LDP/LDP/howto/linuxdoc/Bash-Prog-Intro-HOWTO.sgml

<!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 at dynamo.com.ar/</author>
    <date>
	Thu Jul 27 09:36:18 ART 2000
    </date>
    <abstract>
    This article is intended 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. Any feedback is
    appreciated, especially in the patch or pull request form. :)
    </abstract>

<!-- Table of contents -->
<toc>

<!-- Requisites -->
<sect>
Introduction


<sect1>
Getting the latest version

<p>
<htmlurl url="https://www.tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html"
     name="https://www.tldp.org/HOWTO/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.
	<P> If you get something like <it>./hello.sh: Command not found.</it>
	Probably the first line '#!/bin/bash' is wrong, issue whereis bash or see
	'finding bash' to see how should you write this line.
    </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>

<!-- Redirection -->
<sect>
All about redirection

	<sect1>
	Theory and quick reference
	<p> There are 3 file descriptors, stdin, stdout and stderr (std=standard).

	<p>Basically you can:
	<enum>
	<item> redirect stdout to a file
	<item> redirect stderr to a file
	<item> redirect stdout to a stderr
	<item> redirect stderr to a stdout
	<item> redirect stderr and stdout to a file
	<item> redirect stderr and stdout to stdout
	<item> redirect stderr and stdout to stderr
	</enum>
	1 'represents' stdout and 2 stderr.
	<p> A little note for seeing the output of these commands: 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.
	</sect1>

	<sect1>
	Sample: stdout 2 file
	<p> This will cause the output of a program to be written to a file.
	<tscreen><verb>
	ls -l > ls-l.txt
	</verb></tscreen>
	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.
	</sect1>

	<sect1>
	Sample: stderr 2 file
	<p> This will cause the stderr output of a program to be written to a file.
	<tscreen><verb>
	grep da * 2> grep-errors.txt
	</verb></tscreen>
	Here, a file called 'grep-errors.txt' will be created and it will contain what you would see
	the stderr portion of the output of the 'grep da *' command.
	</sect1>

	<sect1>
	Sample: stdout 2 stderr
	<p> This will cause the stdout output of a program to be written to the same filedescriptor
	as stderr.
	<tscreen><verb>
	grep da * 1>&2
	</verb></tscreen>
	Here, the stdout portion of the command is sent to stderr, you may notice that in different ways.
	</sect1>

	<sect1>
	Sample: stderr 2 stdout
	<p> This will cause the stderr output of a program to be written to the same filedescriptor
	as stdout.
	<tscreen><verb>
	grep * 2>&1
	</verb></tscreen>
	Here, the stderr portion of the command is sent to stdout, if you pipe to less, you'll see that
	lines that normally 'disappear' (as they are written to stderr) are being kept now (because
	they're on stdout).
	</sect1>

	<sect1>
	Sample: stderr and stdout 2 file
	<p> 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.
	<tscreen><verb>
	rm -f $(find / -name core) &> /dev/null
	</verb></tscreen>
	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 command is doing if you are going to wipe its output.
	</sect1>
<!--
redirect stderr and stdout to stdout
redirect stderr and stdout to stderr
-->



<!--
	<sect1>
	Sample:
	<p> This will
	<tscreen><verb>
	command
	</verb></tscreen>
	Explanation
	</sect1>
-->

</sect>

<!-- pipes -->
<sect>
Pipes
	<p> This section explains in a very simple and practical way how to use pipes,
	and why you may want to use them.

	<sect1>
	What they are and why you'll want to use them
		<p> Pipes let you use (very simple, I insist) the output of a program as the
		input of another one
	</sect1>


	<sect1>
	Sample: simple pipe with sed
	<p> This is very simple way to use pipes.
	<tscreen><verb>
	ls -l | sed -e "s/[aeio]/u/g"
	</verb></tscreen>
	Here, the following happens: first the command ls -l is executed, and its output,
	instead of being printed, is sent (piped) to the sed program, which in turn, prints
	what it has to.
	</sect1>

	<sect1>
	Sample: an alternative to ls -l *.txt
	<p> Probably, this is a more difficult way to do ls -l *.txt, but it is here for illustrating pipes,
	not for solving such listing dilemma.
	<tscreen><verb>
	ls -l | grep "\.txt$"
	</verb></tscreen>
	Here, the output of the program ls -l is sent to the grep program, which, in turn, will print
	lines which match the regex "\.txt$".
	</sect1>



<!--
	<sect1>
	Sample:
	<p> This will
	<tscreen><verb>
	command
	</verb></tscreen>
	Explanation
	</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>

	<sect1>
	Local variables

	<p> Local variables can be created by using the keyword <it/local/.

