CST8177 - Linux II
More Scripting Techniques 
Scripting Process 
Example Script 
Todd Kelley 
kelleyt@algonquincollege.com 

CST8207 - Todd Kelley 
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.arguments to scripts 

.positional parameters 

.input using read 

.exit status 

.test program, also known as [ 

.if statements 

.error messages 

.while loops 

.for loops 



scripting so far
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.why write scripts in the first place? 

.lists 

.set and shift 

.integer arithmetic 

.scripting process in general 



scripting to come
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1.use when an alias gets too complex, impossible 

.aliases cannot use arguments in their replacement text 


2.make new, specialized commands 

.look at /bin/gunzip and /bin/zcat 


3.automate long and/or complex tasks 

.example, actually part of the boot system: /etc/rc.d/rc 

.backup scripts, bulk user creation, other sysadmin tasks 

.as we'll see, scripts are the contents of the "regularly scheduled tasks" directories: 



        /etc/cron.{hourly,daily,monthly}/ 
 

Why do we write scripts?
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.In the general form of the control statements(examples: if, for, while) in the bash man page we will see the notion of a list 

.for name [ in word... ] ; do list ; done 

.if list1; then list2; fi 


.From the man page for bash: 

.A list is a sequence of one or more pipelines  separated  by  one  of  the operators  ;,  &,  &&, or ||, and optionally terminated by one of ;, &, or <newline>. 




Lists
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.Hey, we already know what most of those mean: 

.ls -al | more  # there's that | operator 

.| means redirect output of one command into another 


.ls -al > foo &    # there's that & operator 

.& means run the command in the background 

.proceed immediately with any command after the & 


.ls -al ; head myfile  # there's the ; operator 

.; means run one command after another  

.wait for the first command to finish in the foreground, then when it's finished, proceed with any command after the ; 





Lists (cont'd)
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.The new ones are  

.command1 && command2 

.command1 || command2  




&& and ||
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.&& means AND 

.This means stop at the first command that has non-zero exit status 

.command1 && command2 is the same as 

.if command1; then command2; fi 

.example, suppose we want to run process.sh on a file, and then remove it: 



process.sh /root/testfile && rm -f /root/testfile 
.if any thing goes wrong with process.sh (non-zero exit status), the file is NOT removed 

.if the process.sh /root/testfile command goes well (has zero exit status), THEN the file will be removed 




 

&& example
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.|| means OR 

.This means stop at the first command that returns a zero exit status 

.command1 || command2 is the same as 

.if command1; then : ; else command2 fi 

.: is a command that does nothing! 


.example, suppose we are writing a script that needs to process a file, and if that doesn't go well, then the script should terminate abnormally: 

.process /root/testfile || exit 1 

.if anything goes wrong with process, the script exits 





|| example
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.Suppose you might qualify for a scholarship: 

.Those who qualify are: 

.born on the moon, and something 

.if we weren't born on the moon, we don't need to know what the <something> is; we already know we don't qualify  


.Algonquin student and something 

.in this case, because we are an Algonquin student, we do need to know what something is to know whether we qualify 



.or in other words 

.born on the moon && something 

.Algonquin student && something 




&& means "and"
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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 &&
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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 

.if you fail first year, you don't attempt second year 


.-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
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.In our script we can manipulate the positional parameters 

.set command to set them 

.shift command to process them in turn 



Manipulating positional parameters
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.We can set the positional parameters with  

.set - oneword secondword -e etc 


.This will set  

.$1 to oneword 

.$2 to secondword 

.$3 to -e 

.$4 to etc 


.Set does take options (see bash manpage) and that first - tells set there are no more options (-e in this case is not an option)   


 

set
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.shift 

.moves all the arguments to the left 

.shift n moves all the arguments to the left by n 

.shift 

.$# is decreased by 1 

.the pre-shift $1 is lost 

.$1 becomes what was in $2 

.$2 becomes what was in $3 

.$3 becomes what was in $4 

.etc 




 

shift
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.The expr command is a common and traditional way to evaluate arithmetic expressions 

.The arguments to the expr command constitute the arithmetic expression  



Doing integer arithmetic
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.examples of using expr command: 


a=`expr 3 + 4`  # a gets set to 7 
a=`expr 3 - 4`  # a gets set to -1 
a=`expr 3 * 4`  # a gets set to 12 
a=`expr 13 / 5`  # integer division: 2 
a=`expr 13 % 5`  # remainder: 3 
.increment the integer in variable a 


a=`expr $a + 1` # a gets set to a+1 

expr examples
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.let is not the preferred way to do arithmetic in this course 

.let arg [arg...] 

