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<!doctype linuxdoc system>
<!--
An introduction to bash shell programming
-->
<!-- Title information -->
<article>
<title>BASH Programming - Introduction HOW-TO</title>
<author>by Mike G <tt/mikkey@dynamo.com.ar/</author>
<date>v0.04, 31 January 2000</date>
<abstract>
This article intends to help you to start programming
basic-intermediate shell scripts. It does not intend to be an
advanced document (see the title). I am NOT an expert nor guru
shell programmer. I decided to write this because I'll learn a
lot and it might be useful to other people.
</abstract>
<!-- Table of contents -->
<toc>
<!-- Requisites -->
<sect>
Introduction
<sect1>
Getting the latest version
<P> Download latest version
<htmlurl url="http://www.digimedia.com.ar/~mike/contrib/BASH-INTRO-PROG-HOW-TO/BASH-PROG-INTRO-HOWTO.tgz"
name="http://www.digimedia.com.ar/~mike/contrib/BASH-INTRO-PROG-HOW-TO/BASH-PROG-INTRO-HOWTO.tgz">
<P> Get sgml source
<htmlurl url="http://www.digimedia.com.ar/~mike/contrib/BASH-INTRO-PROG-HOW-TO/BASH-PROG-INTRO-HOWTO.sgml"
name="http://www.digimedia.com.ar/~mike/contrib/BASH-INTRO-PROG-HOW-TO/BASH-PROG-INTRO-HOWTO.sgml">
<P> View it online
<htmlurl url="http://www.digimedia.com.ar/~mike/contrib/BASH-INTRO-PROG-HOW-TO/html/BASH-PROG-INTRO-HOWTO.html"
name="http://www.digimedia.com.ar/~mike/contrib/BASH-INTRO-PROG-HOW-TO/html/BASH-PROG-INTRO-HOWTO.html">
</sect1>
<sect1>
Requisites
<P> Familiarity with GNU/Linux command lines, and familiarity
with basic programming concepts is helpful. While this is not
a programming introduction, it explains (or at least tries) many
basic concepts.
<P>
</sect1>
<sect1>
Uses of this document
<P> This document tries to be useful in the following situations
<itemize>
<item> You have an idea about programming and you want to start
coding some shell scripts.
<item> You have a vague idea about shell programming and want
some sort of reference.
<item> You want to see some shell scripts and some comments to
start writing your own
<item> You are migrating from DOS/Windows (or already did)
and want to make "batch" processes.
<item> You are a complete nerd and read every how-to available
</itemize>
</sect1>
</sect>
<!-- Very simple Scripts -->
<sect>
Very simple Scripts
<P> This HOW-TO will try to give you some hints about shell script
programming strongly based on examples.
<P> In this section you'll find some little scripts which
will hopefully help you to understand some techniques.
<sect1>
Traditional hello world script
<!-- __TO-DO__
someone suggested using this instead of the line numbering
but it doesn't work on verion 1.0.9
<pre></pre>
-->
<P>
<tscreen><verb>
#!/bin/bash
echo Hello World!
</verb></tscreen>
<P>
<P> This script has only two lines.
The first indicates the system which program
to use to run the file.
<P> The second line is the only action performed by this script,
which prints 'Hello World! on the terminal.
</sect1>
<sect1>
A very simple backup script
<P>
<tscreen><verb>
#!/bin/bash
tar -cZf /var/my-backup.tgz /home/me/
</verb></tscreen>
<P> In this script, instead of printing a message on the terminal,
we create a tar-ball of a user's home directory. This is NOT intended
to be used, a more useful backup script is presented later in this
document.
</sect1>
</sect>
<!-- Variables -->
<sect>
Variables
<!-- Dry Theory -->
<P> You can use variables as in any programming languages.
There are no data types. A variable in bash can contain a number, a
character, a string of characters.
<P> You have no need to declare a variable, just
assigning a value to its reference will create it.
<!-- Samples -->
<sect1>
Sample: Hello World! using variables
<P>
<tscreen><verb>
#!/bin/bash
STR="Hello World!"
echo $STR
</verb></tscreen>
<P> Line 2 creates
a variable called STR and assigns the string "Hello World!" to
it. Then the VALUE of this variable is retrieved by putting
the '$' in at the beginning. Please notice (try it!)
that if you don't use the '$' sign, the output of the program will
be different, and probably not what you want it to be.