	<tscreen><verb>
		#!/bin/bash
		HELLO=Hello
		function hello {
			local HELLO=World
			echo $HELLO
		}
		echo $HELLO
		hello
		echo $HELLO
	</verb></tscreen>
	<p> This example should be enought to show how to use a local variable.
	</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</bf> <it/expression/ <bf>then</bf> <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>
<!--
	<sect1>
	Sample: testing if a file exists
	    <P> one more thank to mike
	    <tscreen><verb>
	    #!/bin/bash
	    FILE=~/.basrc
	    if [ -f $FILE ]; then
	        echo file $FILE exits
	    else
	        echo file not found
	    fi
	    if [ 'test -f $FILE']
	    </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>
	<sect1>
	C-like for
	<P> fiesh suggested adding this form of looping. It's a for loop
	more similar to C/perl... for.
	<tscreen><verb>
	#!/bin/bash
	for i in `seq 1 10`;
	do
		echo $i
	done
	</verb></tscreen>
	</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, 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>
	   #!/bin/bash
	   function quit {
	       exit
	   }
	   function hello {
	       echo Hello!
	   }
	   hello
	   quit
	   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 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>

     <sect1>
     Functions with parameters sample
           <P> <tscreen><verb>
		#!/bin/bash
		function quit {
 		   exit
		}
		function e {
		    echo $1
		}
		e Hello
		e World
		quit
		echo foo

	   </verb></tscreen>
	   <P> This script is almost identical to the previous one. The main difference is the
		function 'e'. This function, prints the first argument it receives.
		Arguments, within functions, are treated in the same manner as arguments given to the script.
     </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 with read
         <P> In many occasions you may want to prompt the user for some input, and
		there are several ways
		to achive this. This is one of those ways:
	<tscreen><verb>
		#!/bin/bash
		echo Please, enter your name
		read NAME
		echo "Hi $NAME!"
	</verb></tscreen>
	<P> As a variant, you can get multiple values with read, this example may
	clarify this.
	<tscreen><verb>
		#!/bin/bash
		echo Please, enter your firstname and lastname
		read FN LN
		echo "Hi! $LN, $FN !"
	</verb></tscreen>

    </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>


    <sect1>
    Finding bash
        <P> From a message from mike (see Thanks to)
        <P> you always use #!/bin/bash .. you might was to give an example of
        <P> how to find where bash is located.
        <P> 'locate bash' is preferred, but not all machines have locate.
        <P> 'find ./ -name bash' from the root dir will work, usually.
        <P> Suggested locations to check:
        <P>         ls -l /bin/bash
        <P>         ls -l /sbin/bash
        <P>         ls -l /usr/local/bin/bash
        <P>         ls -l /usr/bin/bash
        <P>         ls -l /usr/sbin/bash
        <P>         ls -l /usr/local/sbin/bash
        <P> (can't think of any other dirs offhand...  i've found it in
        <P> most of these places before on different system).
	<P> You may try also 'which bash'.
   </sect1>

    <!-- Capturing a commands output  -->
    <sect1>
    Getting the return value of a program
        <P> In bash, the return value of a program is stored in a special variable called $?.
	<P> This illustrates how to capture the return value of a program, I assume that the directory
	<it/dada/ does not exist. (This was also suggested by mike)
	<tscreen><verb>
	#!/bin/bash
	cd /dada &> /dev/null
	echo rv: $?
	cd $(pwd) &> /dev/null
	echo rv: $?
	</verb> </tscreen>
    </sect1>


    <!-- Capturing a commands output  -->
    <sect1>
    Capturing a commands output
         <P> This little script shows all tables from all databases (assuming you got MySQL installed).
	Also, consider changing the 'mysql' command to use a valid username and password.
	<tscreen><verb>
	#!/bin/bash
	DBS=`mysql -uroot  -e"show databases"`
	for b in $DBS ;
	do
	        mysql -uroot -e"show tables from $b"
	done
	</verb></tscreen>
    </sect1>

    <!-- Multiple source files -->
    <sect1>
    Multiple source files
        <P> You can use multiple files with the command source.
	<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>

