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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

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.

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.

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$".

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.

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.

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

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

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.

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 more 'logical' output. This is to evaluate an
arithmetic expression. You can achieve this also like this:

echo $[1+1]

If you need to use fractions, or more math or you just want it, you
can use bc to evaluate arithmetic expressions.

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.

10.3. Finding bash

From a message from mike (see Thanks to)

you always use #!/bin/bash .. you might was to give an example of

how to find where bash is located.

Suggested locations to check:

ls -l /bin/bash

ls -l /sbin/bash

ls -l /usr/local/bin/bash

ls -l /usr/bin/bash

ls -l /usr/sbin/bash

ls -l /usr/local/sbin/bash

(can't think of any other dirs offhand... i've found it in

most of these places before on different system).

You may try also 'which bash'.

10.4.

Getting the return value of a program

In bash, the return value of a program is stored in a special variable
called $?.

This illustrates how to capture the return value of a program, I
assume that the directory dada does not exist. (This was also
suggested by mike)

#!/bin/bash
cd /dada &> /dev/null
echo rv: $?
cd $(pwd) &> /dev/null
echo rv: $?

10.5. Capturing a commands output

This little scripts show all tables from all databases (assuming you
got MySQL installed). Also, consider changing the 'mysql' command to
use a valid username and password.

#!/bin/bash
DBS=`mysql -uroot -e"show databases"`
for b in $DBS ;
do
mysql -uroot -e"show tables from $b"
done

10.6.

Multiple source files

You can use multiple files with the command source.

__TO-DO__

11.

Tables
11.1.

String comparison operators

(1) s1 = s2

(2) s1 != s2

(3) s1 < s2

(4) s1 > s2

(5) -n s1

(6) -z s1

(1) s1 matches s2

(2) s1 does not match s2

(3) __TO-DO__

(4) __TO-DO__

(5) s1 is not null (contains one or more characters)

(6) s1 is null

11.2.

String comparison examples

Comparing two strings.

#!/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

I quote here a note from a mail, sent buy Andreas Beck, refering to
use if [ $1 = $2 ].

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.

11.3.

Arithmetic operators

% (remainder)

11.4.

Arithmetic relational operators

-lt (<)

-gt (>)

-le (<=)

-ge (>=)

-eq (==)

-ne (!=)

C programmer's should simple map the operator to its corresponding
parenthesis.

11.5.

Useful commands

This section was re-written by Kees (see thank to...)

Some of these command's 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 man pages of each
command.

sed (stream editor)

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:

$sed 's/to_be_replaced/replaced/g' /tmp/dummy

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'.

$sed 12, 18d /tmp/dummy

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

awk (manipulation of datafiles, text retrieval and processing)

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 a
action will be performed.

Again, I've created a dummy file containing the following lines:

"test123

test

tteesstt"

$awk '/test/ {print}' /tmp/dummy

test123

test

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'.

$awk '/test/ {i=i+1} END {print i}' /tmp/dummy

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'.

grep (print lines matching a search pattern)

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.

$grep "look for this" /var/log/messages -c

The string "look for this" has been found 12 times in the file
/var/log/messages.

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

wc (counts lines, words and bytes)

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: "bash introduction
howto test file"

$wc --words --lines --bytes /tmp/dummy

2 5 34 /tmp/dummy

Wc doesn't care about the parameter order. Wc always prints them in a
standard order, which is, as you can see: .

sort (sort lines of text files)

This time the dummy file contains the following text:

$sort /tmp/dummy

This is what the output looks like:

Commands shouldn't be that easy :-) bc (a calculator programming
language)

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

I start bc using the -q parameter to avoid a welcome message.

$bc -q

1 == 5

0.05 == 0.05

5 != 5

2 ^ 8

256

sqrt(9)

while (i != 9) {

i = i + 1;

print i

}

123456789

quit

tput (initialize a terminal or query terminfo database)

A little demonstration of tput's capabilities:

$tput cup 10 4

The prompt appears at (y10,x4).

$tput reset

Clears screen and prompt appears at (y1,x1). Note that (y0,x0) is the
upper left corner.

$tput cols

Shows the number of characters possible in x direction.

It it 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.

[some samples are taken from man pages or FAQs]

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)

#!/bin/bash
SRCD="/home/"
TGTD="/var/backups/"
OF=home-$(date +%Y%m%d).tgz
tar -cZf $TGTD$OF $SRCD

12.3.

File re-namer

#!/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!

12.4.

File renamer (simple)

#!/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

13.

When something goes wrong (debugging)

13.1. Ways Calling BASH

A nice thing to do is to add on the first line

#!/bin/bash -x

This will produce some intresting output information

14.

About the document

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.

14.1.

(no) warranty

This documents comes with no warranty of any kind. and all that

14.2.

Translations

Italian: by William Ghelfi (wizzy at tiscalinet.it) is here

French: by Laurent Martelli is missed

Korean: Minseok Park http://kldp.org

Korean: Chun Hye Jin unknown

Spanish: unknow http://www.insflug.org

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.

14.3.

Thanks to

· People who translated this document to other languages (previous
section).

· Nathan Hurst for sending a lot of corrections.

· Jon Abbott for sending comments about evaluating arithmetic
expressions.

· Felix Hudson for writing the renna script

· Kees van den Broek (for sending many corrections, re-writting
usefull comands section)

· Mike (pink) made some suggestions about locating bash and testing
files

· Fiesh make a nice suggestion for the loops section.

· Lion suggested to mention a common error (./hello.sh: Command not
found.)

· Andreas Beck made several corrections and coments.

14.4.

History

New translations included and minor correcitons.

Added the section usefull commands re-writen by Kess.

More corrections and suggestions incorporated.

Samples added on string comparison.

v0.8 droped the versioning, I guess the date is enought.

v0.7 More corrections and some old TO-DO sections written.

v0.6 Minor corrections.

v0.5 Added the redirection section.

v0.4 disapperd from its location due to my ex-boss and thid doc found
it's new place at the proper url: www.linuxdoc.org.

prior: I don't rememeber and I didn't use rcs nor cvs :(

14.5.

More resources

Introduction to bash (under BE)
http://org.laol.net/lamug/beforever/bashtut.htm

Bourne Shell Programming http://207.213.123.70/book/