CST8177 - Linux II
More Scripting 
Todd Kelley 
kelleyt@algonquincollege.com 

CST8207 - Todd Kelley 

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.trailing topics from last week 

.Scripting reference material 

.more about test program 

.numbers versus strings 

.|| and &&     


 
     

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Today's Topics





.Sobel Chapter 27 

.http://teaching.idallen.com/cst8129/05f/notes/exit_status.txt 

.http://teaching.idallen.com/cst8129/05f/notes/quick_tests.txt 

.http://teaching.idallen.com/cst8129/05f/notes/quotes.txt 

.http://teaching.idallen.com/cst8129/05f/notes/script_checklist.txt 

.http://teaching.idallen.com/cst8129/05f/notes/script_style.txt 

.http://teaching.idallen.com/cst8129/05f/notes/shell_read.txt 

.http://teaching.idallen.com/cst8129/05f/notes/shell_script_execution.txt 

.http://teaching.idallen.com/cst8129/05f/notes/shell_variables.txt  



Scripting Reference Material
CST8177 - Todd Kelley 

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Why scripting? Why would anyone want to know how to write a script? Why is is particularly important for a sysadmin? Here are 7 reasons to consider: 1.avoid complex typing, preventing possible errors  

2.automate repetitive tasks 

3.use when an alias gets too complex or not possible 

4.make new, specialized commands 

5.automate long and/or complex tasks 

6.handle rare but complex activities 

7.create a "wrapper" for a program These are all valid reasons, especially for a sysadmin managing a Linux/Unix server on behalf of an enterprise. The reasonable use of scripting will make you more productive, more accurate, and more efficient, increasing your value to your employer. 








.gunzip 

.vi `which gunzip` 


.vimtutor 

.vi `which vimtutor` 




Production Examples
CST8177 - Todd Kelley 

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.man test (three categories of simple test) 

.tests about files 

.is it a file or a directory 

.is it executable? 

.does it have its SUID bit set? 


.tests about strings 

.is it a null string? 

.is one string equal to another? 

.is one string alphabetically before or after? 


.tests about numbers? 

.equal to, less than, greater than?  





test program examples
CST8177 - Todd Kelley 

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.building complex tests from simple tests 

.test1 -a test2 

.both test1 and test2 must be true 


.test1 -o test2 

.at least one must be true 


.! test1 

.test1 must be false 


.(test1) 

.true if test1 is true 




combining tests together
CST8177 - Todd Kelley 

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.numbers and strings are not the same 

."00" is not the same as "0"  (as strings) 

.one has two characters, the other has one character 

.how can they be the same? 

.if your pin number is 0037 and someone tries, 37, should it work? 

.as strings, "0037" is not the same as "37" so, no, it shouldn't work 


.= and != and < and > are for STRINGS 



numbers versus strings
CST8177 - Todd Kelley 

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.numbers and strings can't be mixed 

.00 is the same as 0 (as numbers) 

.005 is the same as 05 and 5 (as numbers) 

.it's an error to ask if the number 0 is the same as the number xyz (error: there is no such number xyz) 

.-eq -ne -gt -lt -ge -le are for NUMBERS 

.0 -eq xyz  #gives ERROR, xyz not a NUMBER 



Numbers versus Strings
CST8177 - Todd Kelley 

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.We have already seen && in action: 

.[ -f $HOME/.bashrc ] && . $HOME/.bashrc 

.[ -z "$PS1" ] && return 


.And we've seen these are equivalent to 


if [ -f $HOME/.bashrc ]; then 
     . $HOME/.bashrc 
fi 
and 
if [ -z "$PS1" ]; then 
     return 
fi 
  

|| and &&
CST8177 - Todd Kelley 

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.Suppose you might qualify for a scholarship: 

.Those who qualify are: 

.eight feet tall, and ?? 

.born on the moon, and ?? 

.algonquin student and ?? 


.or in other words 

.eight feet tall && ?? 

.born on the moon && ?? 

.algonquin student && ?? 


.In which case do we need to find out what ?? is? 



&& means "and"
CST8177 - Todd Kelley 

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.With "and", we need to keep going as long as we keep encountering "true" 

.As soon as we encounter "false", we can stop 

.born on the moon && ??  # we don't care about ?? 

.algonquin student && ??  # we need to know ?? 


.In the first case, we would not do the ?? command, whatever it is 

.In the second case, we would do the ?? command, whatever it is 

.Often we don't need the exit status of the command ??, we just wanted the command to run (or not)  



&& continued
CST8177 - Todd Kelley 

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.As soon as we encounter "true", we can stop 

.You qualify for a $1000 rebate under the following conditions: 

.born on the moon, or ?? 

.algonquin student, or ?? 


.In the first case, we need to know what the exit status of the ?? is, we need to run the ?? command 

.In the second case, we can stop before running the ?? command  



|| means "or", opposite of &&
CST8177 - Todd Kelley 

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.aa && bb 

.means if aa; then bb; fi 



 
.aa || bb 

.means if not aa; then bb; fi 




&& and || in general
CST8177 - Todd Kelley 

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.You have six numbers, and six numbers are drawn randomly 

.When you're checking the numbers, is it && or || that governs when you stop checking?  


first number matches && 
second number matches && 
third number matches && 
..etc... 

Checking Lottery Numbers
CST8177 - Todd Kelley 

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.After you try out for the basketball team, the list of people who made the team is posted. 

.When you're checking for your name on the list, which of && or || governs when you stop checking?  


first name on the list is yours || 
second name on the list is yours || 
third name on the list is yours || 
...etc... 

Is your name on a list?
CST8177 - Todd Kelley 

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.&& and -a both mean "and" 

.&& is used between commands 

.-a is used between expressions in the test command  


.|| and -o both mean "or" 

.|| is used between commands 

.-o is used between expressions in the test command 




&& versus -a, || versus -o
CST8177 - Todd Kelley 

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.&& and || are used with commands that tend to get things done 

.to graduate, you 

.complete first year && complete second year 


.complete first year is a "command" that gets things done: you learn the first-year material 


.-a and -o are used in test, and don't do things, just affect the exit status of test 

.you are a rich canadian if 

.you are canadian -a you are rich 


.checking whether or not you're canadian doesn't get things done - but it does establish a truth value 




&& and || versus -a and -o
CST8177 - Todd Kelley 

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.If we used &&, we'd be getting things done  

.become a Canadian && earn $1000000  

.When it comes to trying to be a rich Canadian, if we fail to become Canadian, we don't need to bother earning $1000000 



Rich Canadian
CST8177 - Todd Kelley 

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