CSC231 Bash Tutorial 7

--D. Thiebaut (talk) 06:15, 31 March 2017 (EDT)
Updated --D. Thiebaut (talk) 20:15, 25 October 2017 (EDT)

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This lab deals with if-statements in bash scripts. If statements in Bash work the same way they work in Python and Java: if somebooleanexpression then dothis else dothat. You will need to submit your solution to Challenge 5 to Moodle.

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Reference

• This Bash tutorial heavily uses material presented in the excellent set of pages "Ryan's Tutorials" at http://ryanstutorials.net/bash-scripting-tutorial/. Do not hesitate to study these tutorials if you need additional information.

Backing up your files

Just in case you mess up and erase files in your account by mistake, you will make an archive of all your files and save it in your instructor's account:

cd
tar -czvf backup2.tgz *
rsubmit backup backup2.tgz

That's it! An archive of all your file should now be saved and available in case of accident!

The Bash If-Statement

The Bash if-statement looks like this:

 
if  [  someExpression ] ; then
   bash statement
   bash statement
else
   bash statement
   bash statement
fi

Of course, there can be as many bash statements inside the then or else block, including for loops, or other if statements.

• Make sure to end the if statement with a fi keyword.
• The else block is not necessary and can be omitted if the logic doesn't call for it.

Testing Numerical Values

• Create this bash script, make it executable, and run it a couple times:

#! /bin/bash
# if0.sh

#--- get input from user ---
echo -n  "Please enter an integer between 1 and 10 (included): "
read guess

#--- display number back ---
echo "You have entered $guess"

• Let's modify it and test the value the user enters:

#! /bin/bash
# if0.sh

#--- get input from user ---
echo -n  "Please enter an integer between 1 and 10 (included): "
read guess

#--- test if number entered is below or above 5 ---
if [ "$guess" -le "5" ] ; then
   echo "You aim low!"
else
   echo "Above average!" 
fi

• Make sure you have spaces around the brackets and around the operator inside the brackets!
• The "-le" operator stands for "less than or equal to." Bash supports the following operators for comparing numbers:

• -le    (less than or equal to)
• -lt    (less than )
• -ge    (greater than or equal to)
• -gt    (greater than)
• -eq    (equal to)
• -ne    (not equal to)

Challenge 0

• Modify the teller.sh script below, so that it outputs a number of bills only if this number is greater than 0.

#! /bin/bash
#set -x

amount=$1

no20s=$( expr $amount / 20 )
amount=$( expr  $amount % 20 )
no10s=$( expr  $amount / 10 )
amount=$( expr  $amount % 10 )
no5s=$( expr  $amount / 5 )
no1s=$( expr  $amount % 5 )

echo ""
echo $no20s " \$20-bill(s)"
echo $no10s " \$10-bill(s)"
echo $no5s " \$5-bill(s)"
echo $no1s " \$1-bill(s)"

• Here is an example of how your program should work:

cs231a@aurora ~/handout $ ./teller.sh 123

6  $20-bill(s)
3  $1-bill(s)

Nested If-Statements

• Of course, if Bash supports if statements, it will support nested if statements.
• Try this new version of the script:

#! /bin/bash
# if2.sh

#--- get user input ---
echo -n  "Please enter an integer between 1 and 10 (included): "
read guess

#--- test if below or above 5 ---
if [ "$guess" -le "5" ] ; then

   #--- compute even/odd property of guess
   x=$( expr $guess % 2 )

   #--- test if guess is odd or even ---
   if [  "$x" -eq "0" ] ; then
       echo "low even number"
   else
       echo "low odd number"
   fi

else
   echo "Above average!" 
fi

Challenge #1:

Write a script that prompts the user for an integer number between 0 and 100 (inclusive) and outputs a letter grade, using Moodle's default grade table:

Moodle Table

Range	Letter Grade
100.00 -93.00	A
92.99 - 90.00	A-
89.99 - 87.00	B+
86.99 - 83.00	B
82.99 - 80.00	B-
79.99 - 77.00	C+
76.99 - 73.00	C
72.99 - 70.00	C-
69.99 - 67.00	D+
66.99 - 60.00	D
59.99 - 0.00	F

Note 1: You do not have to worry about real numbers here. You can do all your testing with integers.
Note 2: Feel free to test just the top 3 ranges of numbers. If you can do it for 3, you can do it for the full list!

