Arrays in bash

Hello and welcome to this weeks installment of Practical shell scripting. This week I'm going to tackle something which is common in most coding languages, but it's handled a little differently in bash shell scripts, that is arrays. Hopefully most of you know that an arrays is a collection of like pieces of data. The data could be integers, or strings, or floats or whatever. For those of who come from a C/C++ background there are several things which are a little on the annoying side about using arrays in shell script. Firstly bash will allow you to mix data types that's because bash and I believe Bourne and Korn as well are what's referred to as loosely typed languages. So for instance in a language like C/C++ you define an arrays being of a specific data type. For example:

int MyNumbers[5]={1,3,5,7,11};

is a single dimensional array which will hold 5 integers, but if you try and introduce other variable types you will fail to compile. Bash on the other hand will you get away with a lot more. Also in Bash it's perfectly acceptable to declare your array without any predefined size, or variable type. So for instance in bash an array might look like this:

SomeAray="1 3 5 7 eleven";

However, the nice thing about bash arrays is that they follow a similar syntax regarding the syntax of the indexes, and like arrays in C/C++ they both start at 0. The only other tricky thing to remember is that in bash arrays are referenced using ${ }. As opposed to C/C++ where the array is just referenced directly. So to represent the fifth element of the C/C++ array you would refer to it as:

MyNumbers[4];

While in Bash you would reference the fifth element as:

${SomeArray[4]}

There are also a few nifty little things about bash arrays that I'm just starting to explore myself, but here's some code to play with to give you some ideas of what you can do with bash arrays.

#!/bin/bash

##-----------------------------------------------------------------##
##
## A simple shell script to give examples of arrays
##
##-----------------------------------------------------------------##


##--- example one ----##
echo "EXAMPLE 1: array of integers"
Array=(1 3 5 7 eleven)

echo "Here's the values"
for value in ${Array[*]}
do
echo "value is $value"
done


echo ""
echo "Here's the index"
for index in ${!Array[*]}
do
echo "index is $index"
done


echo ""
echo "Here's the index and it's corresponding value"
for index in ${!Array[*]}
do
echo "index is $index, value is ${Array[$index]}"
done


##--- example two ----##
echo ""
echo "So the value at index 2 is ${Array[2]}"
echo ""
echo "EXAMPLE 2: array of strings "
NewArray=(Apples Oranges Pears Beans 99)
echo "NewArray[0] is ${NewArray[0]}"
echo "NewArray[1] is ${NewArray[1]}"
echo "NewArray[2] is ${NewArray[2]}"
echo "NewArray[3] is ${NewArray[3]}"
echo "NewArray[4] is ${NewArray[4]}"



Notice how you can mix and match strings in with integers in the array and bash doesn't even care. There's a couple of other neat tricks that you can do with arrays, but I'll save that for next weeks entry. That's it for this week, as always if you have any questions or comments please feel free to contact me and Happy coding.

Condition testing and braces

Hello and welcome to this weeks edition of Practical Shell Scripting. Recently I was handed a project to work on that was a rewrite of some perl code into shell script and I came across some new tricks that I was unaware of and I wanted to share some of them with you. In particular testing for conditions using metacharacters. I'm sure that all of you are aware of the simple test condition statements using brackets such as:

if [ -f foobar ]; then

which will test for the existence of a the file foobar, but something that you might not be aware of, and something that I was was previously unaware of is that bash can also do much more complex metacharacter testing through the use of double brackets. ie: [[

Here's an example of a simple test using the common metachacter ^.

##----------------------------------------------------------------##
#!/bin/bash

STRING_TO_READ="foobar"

if [ $STRING_TO_READ =~ ^foo ]; then
echo "We have foo"
else
echo "no foo here"
fi

## Note the double braces in the test condition statement ##
if [[ $STRING_TO_READ =~ ^foo ]]; then
echo "We have foo"
else
echo "no foo here"
fi

##--------------------------------------------------------------------------------------------##

For those unfamiliar with metacharacters the ^ symbol is used to specify the something at the start of a line. So here's the tricky part.... by taking a regular set of test brackets an adding a second pair of brackets around them you now have the capability of doing any of the functions that you could do in say sed of other unix tools. I'm just starting to play with this, but I'm sure that this tool will prove incredibly versatile in the future.

That's it for this week, be sure and drop me a line if you have any questions or comments. Happy coding

Simple loops and counters in bash

Welcome to this weeks issue of Practical Shell Scripting. Last week I gave an example and explanation of a shell script that did some reading from a text file in a line by line fashion. This week I'm going to give a discussion of basic looping and simple math comparison. First lets look at the basics of doing simple math and comparison testing. Then I'll give a real world example of when this stuff might be used.

One of the confusing things for me is the syntax that bash uses for incrementing. The basic construct is just like that of basic, and I think maybe pascal and cobol, but it's been a while
since I coded in those languages. Specifically bash uses the word let to carry out arithmetic operations. However, here's where it gets a little funky. Instead of a simple statement like

let A=A+1

Which is how this is done in most formal languages. However bash uses an expression like this:

let $((COUNTER=$COUNTER+1))

In order to get this right and make it work the $(( on the left side must be just like that and the variable COUNTER must be next to them. While the second half must be $COUNTER, referencing the variable as one normally would. If the syntax is not exactly like this you can pretty much count on having some problems.