.each arg is an arithmetic expression to evaluate 

.see ARITHMETIC EVALUATION of bash man page 

.shell variables are allowed with or without $ 

.arithmetic expressions can involve assignment of values to variables, where "=" is the basic assignment operator 


.examples: 


let a=1 
echo $a 
let a=a+1 
echo $a 

let
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.let seems great, why not use it? 

.because it's not as portable as the alternatives 

."portable" means you can run your script unmodified on other systems, maybe running /bin/dash instead of /bin/bash, and it will work just as well 


.expr is "standard" and works everywhere 

.$((arith)) is what you would use if you don't want to use expr (and it's faster than expr) 

.x=0 

.x=$(( x + 1 )) 

.x=$(( $x + 1 ))      




let not preferred
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.What's the difference between ((expression)) and $((expression)) 

.$((expression)) is an expansion like a variable expansion, where $((expression)) is replaced by the results of the evaluated expression 


 
.((expression)) is a compound command equivalent to let expression  



((expression)) and $((expression))
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.You can put $((expression)) where you can put variable expansion like $myvar:  


a=0 
while [ "$a" -lt 10 ]; do 
    echo username: "user$((a+1))" 
    a=$((a+1)) 
done 

expression examples
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.You can use ((expression)) wherever you would use a command: 


((a=0))      # set a to 0 
((a=a+1))  # increment a to 1 
a=((a+1))  # syntax error 
a="((a+1))"  # set a to literal string "((a+1))" 
a="ls"      # set a to literal string "ls" 
.In these last two, there is what looks like a command, ls and ((a+1)), but no command is run 



expression examples (cont'd)
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.Analyze and understand the problem 

.Write pseudocode to document and understand the problem 

.Start with a small number of lines in the script, and get that working properly 

.Add a small amount to the script, and get it working properly again 

.repeat previous step until script is complete 

.run final tests 



Scripting Process
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.Example from Sobell, A Practical Guide to Linux, Commands, Editors, and Shell Programming, 2nd Ed, Chapter 10 

.lnks script: 

.Given a filename, find hard links to that file. 

.Search for hard links under a certain directory if one is given; otherwise, search for hard links in the current working directory and its subdirectories. 




Analyze, understand, pseudocode
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.Generally, for any problem, we need to deal with the following three phases 

1.Input 

2.Processing 

3.Output 



lnks script analysis
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.Input is the information/data that a script needs before it can do its job 

.What is the information? 

.name of a file 

.name of a directory 


.How does our script get its information? 

.the description says "given a filename", and "directory if one is given" 

.this would imply that those items would be command line arguments 

.if the description said "ask for", then the script would prompt for the information after its running 




Input Phase
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.What's the data that will be processed? 

.What work will our script do? 

.the script will look for hard links in the file system 

.therefore, the data is the contents of the file system 

.our script will look for hard links in the file system 

.our script will "read in" this data by running commands that access the file system (ls, find, etc) 




Input Phase (cont'd)
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.OK, we now understand the problem 

.take a file name and an optional directory location as arguments on the command line, and use Linux commands to find hard links at that the specified directory (or the current directory if none was specified) in the file system  


.Simple, let's just tell the computer to do that! 



Input Phase (cont'd)
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.Our scripts are like little robots that will do exactly what we tell them to do. 

.Imagine you move into a new neighborhood and you tell your domestic robot to go to the corner store and bring back a turkey sandwich and a coke 

.The robot comes back with a pound of butter and a can of crab juice 

.You aren't happy, but whose fault is it? 

.the store had ham, but no turkey, and pepsi, but no coke 

.again, whose fault is it that you aren't happy? 




The game of "what if?"
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.If they don't have coke, then get pepsi, and if they don't have pepsi, then nothing, etc, etc 

.When we write our scripts, we end up happiest when we specify absolutely every single detail of every possible circumstance 

.We need to make a conscious effort to predict all possible ways things can go wrong 

.It's a common script beginner's mistake to think only about everything going right 

.The major work is dealing properly with what goes wrong  



What if?
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.What if no directory name is given on command line? 

.use the current directory (hey, this is easy!) 


.What if no file name is given on the command line? 

.print an error message (but then what?) 


.What if the given file name is a directory and not a regular file? 

.print an error message and exit 


.What if too many names are given? 

.What if no hard links are found? 

.What if there are no hard links to the file? 

.What if the directory is unreadable? Doesn't exist? 

.The more "what ifs" our script can handle, the better... 



What ifs for lnks script
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.Speaking of "what if"s... 

