IT 244: Introduction to Linux/Unix

IT 244: Introduction to Linux/Unix
Class 24

Today's Topics

Review

Using test in Scripts
Checking the Arguments to a Script
The if ... then ... else ... Construct
The if ... then ... elif ... Construct
Debugging Scripts

New Material

for ... in ... Loops
for Loops
Three Statement for Loops
while Loops
until Loops
continue
break
Real World for Loop Examples

Homework 11

I have posted homework 11 here.

It is due this coming Sunday at 11:59 PM.

This is the last homework assignment.

Final Exam

The final exam will be given on Tuesday, December 19th from 3:00 - 6:00.

The exam will be given in McCormack 3-430.

There will be 50 questions, which will be similar to the questions on the mid-term and the quizzes.

As with the Mid-term, 60% of the questions will come from the ungraded Class Quizzes.

The questions will cover all of the material for this course.

The last class, Tuesday, December 12th, will be a review session.

You will only be responsible for the material contained in the Class Notes for that class, and the Class Notes for the review session for the Mid-term, which you will find here.

Although the time alloted for the exam is 3 hours, I would expect that most of you would not need that much time.

You will not be able to leave the room until you turn in your exam paper so you should visit the restroom before you take the test.

The final is a closed book exam.

Review

Using `test` in Scripts

To make shell scripting look more like a "real" programming language ...
Bash provides a synonym for test ...
a pair of square brackets, [ ]
To test whether the value of the variable number1
is greater than the value of the variable number2

you could write either

    test $number1 -gt $number2

    [ $number1 -gt $number2 ]

Whenever you use this construction
there must be a space ...
before and after each square bracket
test returns true or false through the exit status
If test returns an exit status of 0 ...
it means the condition was true
If the condition is false ...
test returns an exit status of 1

Checking the Arguments to a Script

If a script must have a certain number of arguments to run ...
it should check for them
If it doesn't get the right number of arguments ...
it should print a usage message and exit

A usage message has a standard form

    Usage:   PROGRAM_NAME  ARG1  ARG2  ...

After the program name, there should be one string for each argument
The string should indicate what sort of value is needed
For example DIR_NAME when a directory name is required
Use the positional parameter 0 along with the basename command ...
to get the program name

The `if ... then ... else ...` Construct

The another conditional is the if ... then ... else ... construct ...

which has the following format

    if  COMMAND
    then
      COMMAND_1
      COMMAND_2
      ....
    else
      COMMAND_A
      COMMAND_B
      ...
    fi

If COMMAND returns an exit status of 0 ...
COMMAND_1, COMMAND_2, ... will be executed ...
otherwise COMMAND_A, COMMAND_B, ..., will be run

The `if ... then ... elif ...` Construct

The if ... then ... elif ... construct allows you to create nested if statements

    if COMMAND
    then
      COMMAND_1
      COMMAND_2
      ...
    elif ANOTHER_COMMAND
    then
      COMMAND_A
      COMMAND_B
      ...
    ...
    else
      COMMAND_N1
      COMMAND_N2
      ...
    fi

elif stands for "else if"
Notice that elif must be followed by then
The then must either be on the next line ...
or the same line separated by a semi-colon, ;
The else statement must be terminated by a fi
elif only requires a single fi at the end

Debugging Scripts

If you run a script using bash with the -x option ...
the shell will print each line of the script ...

just before it executes that line

    $ cat match_three.sh 
    #! /bin/bash
    #
    # takes three stings as input and compares them

    if [ $# -lt 3 ]
    then
        echo Usage: $(basename $0) string1 string2 string3
        exit 1
    fi

    if [ "$1" = "$2" -a "$2" = "$3" ]
    then
        echo "All arguments match"
    elif [ "$1" = "$2" ]
    then
        echo "Arguments 1 and 2 match"
    elif [ "$1" = "$3" ]
    then
        echo  "Arguments 1 and 3 match"
    elif [ "$2" = "$3" ]
    then
        echo  "Arguments 2 and 3 match"
    else
        echo "No arguments match"
    fi
    
    $ bash -x match_three.sh foo bar foo
    + '[' 3 -lt 3 ']'
    + '[' foo = bar -a bar = foo ']'
    + '[' foo = bar ']'
    + '[' foo = foo ']'
    + echo 'Arguments 1 and 3 match'
    Arguments 1 and 3 match

Before bash prints a line from the script ...
it prints a plus sign, +
to let you know that the line is not output from the script

New Material

`for ... in ...` Loops

The most common programming construct after the if statement ...
is the loop
Bash provides many kinds of loops ...
but we'll start with the for ... in loop ...

which has the following format

    for LOOP_VARIABLE  in  LIST_OF_VALUES
    do
        COMMAND_1
        COMMAND_2
        ...
    done

The do must be on a different line from for ...
unless you place a semicolon, ;, before the do ...
just like then in an if statement
The commands between do and done ...
are repeated each time through the loop
In a for ... in loop, Bash
- Assigns the first value in the LIST_OF_VALUES ...
  to the variable specified by LOOP_VARIABLE
- Executes the commands between do and done
- Assigns the next value in the LIST_OF_VALUES ...
  to the LOOP_VARIABLE
- Executes the commands between do and the done again
- And so on until each value in LIST_OF_VALUES has been used

Here is an example

    $ cat fruit.sh
    #! /bin/bash
    #
    # demonstrates the for in loop

    for fruit in apples oranges pears bananas
    do
        echo $fruit
    done
    echo Task complete.
	
