here then #do this if exit code is 0 statements fi

Exit codes 

Also known as an exit status code



Status code 0 means success



 



type echo $? To see the last exit code

echo $? 0

Status code 1 means failure   

ls /etc/issues echo $? 1

While statements 

Do something until condition is false



Ex: Create 10 files



     

Uses same conditional expressions as if = i=1 while [ $i ‐lt $Stoppoint ] do touch myfile$i i=$[$i+1] done

6

10/2/2007

Another while example 

Read input from a pipe 

ls | while read file



do echo $USER has file $file done

 



Results 

student has file myfile1 student has file myfile2



….



7