BASH Programming - Introduction HOW-TO

by Mike G mikkey at

Thu Jul 27 09:36:18 ART 2000

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. Any feedback will be apreciated, specially in the patch form :)

1. Introduction

2. Very simple Scripts

3. All about redirection

4. Pipes

5. Variables

6. Conditionals

7. Loops for, while and until

8. Functions

9. User interfaces

10. Misc

11. Tables

12. More Scripts

13. When something goes wrong (debugging)

14. About the document

1. Introduction

1.1 Getting the latest version

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

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

          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 ./ Command not found. Probably the first line '#!/bin/bash' is wrong, issue whereis bash or see 'finding bash' to see how sould you write this line.

2.2 A very simple backup script

        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 contain 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 command 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 program, 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

            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)

           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 '$(date +%Y%m%d)'. 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.

                function hello {
                        local HELLO=World
                        echo $HELLO
                echo $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. '2<1' is an expresion that evaluates to false, while '2>1' 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 'statement2' is executed.

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];


code if 'expression' is true.


6.2 Sample: Basic conditional example if .. then

            if [ "foo" = "foo" ]; then
               echo expression evaluated as true

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

            if [ "foo" = "foo" ]; then
               echo expression evaluated as true
               echo expression evaluated as false

6.4 Sample: Conditionals with variables

            if [ "$T1" = "$T2" ]; then
                echo expression evaluated as true
                echo expression evaluated as false

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 'words' within a string.

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

        for i in $( ls ); do
            echo item: $i

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).

'done' (4) indicates that the code that used the value of $i has 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.

        for i in `seq 1 10`;
                echo $i

7.3 While sample

         while [  $COUNTER -lt 10 ]; do
             echo The counter is $COUNTER
             let COUNTER=COUNTER+1 

This script 'emulates' the well known (C, Pascal, perl, etc) 'for' structure

7.4 Until sample

         until [  $COUNTER -lt 10 ]; do
             echo COUNTER $COUNTER
             let COUNTER-=1

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

           function quit {
           function hello {
               echo Hello!
           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

                function quit {
                function e {
                    echo $1 
                e Hello
                e World
                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

           OPTIONS="Hello Quit"
           select opt in $OPTIONS; do
               if [ "$opt" = "Quit" ]; then
                echo done
               elif [ "$opt" = "Hello" ]; then
                echo Hello World
                echo bad option

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

          if [ -z "$1" ]; then 
              echo usage: $0 directory
          OF=home-$(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:

                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.

                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.

'locate bash' is preferred, but not all machines have locate.

'find ./ -name bash' from the root dir will work, usually.

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)

        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.

        DBS=`mysql -uroot  -e"show databases"`
        for b in $DBS ;
                mysql -uroot -e"show tables from $b"

10.6 Multiple source files

You can use multiple files with the command source.


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.

        if [ $S1=$S2 ];
                echo "S1('$S1') is not equal to S2('$S2')"
        if [ $S1=$S1 ];
                echo "S1('$S1') is equal to S1('$S1')"

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:




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



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




while (i != 9) {

i = i + 1;

print i




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)

            OF=home-$(date +%Y%m%d).tgz
            tar -cZf $TGTD$OF $SRCD

12.3 File re-namer

             # 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
             # this for loop iterates through all of the files that we gave the program
             # it does one rename per file given
               for file in $*
                 mv ${file} $prefix$file
             #we now exit the program
               exit 0
             # 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
              for file in $*
                mv ${file} $file$suffix
              exit 0
             # check for the replacement rename
             if [ $1 = r ]; then
             # 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
             # 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 $*
                 new=`echo ${file} | sed s/${OLD}/${NEW}/g`
                 mv ${file} $new
             exit 0
             # 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)

     # basic file renamer

     for i in $( ls *$criteria* ); 
         tgt=$(echo $i | sed -e "s/$re_match/$replace/")
         mv $src $tgt

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 is here

French: by Laurent Martelli is missed

Korean: Minseok Park

Korean: Chun Hye Jin unknown

Spanish: unknow

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

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:

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

14.5 More resources

Introduction to bash (under BE)

Bourne Shell Programming