	<sect1>
	String comparison examples
	<p> Comparing two strings.
	<tscreen><verb>
	#!/bin/bash
	S1='string'
	S2='String'
	if [ $S1=$S2 ];
	then
	        echo "S1('$S1') is not equal to S2('$S2')"
	fi
	if [ $S1=$S1 ];
	then
	        echo "S1('$S1') is equal to S1('$S1')"
	fi
	</verb></tscreen>
	<p> I quote here a note from a mail, sent buy Andreas Beck, refering to use
	<it/if [ $1 = $2 ]/.
	<p>  This is not quite a good idea, as if either $S1 or $S2 is empty, you will
	 get a parse error. x$1=x$2 or "$1"="$2" is better.

	</sect1>

    <!-- Arithmetic operators -->
    <sect1>
    Arithmetic operators
    <P> +
    <P> -
    <P> *
    <P> /
    <P> % (remainder)
    </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> This section was re-written by Kees (see thank to...)
	 <P>  Some of these commands almost contain complete programming languages.
	From those commands only the basics will be explained. For a more detailed
	description, have a closer look at the main pages of each command.

	<bf/sed/ (stream editor)

   	<P> Sed is a non-interactive editor. Instead of altering a file by moving the
	cursor on the screen, you use a script of editing instructions to sed, plus the
	name of the file to edit. You can also describe sed as a filter. Let's have
	a look at some examples:

   	<tscreen><verb>
	$sed 's/to_be_replaced/replaced/g' /tmp/dummy
	</verb></tscreen>

   	<P> Sed replaces the string 'to_be_replaced' with the string 'replaced' and
	reads from the /tmp/dummy file. The result will be sent to stdout (normally
	the console) but you can also add '> capture' to the end of the line above so
	that sed sends the output to the file 'capture'.

	<tscreen><verb>
   	$sed 12, 18d /tmp/dummy
	</verb></tscreen>

   	<P> Sed shows all lines except lines 12 to 18. The original file is not altered by this command.

	<bf/awk/ (manipulation of datafiles, text retrieval and processing)

   	<P> Many implementations of the AWK programming language exist (most known interpreters are GNU's
   	gawk and 'new awk' mawk.) The principle is simple: AWK scans for a pattern, and for every
   	matching pattern an action will be performed.
   	<P> Again, I've created a dummy file containing the following lines:
	<P> <it/"test123/
   	<P> <it/test/
	<P> <it/tteesstt"/

   	<tscreen><verb>
	$awk '/test/ {print}' /tmp/dummy
	</verb></tscreen>
   	<P> test123

   	<P> test

   	<P> The pattern AWK looks for is 'test' and the action it performs when it found a line in the file
   	/tmp/dummy with the string 'test' is 'print'.

   	<tscreen><verb>
	$awk '/test/ {i=i+1} END {print i}' /tmp/dummy
	</verb></tscreen>

   	<P> 3

   	<P> When you're searching for many patterns, you should replace the text between the quotes with '-f
   	file.awk' so you can put all patterns and actions in 'file.awk'.

   	<bf/grep/ (print lines matching a search pattern)

   <P> We've already seen quite a few grep commands in the previous chapters, that display the lines
   matching a pattern. But grep can do more.
   	<tscreen><verb>
   	$grep "look for this" /var/log/messages -c
	</verb></tscreen>
   <P> 12
   <P> The string "look for this" has been found 12 times in the file /var/log/messages.

   <P> [ok, this example was a fake, the /var/log/messages was tweaked :-)]

   <bf/wc/ (counts lines, words and bytes)

   <P> In the following example, we see that the output is not what we expected. The dummy file, as used
   in this example, contains the following text:
       	<it/"bash introduction/
     	<it/ howto test file"/

   <tscreen><verb>
	$wc --words --lines --bytes /tmp/dummy
	</verb></tscreen>

   <P> 2 5 34 /tmp/dummy

   <P> Wc doesn't care about the parameter order. Wc always prints them in a standard order, which is,
   as you can see: &lt;lines&gt;&lt;words&gt;&lt;bytes&gt;&lt;filename&gt;.

