Introduction to Shell Scripts - Microsoft Azureclasses.eastus.cloudapp.azure.com/~barr/classes/comp190/lecture/Lesson... · Back to shell scripts Now that you've learned a few commands

Copyright 2006 Stewart WeissCopyright 2009 Stewart Weiss

Scripting

More Shell Scripts

Adapted from Practical Unix and ProgrammingHunter College

41 Comp 190 Scripting Languages

Back to shell scripts

Now that you've learned a few commands and can edit files, you can start to write shell scripts.You need a few more tools to make this possible. For starters, you need to know how a shell script can access the words you type on its command line.For example, suppose I want to write a script named swap that when called like this:

swap word1 word2

would output this: word2 word1


Command line arguments

Bash can access the words on the command line using the variables $1, $2, $3, and so on. The first word is stored in $1, the second, in $2, etc.

The number of words on the line, excluding the command itself, is stored in a variable named $#.

Therefore, our swap script would be as simple as #!/bin/bash echo $2 $1

This displays the second word, then the first word


Adding error-checking: test

Scripts should always check that they have the correct number of arguments and that they are all valid. swap just has to check that there are exactly 2 words, so it needs to test whether $# is equal to 2.

The test command evaluates an expression. For example

test 1 -ne 2

is true because 1 is not equal to 2. test 2 -eq 2

is true because 2 equals 2. The other operators are -le, -lt, -gt, and -ge. Can you guess what these mean?


Other forms of test

If you read the man page for test, you will see that there are other ways to use it. For example

[ 1 -ne 2 ]

is equivalent to test 1 -ne 2

You can put square brackets around an expression but there must be spaces on either side of them:

[1 -ne 2]

would be an error.


Using the if command

Every shell has a command named if. In bash, you have to follow a very specific set of rules to use it. It look like

if test-command then

any sequence of commands fi

where the words if, then, and fi are on separate lines.


Example if command

if test $# -ne 2 then

echo usage: swap word1 word2exit

fiThis piece of shell script will print a usage message if the number of words on the command line is not equal to 2.It will also quit immediately after printing the message, no matter what commands follow the word fi.


Putting it all together

We can put the testing bit of stuff ahead of our echo command to do our input-checking, and we now have a safe script:

#!/bin/bash if test $# -ne 2 then


fi echo $2 $1


Another Type of Test

The test command has many different types of tests. Many are called file tests and they can be used to test whether a file exists, or is of a given type, or size, or has some other property. For example:

#!/bin/bash if test -e $1 then

echo “$1 exists” fi


Negating Tests

There is no test that is true if a file does not exist. If you want to print an error message if the user did not supply a filename, you need to negate the test. The exclamation mark negates expressions:

if test ! -e myfile

is true if myfile does not exist, and is false if it does. If $1 is a command line argument then

if test ! -e $1

is true if it is not the name of a file that exists.


Shell comments

Add your authorship and other information in a comment (a line that starts with #:

#!/bin/bash # Written by Stewart Weiss, 09/24/2009 if test $# -ne 2 then


fi echo $2 $1


Shell comments

Add your authorship and other information in a comment (a line that starts with #)

#!/bin/bash # Written by John Barr, 09/20/2018

if test $# -ne 2 then


fi cat $2 $1 > newFile.txt # Any shell command works!


Adding comments to the script

A line that starts with # and no ! after it is called a comment. The shell ignores everything to the right of the #.

In fact, the # can be written to the right of a command and the shell will ignore the rest of the line after the #:

# Written by Stewart Weiss, 09/24/2009 # This script swaps words. echo $2 $1 # swap first and second words


Arithmetic

$((expression))

Must enclose the expression in double quotes

#!/bin/bashecho $(( 8#100 ))echo $(( 2#1010 ))

i=10echo $(( i+5 ))

# can use spaces in expression but not in =i=$(( i *=5 ))echo $i


More Arithmetic

$((expression))

Must enclose the expression in double quotes

#!/bin/bashecho $a# spaces optional within parena=$(( a + 2 ))echo $a

b=$(( a < 100 ))# 0 = false, 1 = trueecho $b


Loops

while command

do

command

command ...

done

# Written by John Barr, 09/20/2018 i=1 # no spaces allowed! No $ necessarywhile [ $i -le 5 ]do

echo $i # this is a command; need $i=$((i+1)) #aritmetic expression!

done


Example: a phonebook

The phonebook:Alice Chebba 971-555-2015

Barbara Swingle 201-555-9257

Liz Stachiw 212-555-2398

Susan Goldberg

201-555-7776Susan Topple 212-555-4932

Tony Iannino 973-555-1295

Create this in your account.