    $ ./fruit.sh 
    apples
    oranges
    pears
    bananas
    Task complete.

Notice that the variable fruit ...
does not have a dollar sign in front of it ...
when it appears after for
That's because here we are dealing with the variable ...
not its value
We are telling Bash which variable to use ...
when storing the values in the list

`for` Loops

The for loop ...
is simpler than the for ... in ... loop

and has the following format

    for LOOP_VARIABLE
    do
        COMMAND_1
        COMMAND_2
        ...
    done

The difference between the two for loops ...
is where they get the values ...
assigned to the loop variable
The for ... in ... loop gets values ...
from the list that follows in
These values are "hard coded" into the script
They never change
In practice, the list of values is often created using meta-characters ...
so the actual values changed depending on where the for loop is run
The plain for loop gets its values from the command line
The plain for loop ...
can have different values each time it is run

Here is an example

    $ cat for_test.sh 
    #! /bin/bash
    #
    # demonstrates the simple for loop

    for arg
    do
        echo $arg
    done
    
    $ ./for_test.sh foo bar bletch
    foo
    bar
    bletch
    
    $ ./for_test.sh bing bang boom
    bing
    bang
    boom

Three Statement `for` Loops

The for loops above are very different ...
from the for loops in programming languages
In programming languages, the for statement
- Initializes a loop variable
- Tests the value of the loop variable ...
  to decide whether to run the loop one more time
- Changes the loop variable ...
  at the end of the loop code
The for statements in programming lanugages ...
create the values used in the loop
But the for loops above ...
must be given the values ...
used in the loop
In the for ... in ... statement ...
the values come after in ...
in the script itself
In the plain for statement ...
the values are given at the command line
But there is a third form of for loop in Bash
This form creates the values for the loop variable the same way ...
as the for loop in programming languages

It has the following form

    for (( STMT_1; STMT_2; STMT_3 ))
    do
        COMMAND_1
        COMMAND_2
        ...
    done

Notice that the three statements ...
are inside double parentheses
That means that anything inside ...
will be treated as numbers ...
not text
The first statement sets the value of the loop variable
The second is a logical statement
As long as it is true ...
the loop will continue
The third statement changes the value ...
of the loop variable ...
after each pass through of the loop

Let's look at an example

    $ cat count_to_five.sh 
    #! /bin/bash
    #
    # this script demonstrates the three statement for loop

    for (( count=1; count<=5; count++ ))
    do
        echo $count
    done
    
    $ ./count_to_five.sh 
    1
    2
    3
    4
    5

The statement count++ ...
increases the value of count by one
Without this third statement ...
the loop would never end ...
and we would have an infinite loop

`while` Loops

The first two for loops keep running ...
until all values supplied to them ...
have been used in the loop
A while loop continue running ...
as long as the test command returns a status code of 0

while loops have the form

    while  COMMAND
    do
        COMMAND_1
        COMMAND_2
        ...
    done

As long as the COMMAND returns a status code of 0
the commands between do and done ...
will be run

Let's look at an example

    $ cat count_to_nine.sh 
    #! /bin/bash
    #
    # counts to 9, then stops
	
    number=0
    while [ $number -lt 10 ] 
    do
        echo -n $number
        (( number += 1))
    done
	
    $ ./count_to_nine.sh 
    0123456789

The line after echo -n ...
the one with the double parentheses ...
tells Bash to interpret the expression as a number...
not a string
The += operator ...
tells Bash to add the number that follows ...
in this case 1 ...
to the current value of number
The echo command in the while loop uses the -n option ...
and does not print a newline
This allows the script to print the numbers ...
one right after the other ...
all on the same line
To end the output and get a new line ...
we need the final echo command
The condition that the while loop tests ...
must change ...

or the while loop will run forever

    $ cat forever.sh 
    #! /bin/bash
    #
    # this loop runs forever
    
    number=0
    while [ $number -lt 10 ]
    do
      echo  $number
    done
    
    $ ./forever.sh 
    0
    0
    0
    0
    ^C

The value of number never changes ...
so the expression given to test never becomes false ...
and the script loops forever ...
until it is aborted

`until` Loops

The until loop is similar the while loop ...
except that the until loop ends ...
when the test condition becomes true
Whereas the while loop stops ...
when the text condition becomes false

The until loop has the form

    until COMMAND 
    do
        COMMAND_1
        COMMAND_2
        ...
    done

Here is an example

    $ cat count_until.sh
    #! /bin/bash
    #
    # counts from 1 to its argument, then stops 
    
    if [ $# -eq 0 ]
    then
        echo Usage:  $(basename $0)  NUMBER
        exit 1
    fi
    
    number=1
    until [ $number -gt $1 ]
    do
      echo  $number
      (( number += 1))
    done
    