</sect1>
<sect1>
Sample: A very simple backup script (little bit better)
<P>
<tscreen><verb>
#!/bin/bash
OF=/var/my-backup-$(date +%Y%m%d).tgz
tar -cZf $OF /home/me/
</verb></tscreen>
<P> This script introduces another thing. First
of all, you should be familiarized with the variable
creation and assignation on line 2. Notice the expression
'$(date +%Y%m%d)'.
If you run the script you'll notice that
it runs the
command inside the parenthesis, capturing its output.
<P> Notice that in this script, the output filename will
be different
every day, due to the format switch to the date command(+%Y%m%d).
You can change this by specifying a different format.
<P> Some more examples:
<P> echo ls
<P> echo $(ls)
</sect1>
</sect>
<!-- Conditionals -->
<sect>
Conditionals
<P> Conditionals let you decide whether to perform an action
or not, this decision is taken by evaluating an expression.
<!-- Dry Theory -->
<sect1>
Dry Theory
<P> Conditionals have many forms. The most basic form is:
<bf/if/ <it/expression/ <bf/then/ <it/statement/
where 'statement' is only executed if 'expression'
evaluates to true.
'2<1' is an expresion that evaluates to false, while '2>1'
evaluates to true.xs
<P> Conditionals have other forms such as:
<bf/if/ <it/expression/
<bf/then/ <it/statement1/ <bf/else/ <it/statement2/.
Here 'statement1' is executed if 'expression' is true,otherwise
'statement2' is executed.
<P> Yet another form of conditionals is:
<bf/if/ <it/expression1/
<bf/then/ <it/statement1/
<bf/else if/ <it/expression2/ <bf/then/ <it/statement2/
<bf/else/ <it/statement3/.
In this form there's added only the
"ELSE IF 'expression2' THEN 'statement2'" which makes statement2 being
executed if expression2 evaluates to true. The rest is as you may
imagine (see previous forms).
<P> A word about syntax:
<P> The base for the 'if' constructions in bash is this:
<P> if [expression];
<P> then
<P> code if 'expression' is true.
<P> fi
</sect1>
<!-- Samples -->
<sect1>
Sample: Basic conditional example if .. then
<P>
<tscreen><verb>
#!/bin/bash
if [ "foo" = "foo" ]; then
echo expression evaluated as true
fi
</verb></tscreen>
<P> The code to be executed if the expression within braces
is true can
be found after the 'then' word and before 'fi' which indicates the end
of the conditionally executed code.
</sect1>
<sect1>
Sample: Basic conditional example if .. then ... else
<P>
<tscreen><verb>
#!/bin/bash
if [ "foo" = "foo" ]; then
echo expression evaluated as true
else
echo expression evaluated as false
fi
</verb></tscreen>
</sect1>
<sect1>
Sample: Conditionals with variables
<P>
<tscreen><verb>
#!/bin/bash
T1="foo"
T2="bar"
if [ $T1 = $T2 ]; then
echo expression evaluated as true
else
echo expression evaluated as false
fi
</verb></tscreen>
</sect1>
</sect>
<!-- Loops : for, while and until -->
<sect>
Loops for, while and until
<!-- Dry Theory -->
<P> In this section you'll find for, while and until loops.
<P> The <bf/for/ loop is a little bit different from other programming
languages. Basically, it let's you iterate over a series of
'words' within a string.
<P> The <bf/while/ executes a piece of code if the control expression
is true, and only stops when it is false (or a explicit break is found
within the executed code.
<P> The <bf/until/ loop is almost equal to the while loop, except that
the code is executed while the control expression evaluates to false.
<P> If you suspect that while and until are very similar you are right.
<!-- Samples -->
<sect1>
For sample
<P>
<tscreen><verb>
#!/bin/bash
for i in $( ls ); do
echo item: $i
done
</verb></tscreen>
<p>
<p> On the second line, we declare i to be the variable that
will take the
different values contained in $( ls ).
<p> The third line could be longer if needed, or there could
be more lines
before the done (4).
<p> 'done' (4) indicates that the code that used the value of $i has
finished and $i can take a new value.
<p> This script has very little sense, but a more useful way to use the
for loop would be to use it to match only certain files on the previous
example
<p>
</sect1>
<!-- while -->
<sect1>
While sample
<P> <tscreen><verb>
#!/bin/bash
COUNTER=0
while [ $COUNTER -lt 10 ]; do
echo The counter is $COUNTER
let COUNTER=COUNTER+1
done
</verb></tscreen>
<P>
<P> This script 'emulates' the well known
(C, Pascal, perl, python, etc) 'for' structure
</sect1>
<!-- until -->
<sect1>
Until sample
<P> <tscreen><verb>
#!/bin/bash
COUNTER=20
until [ $COUNTER -lt 10 ]; do
echo COUNTER $COUNTER
let COUNTER-=1
done
</verb></tscreen>
</sect1>
</sect>
<!-- Functions -->
<sect>
Functions
<P> As in almost any programming language, you can use functions to group
pieces of code in a more logical way or practice the divine art of recursion.