   <bf/sort/ (sort lines of text files)

   <P> This time the dummy file contains the following text:
   <P> <it/"b/
   <P> <it/c/
   <P> <it/a"/
   	<tscreen><verb>
	$sort /tmp/dummy
	</verb></tscreen>

   <P> This is what the output looks like:

   <P> <it/a/
   <P> <it/b/
   <P> <it/c/

   <P> Commands shouldn't be that easy :-)
   <bf/bc/ (a calculator programming language)

   <P> Bc is accepting calculations from command line (input from file. not from redirector or pipe),
   but also from a user interface. The following demonstration shows some of the commands. Note that
   <P> I start bc using the -q parameter to avoid a welcome message.

   	<tscreen><verb>
   $bc -q
	</verb></tscreen>

   <P> <it/1 == 5/
   <P> <it/0/
   <P> <it/0.05 == 0.05/
   <P> <it/1/
   <P> <it/5 != 5/
   <P> <it/0/
   <P> <it/2 ^ 8/
   <P> <it/256/
   <P> <it/sqrt(9)/
   <P> <it/3/
   <P> <it/while (i != 9) {/
   <P> <it/i = i + 1;/
	<P> <it/print i/
   <P> <it/}/
   <P> <it/123456789/
   <P> <it/quit/

   <bf/tput/ (initialize a terminal or query terminfo database)

   <P> A little demonstration of tput's capabilities:
   	<tscreen><verb>
	$tput cup 10 4
	</verb></tscreen>
   <P> The prompt appears at (y10,x4).
   	<tscreen><verb>
	$tput reset
	</verb></tscreen>
   <P> Clears screen and prompt appears at (y1,x1). Note that (y0,x0) is the upper left corner.
   	<tscreen><verb>
	$tput cols
	</verb></tscreen>
   <it/80/
   <P> Shows the number of characters possible in x direction.
   <P> It is higly recommended to be familiarized with these programs (at least). There are tons of
   	little programs that will let you do real magic on the command line.
   <P> [some samples are taken from man pages or FAQs]
    </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 simple 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>
     Translations
     	<p> Italian: by William Ghelfi (wizzy at tiscalinet.it)
     		<htmlurl name="is here" url="http://web.tiscalinet.it/penguin_rules">
     	<p> French: by Laurent Martelli
		<htmlurl name="is missed" url="http://">
	<p> Korean: Minseok Park
		<htmlurl url="http://kldp.org" name="http://kldp.org">
	<p> Korean: Chun Hye Jin
		<htmlurl url="" name="unknown">
	<p> Spanish: unknow
		<htmlurl url="http://www.insflug.org" name="http://www.insflug.org">
     <p> I guess there are more translations, but I don't have any info of them,
     if you have it, please, mail it to me so I update this section.
     </sect1>
     <sect1>
     Thanks to
     <P>
     <itemize>
     	<item> People who translated this document to other languages (previous section).
        <item> Nathan Hurst for sending a lot of corrections.
	<item> Jon Abbott for sending comments about evaluating arithmetic expressions.
	<item> Felix Hudson for writing the <it/renna/ script
	<item> Kees van den Broek
		(for sending many corrections, re-writting useful commands section)
	<item> Mike (pink) made some suggestions about locating bash and testing files
	<item> Fiesh make a nice suggestion for the loops section.
	<item> Lion suggested to mention a common error (./hello.sh: Command not found.)
	<item> Andreas Beck made several corrections and comments.
     </itemize>
     </sect1>
	<sect1>
	History
	<p> New translations included and minor corrections.
	<p> Added the section useful commands re-writen by Kess.
	<p> More corrections and suggestions incorporated.
	<p> Samples added on string comparison.
	<p> v0.8 droped the versioning, I guess the date is enough.
	<p> v0.7 More corrections and some old TO-DO sections written.
	<p> v0.6 Minor corrections.
	<p> v0.5 Added the redirection section.
	<p> v0.4 disappeared from its location due to my ex-boss and this doc found its new place
	at the proper url: www.linuxdoc.org.
	<p> prior:  I don't rememeber and I didn't use rcs nor cvs :(
	</sect1>

     <sect1>
     More resources
          <P>
	  <P> Introduction to bash (under BE)
	      <htmlurl url="http://org.laol.net/lamug/beforever/bashtut.htm"
		name="http://org.laol.net/lamug/beforever/bashtut.htm">
              <htmlurl url="https://web.archive.org/web/20031203073810/http://org.laol.net/lamug/beforever/bashtut.htm"
                name="Link defunct, but here's a link from the Wayback Machine">
          <P> Bourne Shell Programming
	      http://207.213.123.70/book/
     </sect1>
</sect>

</article>