Testing and Comparing Strings

Bash uses different operators for strings: == for equality, and != for inequality. Here is an example:

#! /bin/bash
# if3.sh

for name in "Smith" "Hampshire" "Umass"  "Amherst" "MtHolyoke" ; do

   if [ "$name" == "Smith" ]; then
       town="Northampton"
       suffix="College"
   fi

   if [ "$name" == "Amherst" ]; then
       town="Amherst"
       suffix="College"
   fi
   if [ "$name" == "Umass" ]; then
       town="Amherst"
       suffix=""
       name="University of Mass."
   fi
   if [ "$name" == "MtHolyoke" ]; then
       town="South Hadley"
       suffix="College"
   fi
   if [ "$name" == "Hampshire" ]; then
       town="Amherst"
       suffix="College"
   fi

   echo "$name $suffix, $town, Massachusetts"

done

• Try it!
• You will have noticed that the then-blocks for "Amherst" and "Hampshire" are the same, so we could simplify our code by using a logical OR operator for "Amherst" and "Hampshire". The syntax for logical operators in Bash is a bit weird, though... logical:

if [ "$name" == "Amherst" ] || [ "$name" == "Hampshire" ]; then
       town="Amherst"
       suffix="College"
fi

• The OR operator is || and the AND operator is &&.

Challenge #2:

Modify the previous script, noticing that "Smith", "Amherst", "Hampshire", and "MtHolyoke" all have suffix College, and therefore one if statement with a then and else block can be used to set the suffix. You will need other if statements for the towns.

Making Bash Scripts Friendlier

When you write a script that expects some parameters on the command line, it is a good idea to tell the user if she enters the wrong number, and to display the correct syntax for using the script. Here is an example of a friendly script that expects the names of 3 fruits on the command line. The first time the user doesn't enter anything, the second time she enters 3 strings:

cs231a@aurora ~/ $ ./if5.sh 
Syntax: ./if5.sh  fruit1 fruit2 fruit3

cs231a@aurora ~/ $ ./if5.sh banana apple pear
You entered the right number of parameters
1) banana
2) apple
3) pear

• And here is the script that does that:

#! /bin/bash
# if5.sh
# D. Thiebaut
# Test if shell file executed with the appropriate
# number of parameters

if [ "$#" -ne "3" ] ; then 
   echo "Syntax: $0  fruit1 fruit2 fruit3"
   exit
fi

echo "You entered the right number of parameters"
echo "1) $1"
echo "2) $2"
echo "3) $3"

• Create and test this script a few times.
• Modify it so that it will work only if the user enters 5 different parameters on the command line (remember to change the syntax part as well!)

Challenge #3:

• Modify the script that you wrote for Challenge 1, that gets a number grade from the user, so that it now gets it directly from the command line. Make your script test that the user has actually entered a grade, and if not, make your script output the correct syntax.

Testing Files

Because Bash interacts directly with Linux, it has a rich set of operators for testing files, and various file properties. See the list of operators on this page from tldp.org for a list of all of them.
Let's try one, and list all the executable files in your directory. The operator -x operator, if applied to a file, will return True if the file is executable (the files generated by ld and the files you applied chmod -x to).
You can probably figure out how to write this script, but here it is:

#! /bin/bash
# if4.sh
# prints executable files only
#

for file in * ; do
   if [ -x $file ] ; then
      echo $file
   fi
done

• Test it in your home directory.
• Verify that it is listing all your executables.

Challenge #4:

Given the name of a file, say in variable $file, Bash can extract its extension with this (strange looking) expression: ${file##*.}

• See how it works by trying this script in your main directory:

#! /bin/bash
# challenge3_prep.sh
# D. Thiebaut
# displays the files and their extension in the current directory

# for all files in the current directory
for file in * ; do
    # get its extension
    extension=${file##*.}
    # print extension and file-name
    echo "$extension -- $file"
done

• Your challenge: write a script that will list all the files in your directory, divided into 3 sections: asm files,

object files (with a .o extension), and scripts (with a .sh extension). Below is the output of the solution program running in my handout directory:

--- o ---
GameOfLife.o

--- asm ---
231Lib.asm
addmovExample.asm
fib.asm  
GameOfLife.asm 
GameOfLifeDT.asm
mystery.asm
newFile.asm
printRegsExample.asm
prog1.asm
prog2.asm
prog3.asm
prog4.asm
quiz1.asm
skel.asm
teller.asm
test_skel.asm

--- sh ---
dateTag.sh
randomWord2.sh
randomWord3.sh
randomWord.sh
runNQueens.sh
runTeller.sh
script1.sh
script2.sh
script3.sh
teller.sh
test1.sh 

Challenge #5:

Computing Fibonaccis: Write a shell script that prints all the Fibonacci terms less than 100. We haven't see the while loop in bash yet but you can use a for-loop that goes a large number of times, larger than the number of Fibonaccis less than 100. Then, when you have found the current Fibonacci term to be greater than 100, just exit the script.

for i in `seq ... ` ; do   # figure out what expression to replace the ... with!

    if [  ...  ] ; then   # if term greater than 100
        exit 0             # then exit with code 0
    fi
done

Note 1: Remember that you can add 2 bash variables together with $( expr $var1 + $var2 ).
Node 2: Here's a Python version of what you need to do:

#! /usr/bin/env python3
# fibonacci.py
# D. Thiebaut
# prints first 10 Fibonacci terms,
# starting with 1, 2, 3...
from __future__ import print_function

fibn = 1
fibn_1 = 1

print( fibn )
print( fibn_1 )

for i in range( 100 ):
    # pass down values 
    fibn_2 = fibn_1
    fibn_1 = fibn

    # update fibn
    fibn = fibn_1 + fibn_2

    if fibn > 100:
        sys.exit()

    # print fibn
    print( fibn )

Moodle Submission

• Submit your submission to Challenge 5 to Moodle. Your script should output one number on each line. The output will start with 1, 1, and end with 89. You have until Saturday 10/28/17 noon time to submit your script.