How and why this works is something that I have no idea. I only know that it does, perhaps someday in my travels this will be revealed to me, but for now I'm willing to take it for granted
that this is simply how we do things in bash. There is another syntax for doing math that I have come across in some bash scripts, however I have found that it doesn't always work with every version of bash and the example that I have given seems to be very portable across older and newer version.

I have one more topic that I need to bring up here before we get into the whole code example, and that is comparison operators. Bash uses a collection of acronym like operators to do comparison, you see the same syntax in perl. However, for us C/C++ guys it looks pretty strange. The basic operators are:

-eq ##equal to
-ne ## not equal to
-le ## less than or equal to
-lt ## less than
-gt ## greater than
-ge ## greater than or equal to

Now here's an example of a very simple while loop that uses both a math variable that gets incremented and a comparison operator to break the loop.

#!/bin/bash
##---------------------------------------------------------------------##
##
## This is a simple looping example with a counter and
## a math comparison operator.
##
##
##---------------------------------------------------------------------##

COUNTER=0
MAX_SIZE=10

while [ $COUNTER -lt $MAX_SIZE ]
do
let $((COUNTER=$COUNTER+1))
echo $COUNTER
sleep 1
done


Now I'd like to share a real world example that I had to write a few years back to do some house cleaning in a directory to make sure that we didn't overflow the drive. The company I work does automated nightly builds using a cron job to make sure that the coders didn't break anything during the days work. Also, there are additional day builds in the same directory to test quick fixes and branch merges. Each build takes up around 100 to 150 megabytes of space. So the problem turns up when after a while the builds end up taking up the whole drive and then at some point the next nightly build fails and some one has to go about the time consuming and tedious job of purging off the old builds. So we came up with a rule that we would keep the most current 10 builds and all the previous ones would be deleted. If anyone wanted a specific build to be saved off that was his responsibility to do so. The house-cleaning script would be run before the regular builds were run to make sure that there was enough room.

#!/bin/bash
# this is the house-cleaning.sh
# it makes sure that there are not a lot
# of silly old bulds laying around taking up
# space
#
#
#-----------------------------------------------------
#
# $Id: $
# $Change: $
# $Author: tglaser $
# $Date: 2008/11/06 $
# $NoKeyword: $
#
#
#-------------------------------------------------------

HOME=/home/tglaser
DERIVEDDIR=$HOME/BuildDir

cd $DERIVEDDIR

pwd

counter=0
too_many_files=5


for file in `ls -t`
do
echo $file
let $((counter=$counter+1))
if [ $counter -gt $too_many_files ]; then
echo "there are $counter files or directories"
echo "So we are going to delete $file";
rm -rf $file;
else
echo "you've still got room to work here"
echo "No need to delete anything yet"
fi

echo " "
echo " "
sleep 3
done

cd $HOME

exit

##----------------------------------------------------##



The only important difference between this script and the first one is the for loop which I will go into in one of the next issues. So that's it for this week. As always if you have any questions or comments please feel free to contact me. Happy coding.

Reading a file line by line using bash

Welcome to this weeks edition of practical shell scripting. This week I'm going to give a small simple example of a how to read a file one line at a time using bash. As with my other previous and hopefully future blog entries I'm trying to keep this down to just a single small topic and try to give a good example of it without clouding it with a lot of other things.

What prompted this weeks entry was a side project I had over the weekend to update one of my servers at home that required me to download a large number of files from an http server. However, I did not want to go to each file one at a time and click on it and download through the web so I viewed the source of the html file, saved it, and stripped out the html until I had a file that was nothing more than a list of all the files that I wanted to download. Once I had that file I fed it into a much more complicated script that retrieved those files from the http server using wget.

For the sake of this example we're going to be looking at just a tiny part of that and I've turned it into a generic example to show how bas reads line by line from a file called grocery.list.

First, here's what's in the file called grocery.list :

apples
oranges
bananas
pears
juice

Now here's the bash script to read it:

#!/bin/bash
###-------------------------------------------------###
#
# this is a simple bash script to show
# how to read from a file
#
###-------------------------------------------------###

INPUT_FILE="$1"

while read curline; do
echo $curline

done < "$INPUT_FILE"



....and that's all there is to it. So what we have here is a simple looping construct that goes one line at a time and steps through the input file that is given as the first argument or $1. In his example "curline" is the variable that the data is read into. This can be confusing because "curline" does not need to be declared a head of time, and in fact in all the shell scripts I have seen I can not recall a single instance of ever being declared up front. The other thing that has always been a little counter intuitive to me is that file being read from is always at the bottom of the while loop which is taking it's input in the done line. Most of us who come from a more formal language like C or C++ are used to having the file declared and read from someplace closer to the top, and this always seems odd to me.


So to execute this example you would type:
./ReadLineExample.sh grocery.list

And the screen should show exactly what is in the file. While there are a great many tools in bash for parsing and working with files this is one of the most versatile since it allows simple usage of a looping construct.

So that's it for this week. As always if you have any questions or comments please feel free to contact me. Happy coding.