    $ ./count_until.sh 10
    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    
    $

while loops are used much more often ...
than until loop

`continue`

Normally, a loop will run through all the commands ...
between do and done ...
for each pass through the loop
Sometimes, however, you want to skip ...
all or part of the loop commands ...
for a specific pass through the loop
Let's say you are calculating interest ...
for a group of savings accounts
In doing this you perform a series of operations ...
on all savings accounts
But if you get to an account that has been closed ...
you don't want to perform these operations
In other words, you want to stop working on a specific account ...
but continue looping through the other accounts
It is for situations like this that continue was created
When the shell comes to continue inside a loop ...
it stops running the loop code ...
and jumps to the top of the loop ...
to begin another pass through the loop

Here is an example

    $ cat continue.sh
    #! /bin/bash
    #
    # demonstrates how continue works
    
    total=0    
    for number in 1 2 3 4 5 
    do
        if [ $number -eq 2 -o $number -eq 4  ]
        then  
            continue
        fi    
        echo Adding $number to $total
        (( total += $number ))
    done
    echo 
    echo total: $total
    
    $ ./continue.sh
    Adding 1 to 0
    Adding 3 to 1
    Adding 5 to 4

    total: 9

Whenever the variable number is 2 or 4, ...
execution of the rest of the code stops ...
and the next pass through the loop begins
continue does not cause the script ...
to break out of the loop ...
it merely stops execution ...
of the loop code for one iteration

`break`

Every time you start a loop ...
you specify what will cause the loop to end
With for ... in and simple for loops ...
the code exits the loop ...
when every value in the argument list ...
has been used
In the while, until, and three statement for loops ...
the code exits the loop when a logical condition is met
In all cases ...
the terminating condition is specified ...
at the top of the loop
But what if you encountered some unusual condition ...
and wanted to break out of the loop entirely?
To do this, you would have to use break
When bash comes across the break ...
in the code inside a loop ...
it jumps out of the loop completely ...
and proceeds with the commands following the loop

Let's look at an example

    $ cat break.sh
    #! /bin/bash
    #
    # demonstrates how break works

    for filename in *
    do
        if  [ -x $filename ]
        then
            echo First executable file: $filename
            break
        fi
    done
    
    $ ./break.sh
    First executable file: bother.sh

This script looks at each file in the current directory
When it finds a file that is executable,
it prints that filename and stops
The loop will end either
- When it finds an executable file
- Or when it has examined every file ...
  and found no executable file
Notice how I got the values for the variable filename ...
in the for ... in loop
I used *
When the script is run the value of * ...
on this line of the script ...
is replaced with the name of every file or directory ...
in the current directory

Real World `for` Loop Examples

I use for loop often ...
when doing administrative work for my classes
For example, assignment_collect.sh collects all files with the .sh extension ...
from your homework directory ...
if I run it like this
```
assignment_collect.sh  sh
```
I run this script to collect your homework assignments ...
from the appropriate homework directory in your class directory
But sometimes this script collects more than I want
For example homework 5 asked you to copy the file work.sh ...
to your homework directory
This means that when I collect this homework assignment I will get work.sh ...
as well as your hw5.sh file
Since I commit all your homework assignments to my Supathersion repository ...
I want to remove all work.sh files ...
before running the commit command
I can do this with the following for loop ...

which I enter at the command line

$ for id in $lncl
> do
> rm $id/work.sh
> done

The shell variable lncl ...
contains the username of all my IT 244 students ...

as you can see from the following for loop

$ for id in $lncl
> do
> echo $id
> done

adelasse
ali
andre001
ashish1
...

When I want to get the latest versions of all files I have backed up in Supathersion ...

I run the script upsvn.sh

cat -n upsvn.sh
     1	pushd $wksp &> /dev/null
     2	for entry in *
     3	do
     4	    if [ -d $entry  ]
     5	    then
     6	    	echo $entry
     7	    	cd $entry
     8	        svn update
     9	        cd ..    	
    10		fi
    11	done
    12	echo notes
    13	cd $notes
    14	svn update
    15	popd &> /dev/null

The first line takes me to the directory where I keep many of my files
The second line begins a for loop using the variable entry
Each time through the loop, entry gets a pathname ...
of something in this directory
Line 4 starts an if statement ...
whose body will be executed only if entry is a directory
Line 7 goes into this directory ...
where I run the Supathersion update command ...
and then pop back up to the parent directory ...
for the next pass through the loop
The last few lines update another directory

Today's Topics

Review

New Material

Homework 11

Final Exam

Review

Using test in Scripts

Checking the Arguments to a Script

The if ... then ... else ... Construct

The if ... then ... elif ... Construct

Debugging Scripts

New Material

for ... in ... Loops

for Loops

Three Statement for Loops

while Loops

until Loops

continue

break

Real World for Loop Examples

Attendance

Class Quiz

Using `test` in Scripts

The `if ... then ... else ...` Construct

The `if ... then ... elif ...` Construct

`for ... in ...` Loops

`for` Loops

Three Statement `for` Loops

`while` Loops

`until` Loops

`continue`

`break`

Real World `for` Loop Examples