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  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-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



  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.


  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

  �  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.


  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


  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


  All about redirection


  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

  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.


  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

               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.



  This section explains in a very simple and practical way how to use
  pipes, nd why you may want it.


  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$".



  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.


  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.


  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 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)


  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.



  Conditionals let you decide whether to perform an action or not, this
  decision is taken by evaluating an expression.


  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

  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.



  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.


  Sample: Basic conditional example if .. then ... else

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


  Sample: Conditionals with variables

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


  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.


  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).

  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


  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


  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'


  Until sample

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



  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

  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.


  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

  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.


  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.


  User interfaces


  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.




  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 !"


  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'.


  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"


  Multiple source files

  You can use multiple files with the command source.




  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


  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.


  Arithmetic operators





  % (remainder)


  Arithmetic relational operators

  -lt (<)

  -gt (>)

  -le (<=)

  -ge (>=)

  -eq (==)

  -ne (!=)

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


  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

  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

  [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

  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]


  More Scripts

  12.1.  Applying a command to all files in a directory.


  Sample: A very simple backup script (little bit better)

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


  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!


  File renamer (simple)

            # basic file renamer


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


  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


  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.


  (no) warranty

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



  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.


  Thanks to

  �  People who translated this document to other languages (previous

  �  Nathan Hurst for sending a lot of corrections.

  �  Jon Abbott for sending comments about evaluating arithmetic

  �  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

  �  Fiesh make a nice suggestion for the loops section.

  �  Lion suggested to mention a common error (./ Command not

  �  Andreas Beck made several corrections and coments.



  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 :(


  More resources

  Introduction to bash (under BE)

  Bourne Shell Programming

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