nu je ico logo waar maken >:)<blockquote><font size="1" face="verdana, arial, helvetica">code:</font><hr><font face="courier, fixedsys, lucida console"><nobr> BASH Programming - Introduction HOW-TO
by Mike G mikkey at dynamo.com.ar
Thu Jul 27 09:36:18 ART 2000
This article intends to help you to start programming basic-inter
mediate shell scripts. It does not intend to be an advanced docu
ment (see the title). I am NOT an expert nor guru shell program
mer. I decided to write this because I'll learn a lot and it might
be useful to other people. Any feedback will be apreciated, spe
cially in the patch form :)
______________________________________________________________________
Table of Contents
1. Introduction
1.1 Getting the latest version
1.2 Requisites
1.3 Uses of this document
2. Very simple Scripts
2.1 Traditional hello world script
2.2 A very simple backup script
3. All about redirection
3.1 Theory and quick reference
3.2 Sample: stdout 2 file
3.3 Sample: stderr 2 file
3.4 Sample: stdout 2 stderr
3.5 Sample: stderr 2 stdout
3.6 Sample: stderr and stdout 2 file
4. Pipes
4.1 What they are and why you'll want to use them
4.2 Sample: simple pipe with sed
4.3 Sample: an alternative to ls -l *.txt
5. Variables
5.1 Sample: Hello World! using variables
5.2 Sample: A very simple backup script (little bit better)
5.3 Local variables
6. Conditionals
6.1 Dry Theory
6.2 Sample: Basic conditional example if .. then
6.3 Sample: Basic conditional example if .. then ... else
6.4 Sample: Conditionals with variables
7. Loops for, while and until
7.1 For sample
7.2 C-like for
7.3 While sample
7.4 Until sample
8. Functions
8.1 Functions sample
8.2 Functions with parameters sample
9. User interfaces
9.1 Using select to make simple menus
9.2 Using the command line
10. Misc
10.1 Reading user input with read
10.2 Arithmetic evaluation
10.3 Finding bash
10.4 Getting the return value of a program
10.5 Capturing a commands output
10.6 Multiple source files
11. Tables
11.1 String comparison operators
11.2 String comparison examples
11.3 Arithmetic operators
11.4 Arithmetic relational operators
11.5 Useful commands
12. More Scripts
12.1 Applying a command to all files in a directory.
12.2 Sample: A very simple backup script (little bit better)
12.3 File re-namer
12.4 File renamer (simple)
13. When something goes wrong (debugging)
13.1 Ways Calling BASH
14. About the document
14.1 (no) warranty
14.2 Translations
14.3 Thanks to
14.4 History
14.5 More resources
______________________________________________________________________
1. Introduction
1.1. Getting the latest version
http://www.linuxdoc.org/HOWTO/Bash-Prog-Intro-HOWTO.html
1.2.
Requisites
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.
1.3.
Uses of this document
This document tries to be useful in the following situations
· You have an idea about programming and you want to start coding
some shell scripts.
· You have a vague idea about shell programming and want some sort of
reference.
· You want to see some shell scripts and some comments to start
writing your own
· You are migrating from DOS/Windows (or already did) and want to
make "batch" processes.
· You are a complete nerd and read every how-to available
2.
Very simple Scripts
This HOW-TO will try to give you some hints about shell script
programming strongly based on examples.
In this section you'll find some little scripts which will hopefully
help you to understand some techniques.
2.1.
Traditional hello world script
#!/bin/bash
echo Hello World
This script has only two lines. The first indicates the system which
program to use to run the file.
The second line is the only action performed by this script, which
prints 'Hello World' on the terminal.
If you get something like ./hello.sh: Command not found. Probably the
first line '#!/bin/bash' is wrong, issue whereis bash or see
2.2.
A very simple backup script
#!/bin/bash
tar -cZf /var/my-backup.tgz /home/me/
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.
3.
All about redirection
3.1.
Theory and quick reference
There are 3 file descriptors, stdin, stdout and stderr (std=standard).