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Solutions to Challenges

Challenge 0

#! /bin/bash
# teller.sh
# D. Thiebaut
# takes the integer given on the command line and breaks it
# down into a number of $20s, $10s, $5s, and $1s.

amount=$1

no20s=$( expr $amount / 20 )
amount=$( expr  $amount % 20 )
no10s=$( expr  $amount / 10 )
amount=$( expr  $amount % 10 )
no5s=$( expr  $amount / 5 )
no1s=$( expr  $amount % 5 )

echo ""
if [ "$no20s" -ne "0" ] ; then
    echo $no20s " \$20-bill(s)"
fi
if [ "$no10s" -ne "0" ] ; then
    echo $no10s " \$10-bill(s)"
fi
if [ "$no5s" -ne "0" ] ; then
    echo $no5s " \$5-bill(s)"
fi
if [ "$no1s" -ne "0" ] ; then
    echo $no1s " \$1-bill(s)"
fi

Challenge 1

#! /bin/bash
# challenge1.sh
# D. Thiebaut

# get integer grade
echo -n "Enter the grade (0-100): "
read grade

if [ "$grade" -gt "93" ]; then
   echo "A"
else
   if [ "$grade" -gt "90" ]; then
      echo "A-"
   else
      if [ "$grade" -gt "87" ]; then
         echo "B+"
      else
         if [ "$grade" -gt "83" ]; then
            echo "B"
         else
            echo "Grade lower than B"
         fi
      fi
   fi
fi

Challenge 2

#! /bin/bash
# Challenge2.sh
# D. Thiebaut

# displays the name of the 5 colleges and where they are located
for name in "Smith" "Hampshire" "Umass"  "Amherst" "MtHolyoke" ; do

   if [ "$name" == "Amherst" ] || [ "$name" == "Hampshire" ] || [ "$name" == "Umass" ]; then
       town="Amherst"
   else
       if [ "$name" == "Smith" ]; then
          town="Northampton"
       else
          town="South Hadley"
       fi
   fi

   if [ "$name" == "Smith" ] ||  [ "$name" == "Amherst" ] || [ "$name" == "Hampshire" ] \
      || [ "$name" == "MtHolyoke" ] ; then
       suffix="College"
   else
       name="University of Mass."
       suffix=""
   fi

   echo "$name $suffix, $town, Massachusetts"

done

Challenge 3

#! /bin/bash
# challenge3.sh
# D. Thiebaut

# Test that we got a parameter, else exit
if [ "$#" -ne "1" ]; then
    echo  "Syntax: $0 grade"
    exit
fi

# save command line param in $grade
grade=$1

if [ "$grade" -gt "93" ]; then
   echo "A"
else
   if [ "$grade" -gt "90" ]; then
      echo "A-"
   else
      if [ "$grade" -gt "87" ]; then
         echo "B+"
      else
         if [ "$grade" -gt "83" ]; then
            echo "B"
         else
            echo "Grade lower than B"
         fi
      fi
   fi
fi

Challenge 4

#! /bin/bash
# listFilesByExt.sh
# D. Thiebaut
# list the object, asm, and script files found in the
# current directory

for extension in o asm sh ; do
    echo "---" $extension "---"
    for file in `ls *` ; do
        ext=${file##*.} 
        if [ "$ext" == "$extension" ] ; then
           echo $file
        fi
    done
    echo ""
done

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Moodle Challenge

#! /bin/bash
# fibonacci.sh
# D. Thiebaut
# display the first terms of the Fibonacci series
# and limits itself to the terms less than 100.


# initialize fibn fibn_1fibn_2

fibn=0
fibn_1=1
fibn_2=1

# display first two terms
echo $fibn_2
echo $fibn_1

# loop 100 times
for i in `seq 1 100` ; do

    # compute nth term as sum of previous 2
    fibn=$( expr $fibn_1 + $fibn_2 )

    # stop if term is greater than 100
    if [ $fibn -gt 100 ] ; then
       exit 0
    fi

    # display term if less than 100
    echo $fibn

    # advance to next term in the series
    fibn_2=$fibn_1
    fibn_1=$fibn

done

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