<P> Declaring a function is just a matter of writing function my_func { my_code }.
<P> Calling a function is just like calling another program, you just write its name.
<P>
<sect1>
Functions sample
<P> <tscreen><verb>
1) #!/bin/bash
2) function quit {
3) exit
4) }
5) function hello {
6) echo Hello!
7) }
8) hello
9) quit
10) echo foo
</verb></tscreen>
<P> Lines 2-4 contain the 'quit' function. Lines 5-7 contain the 'hello' function
If you are not absolutely sure about what this script does, please try it!.
<P> Notice that a functions don't need to be declared in any specific order.
<P> When running the script you'll notice that first: the function 'hello' is
called, second the 'quit' function, and the program never reaches line 10.
</sect1>
</sect>
<!-- User interfaces -->
<sect>
User interfaces
<!-- Simple menus with select -->
<sect1>
Using select to make simple menus
<P>
<tscreen><verb>
#!/bin/bash
OPTIONS="Hello Quit"
select opt in $OPTIONS; do
if [ $opt = "Quit" ]; then
echo done
exit
elif [ $opt = "Hello" ]; then
echo Hello World
else
clear
echo bad option
fi
done
</verb></tscreen>
<P> If you run this script you'll see that it is a
programmer's dream for text based menus. You'll probably notice
that it's very similar to the 'for' construction, only rather
than looping for each 'word' in $OPTIONS, it prompts the user.
<P>
</sect1>
<!-- Using the command line -->
<sect1>
Using the command line
<P>
<tscreen><verb>
#!/bin/bash
if [ -z $1 ]; then
echo usage: $0 directory
exit
fi
SRCD=$1
TGTD="/var/backups/"
OF=home-$(date +%Y%m%d).tgz
tar -cZf $TGTD$OF $SRCD
</verb></tscreen>
<P>
<P> What this script does should be clear to you. The expression
in the first conditional tests if the program has received an argument
($1) and quits if it didn't, showing the user a little usage message.
The rest of the script should be clear at this point.
</sect1>
</sect>
<!-- Misc -->
<sect>
Misc
<!-- Reading user input -->
<sect1>
Reading user input
<P> __TO-DO__
</sect1>
<!-- Arithmetic evaluation -->
<sect1>
Arithmetic evaluation
<P> On the command line (or a shell) try this:
<P> echo 1 + 1
<P> If you expected to see '2' you'll be disappointed. What if
you want BASH to evaluate some numbers you have? The solution
is this:
<P> echo $((1+1))
<P> This will produce a more 'logical' output. This is to evaluate an
arithmetic expression. You can achieve this also like this:
<P> echo $[1+1]
<P>
<P> If you need to use fractions, or more math or you just want it, you
can use bc to evaluate arithmetic expressions.
<P> if i ran "echo $[3/4]" at the command prompt, it would return 0
because bash only uses integers when answering. If you ran
"echo 3/4|bc -l", it would properly return 0.75.
</sect1>
<!-- Capturing a commands output -->
<sect1>
Capturing a commands output
<P> __TO-DO__
</sect1>
<!-- Multiple source files -->
<sect1>
Multiple source files
<P> __TO-DO__
</sect1>
</sect>
<!-- Tables -->
<sect>
Tables
<!-- String operators -->
<sect1>
String comparison operators
<P> (1) s1 = s2
<P> (2) s1 != s2
<P> (3) s1 < s2
<P> (4) s1 > s2
<P> (5) -n s1
<P> (6) -z s1
<P>
<P> (1) s1 matches s2
<P> (2) s1 does not match s2
<P> (3) __TO-DO__
<P> (4) __TO-DO__
<P> (5) s1 is not null (contains one or more characters)
<P> (6) s1 is null
</sect1>
<!-- Arithmetic operators -->
<sect1>
Arithmetic operators
<P> +
<P> -
<P> *
<P> /
<P> % (reminder)
</sect1>
<!-- Arithmetic relational operators -->
<sect1>
Arithmetic relational operators
<P> -lt (<)
<P> -gt (>)
<P> -le (<=)
<P> -ge (>=)
<P> -eq (==)
<P> -ne (!=)
<P> C programmer's should simple map the operator to its corresponding
parenthesis.