Basically you can:
1. redirect stdout to a file
2. redirect stderr to a file
3. redirect stdout to a stderr
4. redirect stderr to a stdout
5. redirect stderr and stdout to a file
6. redirect stderr and stdout to stdout
7. redirect stderr and stdout to stderr
1 'represents' stdout and 2 stderr.
A little note for seeing this things: with the less command you can
view both stdout (which will remain on the buffer) and the stderr that
will be printed on the screen, but erased as you try to 'browse' the
buffer.
3.2. Sample: stdout 2 file
This will cause the ouput of a program to be written to a file.
ls -l > ls-l.txt
Here, a file called 'ls-l.txt' will be created and it will contain
what you would see on the screen if you type the command 'ls -l' and
execute it.
3.3. Sample: stderr 2 file
This will cause the stderr ouput of a program to be written to a file.
grep da * 2> grep-errors.txt
Here, a file called 'grep-errors.txt' will be created and it will con
tain what you would see the stderr portion of the output of the 'grep
da *' command.
3.4.
Sample: stdout 2 stderr
This will cause the stderr ouput of a program to be written to the
same filedescriptor than stdout.
grep da * 1>&2
Here, the stdout portion of the command is sent to stderr, you may
notice that in differen ways.
3.5. Sample: stderr 2 stdout
This will cause the stderr ouput of a program to be written to the
same filedescriptor than stdout.
grep * 2>&1
Here, the stderr portion of the command is sent to stdout, if you pipe
to less, you'll see that lines that normally 'dissapear' (as they are
written to stderr) are being kept now (because they're on stdout).
3.6. Sample: stderr and stdout 2 file
This will place every output of a program to a file. This is suitable
sometimes for cron entries, if you want a command to pass in absolute
silence.
rm -f $(find / -name core) &> /dev/null
This (thinking on the cron entry) will delete every file called 'core'
in any directory. Notice that you should be pretty sure of what a com
mand is doing if you are going to wipe it's output.
4.
Pipes
This section explains in a very simple and practical way how to use
pipes, nd why you may want it.
4.1.
What they are and why you'll want to use them
Pipes let you use (very simple, I insist) the output of a program as
the input of another one
4.2. Sample: simple pipe with sed
This is very simple way to use pipes.
ls -l | sed -e "s/[aeio]/u/g"
Here, the following happens: first the command ls -l is executed, and
it's output, instead of being printed, is sent (piped) to the sed pro
gram, which in turn, prints what it has to.
4.3. Sample: an alternative to ls -l *.txt
Probably, this is a more difficult way to do ls -l *.txt, but it is
here for illustrating pipes, not for solving such listing dilema.
ls -l | grep "\.txt$"
Here, the output of the program ls -l is sent to the grep program,
which, in turn, will print lines which match the regex "\.txt$".
5.
Variables
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.
You have no need to declare a variable, just assigning a value to its
reference will create it.
5.1.
Sample: Hello World! using variables
#!/bin/bash
STR="Hello World!"
echo $STR
Line 2 creates a variable called STR and assigns the string "Hello
World!" to it. Then the VALUE of this variable is retrieved by putting
the '$' in at the beginning. Please notice (try it!) that if you
don't use the '$' sign, the output of the program will be different,
and probably not what you want it to be.
5.2.
Sample: A very simple backup script (little bit better)
#!/bin/bash
OF=/var/my-backup-$(date +%Y%m%d).tgz
tar -cZf $OF /home/me/
This script introduces another thing. First of all, you should be
familiarized with the variable creation and assignation on line 2.
Notice the expression If you run the script you'll notice that it runs
the command inside the parenthesis, capturing its output.
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.
Some more examples:
echo ls
echo $(ls)
5.3.
Local variables
Local variables can be created by using the keyword local.
#!/bin/bash
HELLO=Hello
function hello {
local HELLO=World
echo $HELLO
}
echo $HELLO
hello
echo $HELLO
This example should be enought to show how to use a local variable.
6.
Conditionals
Conditionals let you decide whether to perform an action or not, this
decision is taken by evaluating an expression.