Example: a phonebook

The lu script:1 #! /bin/bash2 grep $1 phonebook

Try these:

$ lu Alice$ lu Susan$ lu Susan T$ lu "Susan T"

Why does the last command still give 2 results?


Example: a phonebook extended

The lu script version 2:1 #! /bin/bash2 grep "$1" phonebook

Try these:

$ lu Tony$ lu Susan$ lu Susan T$ lu "Susan T"

What do you get now?

grep saw "Susan T" as 2 different arguments!What difference do the quotes around $1 make?


Example: a phonebook adding

Write a script named "an" to add an entry to the phonebook.Remember that there is a tab between the name and the number!

Try these comands:

$ an 'Stromboli Pizza' 973-555-9478$ more phonebook



The an script version 1:1 #! /bin/bash2 echo "$1 $2" >> phonebook

Note that there is a tab between the arguments. Try:

$ an 'Stromboli Pizza' 973-555-9478

Is the phonebook still sorted?



The an script version 2:1 #! /bin/bash2 echo "$1 $2" >> phonebook3 sort -o phonebook phonebook

what does the -o flag do?


Example: a phonebook removing

Write a script named "rem" to add an entry to the phonebook.Remember that there is a tab between the name and the number!Note: do not name your script rm! Why?

Try these comands:

$ rem 'Stromboli Pizza' $ more phonebook


Example: a phonebook remove

The rem script version 1:

1 #! /bin/bash 2 grep -v "$1" phonebook > /tmp/phonebook 3 mv /tmp/phonebook phonebook

Try:$ rem "Stromboli Pizza"$ rem Susan

What happens with the last command?How would we fix this?




1 #! /bin/bash2 grep -v "$1" phonebook > /tmp/phonebook 3 mv /tmp/phonebook phonebook

How do we fix this? Must recognize:1. If there are no matches2. If there is more than 1 match

To find the number of matches:grep -o "$1" phonebook | wc -l



The rem script version 1

How do we fix this? Must recognize:

1. If there are no matches2. If there is more than 1 match

To do find the number of matches:grep -o "$1" phonebook | wc -l

now must save the number of matches in a variable: lines=$(grep -o "$1" phonebook | wc -l)



The rem script version 1

How do we fix this? Must recognize:

1. If there are no matches2. If there is more than 1 match

Now we just need to test the value in $line

See the next 2 slides.




1 #!/bin/bash 2 if [ "$#" -ne 1 ] 3 then 4 echo "Incorrect number of arguments." 5 echo "Usage: rem name" 6 exit 1 7 fi 8 9 lines=$(grep -o "$1" phonebook | wc -l) 10

Check for the correct number of arguments

Line numbers are part of the editor, not the code!



The rem script version 2 (cont):

11 if [ $lines -eq 0 ] 12 then 13 echo "no match for " $1 14 exit 1 15 elif [ $lines -gt 1 ] 16 then 17 echo "Too many matches" 18 grep "$1" phonebook 19 exit 1 20 else 21 grep -v "$1" phonebook > \tmp\phonebook 22 mv \tmp\phonebook phonebook 23 fi


Things to try

Try creating a few simple scripts of your own. It will give you practice using gedit if you are at a UNIX console, or vi or nano if you are not.

Read about the test command and learn its tricky syntax.

Play around with > to store the output of various commands.

Documents

Introduction to Shell Scripts - Microsoft Azureclasses.eastus.cloudapp.azure.com/~barr/classes/comp190/lecture/Lesson... · Back to shell scripts Now that you've learned a few commands