515 Loops and Control Structures:

   515 Loops

   516 -----------------------------

   516 -----

   517 

   518 Bash has three different commands for looping -- ``for``, ``while`` and ``until``. 

   519 

   520 ``for`` loop

   521 ~~~~~~~~~~~~

   522 

   523 Suppose we have a set of files, that have names beginning with numbers followed by their names - ``08 - Society.mp3``. We would like to rename these files to remove the numbering. How would we go about doing that? It is clear from the problem statement that we could use a ``for`` loop, to loop through the list of files and rename each of the files.  

   524 

   525 Let's first look at a simple ``for`` loop, to understand how it works. 

   526 ::

   527 

   528   for animal in rat cat dog man

   529   do 

   530     echo $animal

   531   done

   532 

   533 We just wrote a list of animals, each animal's name separated by a space and printed each name on a separate line. The variable ``animal`` is a dummy variable and has no significance. You could use something as lame as ``i`` in place of ``animal``.  

   534 

   535 Now, we use a simple ``for`` loop to list the files that we are interested in. 

   536 ::

   537 

   538   ls *.mp3 > list

   539   for i in `cat list`

   540   do

   541     echo "$i"

   542   done

   543 

   544 If your filenames contain spaces, ``for`` assumes each space separated word to be a single item in the list and prints it in a separate line. We could change the script slightly to overcome this problem. 

   545 ::

   546 

   547   for i in *.mp3

   548   do

   549     echo "$i"

   550   done

   551 

   552 Now, we have each file printed on a separate line. Depending on the files that we have we could use grep to get the relevant portion of the filenames and rename the files. 

   553 ::

   554 

   555   for i in *.mp3

   556   do 

   557     j=$(echo "$i"|grep -o "[A-Za-z'&. ]*.mp3")

   558     echo "$i -> $j"

   559   done

   560 

   561 Now we just replace the echo command with a ``mv`` or a ``cp`` command. 

   562 ::

   563 

   564   for i in *.mp3

   565   do 

   566     j=$(echo "$i"|grep -o "[A-Za-z'&. ]*.mp3")

   567     cp "$i" "$j"

   568   done

   569 

   570 As an exercise, you could try sorting the files in reverse alphabetical order and then prefix numbers to each of the filenames.  

   571 

   572 ``while``

   573 ~~~~~~~~~

   574 

   575 The ``while`` command allows us to continuously execute a block of commands until the command that is controlling the loop is executing successfully. 

   576 

   577 Let's start with the lamest example of a while loop.

   578 ::

   579 

   580   while true

   581   do

   582     echo "True"

   583   done

   584 

   585 This, as you can see, is an infinite loop that prints the ``True``. 

   586 

   587 Say we wish to write a simple program that takes user input and prints it back, until the input is ``quit``, which quits the program. 

   588 ::

   589 

   590   while [ "$variable" != "quit" ]

   591   do

   592     read variable

   593     echo "Input - $variable"

   594   done

   595   exit 0

   596 

   597 ``until``

   598 ~~~~~~~~~

   599 

   600 The ``until`` loop is similar to the ``while`` loop, except that it executes until the conditional command does not execute properly. 

   601 

   602 The infinite loop changes to the following, when ``until`` is used.

   603 ::

   604 

   605   until false

   606   do

   607     echo "True"

   608   done