6.1.
Dry Theory
Conditionals have many forms. The most basic form is: if expression
then statement where 'statement' is only executed if 'expression'
evaluates to true. evaluates to true.xs
Conditionals have other forms such as: if expression then statement1
else statement2. Here 'statement1' is executed if 'expression' is
true,otherwise
Yet another form of conditionals is: if expression1 then statement1
else if expression2 then statement2 else statement3. In this form
there's added only the "ELSE IF 'expression2' THEN 'statement2'" which
makes statement2 being executed if expression2 evaluates to true. The
rest is as you may imagine (see previous forms).
A word about syntax:
The base for the 'if' constructions in bash is this:
if [expression];
then
code if 'expression' is true.
fi
6.2.
Sample: Basic conditional example if .. then
#!/bin/bash
if [ "foo" = "foo" ]; then
echo expression evaluated as true
fi
The code to be executed if the expression within braces is true can be
found after the 'then' word and before 'fi' which indicates the end of
the conditionally executed code.
6.3.
Sample: Basic conditional example if .. then ... else
#!/bin/bash
if [ "foo" = "foo" ]; then
echo expression evaluated as true
else
echo expression evaluated as false
fi
6.4.
Sample: Conditionals with variables
#!/bin/bash
T1="foo"
T2="bar"
if [ "$T1" = "$T2" ]; then
echo expression evaluated as true
else
echo expression evaluated as false
fi
7.
Loops for, while and until
In this section you'll find for, while and until loops.
The for loop is a little bit different from other programming
languages. Basically, it let's you iterate over a series of
The 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.
The until loop is almost equal to the while loop, except that the code
is executed while the control expression evaluates to false.
If you suspect that while and until are very similar you are right.
7.1.
For sample
#!/bin/bash
for i in $( ls ); do
echo item: $i
done
On the second line, we declare i to be the variable that will take the
different values contained in $( ls ).
The third line could be longer if needed, or there could be more lines
before the done (4).
finished and $i can take a new value.
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
7.2.
C-like for
fiesh suggested adding this form of looping. It's a for loop more
similar to C/perl... for.
#!/bin/bash
for i in `seq 1 10`;
do
echo $i
done
7.3.
While sample
#!/bin/bash
COUNTER=0
while [ $COUNTER -lt 10 ]; do
echo The counter is $COUNTER
let COUNTER=COUNTER+1
done
This script 'emulates' the well known (C, Pascal, perl, etc) 'for'
structure
7.4.
Until sample
#!/bin/bash
COUNTER=20
until [ $COUNTER -lt 10 ]; do
echo COUNTER $COUNTER
let COUNTER-=1
done
8.
Functions
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.
Declaring a function is just a matter of writing function my_func {
my_code }.
Calling a function is just like calling another program, you just
write its name.
8.1.
Functions sample
#!/bin/bash
function quit {
exit
}
function hello {
echo Hello!
}
hello
quit
echo foo
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!.
Notice that a functions don't need to be declared in any specific
order.
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.
8.2.
Functions with parameters sample
#!/bin/bash
function quit {
exit
}
function e {
echo $1
}
e Hello
e World
quit
echo foo
This script is almost identically to the previous one. The main
difference is the funcion 'e'. This function, prints the first
argument it receives. Arguments, within funtions, are treated in the
same manner as arguments given to the script.
9.
User interfaces
9.1.
Using select to make simple menus
#!/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
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.
9.2. Using the command line
#!/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
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.
10.
Misc
10.1.
Reading user input with read
In many ocations you may want to prompt the user for some input, and
there are several ways to achive this. This is one of those ways:
#!/bin/bash
echo Please, enter your name
read NAME
echo "Hi $NAME!"
As a variant, you can get multiple values with read, this example may
clarify this.
#!/bin/bash
echo Please, enter your firstname and lastname
read FN LN
echo "Hi! $LN, $FN !"
10.2.
Arithmetic evaluation
On the command line (or a shell) try this:
echo 1 + 1
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:
echo $((1+1))
This will produce a&
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