.A Test Plan is a document that describes a process for verifying that the script does the right thing every time no matter what 

.In simple cases it can be a list of tests, which would have this form: 

.test name 

.short description of what's being tested 

.command (and/or instructions) for running the test 

.expected result and/or output from running the test 

.the actual result (filled in when a test plan is executed) 




Test Plans
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.As we have seen, the process of understanding the problem and generating a test plan both involve "what if" analysis, and therefore can proceed at the same time 

.The plan for testing the final result begins to form early (maybe even before the problem is fully understood!) 



Test Plan's (cont'd)
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.Start small and get that working, and then add to it 

.Let's start with just the "input" phase: 


 
#!/bin/sh -u 
PATH=/bin:/usr/bin ; export PATH 
umask 022 
 
# identify links to a file 
# Usage: $0 file [directory] 
 
if [ $# -eq 0 -o $# -gt 2 ]; then 
    echo 1>&2 "Usage $0 file [directory]" 
    exit 1 
fi 
.Continue on real Linux system.... 



Back to lnks script
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.After finishing the Input stage, we have our file name and directory under which to search 

.How do we find the hard links? 

.Think about hard links... 

.If we know the inode of the file, then all hard links to the same file will have the same inode 

.We can get the inode of the file using ls 

.We can look for files with the same inode using find 




Processing
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.Getting the inode number into a variable named inode so we can refer to it as $inode 

.Consider the output of ls -i 

.Assuming the filename is in $file, Each of the following will do the trick: 

.inode=`ls -i "$file" | awk '{print $1}'` 



OR 
.set - `ls -i "$file"` 

.inode=$1 



OR 
.inode=$(ls -i "$file" | cut -d' ' -f1) 




Processing (cont'd)
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.Now that we have the inode, how do we find other files with that inode? 

.This literally has the find command written all over it!  (man find) 


find "$directory" -inum "$inode" -print  

Processing (cont'd)
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.How does our script deliver its results? 

.Should the names be put in a special file somewhere? 

.It depends. 

.In keeping with the Unix filter/pipeline philosophy, lets have our script print the names on the standard output 

.Hey, find is already printing the names on its standard output 

.Recall that the standard output of the script is the same as the standard output of the commands inside the script 



Output
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.Notice that errors and other messages should be printed on standard error 

.The "goods" (the actual filenames we were looking for) are printed on standard output 

.That's how we like it, because now we can redirect standard output to a file, or run it through grep or any other utility, and we won't have those messages mixed in 

.All of those messages will go to standard error, which we can also redirect elsewhere if we want 



Output (cont'd)
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.Indenting: look at how the if statements are indented: 


if list; then 
    ls -l # indented 4 spaces 
fi 
.The same would be true of other control statements: 


while list; do 
    #indent 4 spaces 
done 

Indenting
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.Variables in our scripts have lower case names 

.Environment variables are indicated by their UPPER CASE names: SHELL, VISUAL, etc 

.It's usually best to put variable expansions inside double quotes, to protect any special characters that might be inside the variable: 


echo "$somevar" 
.if somevar contained the * character, the double quotes stop the shell from globbing it 




Variables (review)
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.The following all mean different things: 

.run the myvar command with two arguments, = and value: 



myvar = value 
.set the myvar variable to have value "", then run the value command with that variable setting in effect 



myvar= value 
.run the myvar command with one argument, namely =value: 



myvar =value 
.set the variable myvar to have value value 



myvar=value 
 

Variables (cont'd)
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.-v option for bash/sh 

.sh -v myscript 

.shell will print each line as its read 

.loop statements are printed once 



 
.-x option for bash/sh 

.sh -x myscript 

.shell will display $PS4 prompt and the expanded command before executing it 

.each loop iteration is shown individually   




Debugging shell scripts
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.test plan creation involves identifying the "Boundary Conditions" 

.The "typical case" or "normal case" is a necessary test, and all such cases are considered equivalent (test one and you've tested them all) 

.Boundary cases are all interesting: 

.present or missing 

.too small, just big enough, typical, almost too big, too big 

.MINIMUM, ... -1,0,1,... MAXIMUM  




testing Boundary Conditions
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.Suppose you're looking for 8.3 filenames, where the "main part" is up to 8 characters, and the extension is exactly 3 characters 

.main part of filename 

. boundaries 

.0, 1, 8, 9 characters 


.typical case (only one needed) 

.2,3,4,5,6,7 



.extension 

.boundaries 

.0,3,4 

.typical case (same as the boundary) 3 





   

Boundary Conditions example
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