</sect1>
<!-- Useful commands -->
<sect1>
Useful commands
<P>
<P> sed (stream editor - very useful)
<P> gawk
<P> grep (show line matching this or not matching that)
<P> wc (count words, lines)
<P> sort
<P> bc (more than a calculator)
<P> cut (edit columns)
<P> It it higly recommended to be familiarized with
this programs (at least). There are tons of little
programs that will let you do real magic in a
command line.
</sect1>
</sect>
<!-- More Scripts -->
<sect>
More Scripts
<sect1>
Applying a command to all files in a directory.
<P>
</sect1>
<sect1>
Sample: A very simple backup script (little bit better)
<P>
<tscreen><verb>
#!/bin/bash
SRCD="/home/"
TGTD="/var/backups/"
OF=home-$(date +%Y%m%d).tgz
tar -cZf $TGTD$OF $SRCD
</verb></tscreen>
<P>
</sect1>
<sect1>
File re-namer
<P>
<tscreen><verb>
#!/bin/sh
# renna: rename multiple files according to several rules
# written by felix hudson Jan - 2000
#first check for the various 'modes' that this program has
#if the first ($1) condition matches then we execute that portion of the
#program and then exit
# check for the prefix condition
if [ $1 = p ]; then
#we now get rid of the mode ($1) variable and prefix ($2)
prefix=$2 ; shift ; shift
# a quick check to see if any files were given
# if none then its better not to do anything than rename some non-existent
# files!!
if [$1 = ]; then
echo "no files given"
exit 0
fi
# this for loop iterates through all of the files that we gave the program
# it does one rename per file given
for file in $*
do
mv ${file} $prefix$file
done
#we now exit the program
exit 0
fi
# check for a suffix rename
# the rest of this part is virtually identical to the previous section
# please see those notes
if [ $1 = s ]; then
suffix=$2 ; shift ; shift
if [$1 = ]; then
echo "no files given"
exit 0
fi
for file in $*
do
mv ${file} $file$suffix
done
exit 0
fi
# check for the replacement rename
if [ $1 = r ]; then
shift
# i included this bit as to not damage any files if the user does not specify
# anything to be done
# just a safety measure
if [ $# -lt 3 ] ; then
echo "usage: renna r [expression] [replacement] files... "
exit 0
fi
# remove other information
OLD=$1 ; NEW=$2 ; shift ; shift
# this for loop iterates through all of the files that we give the program
# it does one rename per file given using the program 'sed'
# this is a sinple command line program that parses standard input and
# replaces a set expression with a give string
# here we pass it the file name ( as standard input) and replace the nessesary
# text
for file in $*
do
new=`echo ${file} | sed s/${OLD}/${NEW}/g`
mv ${file} $new
done
exit 0
fi
# if we have reached here then nothing proper was passed to the program
# so we tell the user how to use it
echo "usage;"
echo " renna p [prefix] files.."
echo " renna s [suffix] files.."
echo " renna r [expression] [replacement] files.."
exit 0
# done!
</verb></tscreen>
</sect1>
<sect1>
File renamer (simple)
<p>
<tscreen><verb>
#!/bin/bash
# renames.sh
# basic file renamer
criteria=$1
re_match=$2
replace=$3
for i in $( ls *$criteria* );
do
src=$i
tgt=$(echo $i | sed -e "s/$re_match/$replace/")
mv $src $tgt
done
</verb></tscreen>
</sect1>
</sect>
<sect>
When something goes wrong (debugging)
<sect1>
Ways Calling BASH
<p> A nice thing to do is to add on the first line
<tscreen><verb>
#!/bin/bash -x
</verb></tscreen>
<P> This will produce some intresting output information
</sect1>
</sect>
<sect>
About the document
<P> Feel free to make suggestions/corrections, or whatever you think
it would be interesting to see in this document. I'll try to update
it as soon as I can.
<sect1>
(no) warranty
<P> This documents comes with no warranty of any kind.
and all that
</sect1>
<sect1>
Thanks to
<P>
<itemize>
<item> Nathan Hurst for sending a lot of corrections.
<item> Jon Abbott for sending comments about evaluating arithmetic expressions.
<item> Laurent Martelli for translating this document to French (soon here the URL)
<item> Felix Hudson for writing the <it/renna/ script
</itemize>
</sect1>
<sect1>
More resources
<P>
<P> Introduction to bash (under BE)
<htmlurl http://org.laol.net/lamug/beforever/bashtut.htm
<P> Bourne Shell Programming
http://207.213.123.70/book/
</sect1>
</sect>
</article>