Script Programming: Control and Iteration

• if - Used to execute one or more statements on a condition. An example:

  if [ ! -d /mnt ]  # be sure the directory /mnt exists
  then
    mkdir /mnt
  fi
  

• case - Used to execute specific commands based on the value of a variable. An example:

  case $NUM
  1)
    echo The number is 1
    ;;
  2)
    echo The number is 2
    ;;
  *)
    echo The number is not 1 or 2
    ;;
  esac
  

• for - Used to loop for all cases of a condition. In the example below, it is used to copy all files found in /mnt/floppy to the /etc directory. The lines were numbered for reference with descriptions:
  1. The for loop statement will loop until all files have been found.
  2. A test to be sure the file is a normal file and not a directory.
  3. A comment line.
  4. This line extracts the name of the file from its full path pointed to by the variable $i and puts it in the variable $filename. The method used here is called parameter expansion and is documented in the bash man page. For more information on parameter expansion read the "Linux Programmer's Guide".
  5. This line sends a statement to the standard output, telling what file is being copied.
  6. This line performs the copy command using the -p option to preserve file attributes. Note: Much ability to perform script programming is couched in the ability to know and use the various commands, programs and tools available in Linux rather than a strict understanding of syntax. This is obvious to anyone who reads the system startup script files in /etc/rc.d and associated directories.
  7. This line ends the if statement.
  8. This line ends the for statement.

  1.  for i in /mnt/floppy/*; do
  2.     if [ -f $i ]; then
  3.     # if the file is there
  4.        filename=${i#/mnt/floppy/}
  5.        echo copying $i to /etc/$filename
  6.        cp -p $i /etc/$filename
  7.     fi
  8.  done
  

• until - Cycles through a loop until some condition is met. The syntax for the command is shown below:

  until [ expression ]
  do
    statements
  done
  

• while - Cycles through a loop while some condition is met. The below example will cycle through a loop forever:

  while [ 1 ]
  do
    statement(s)
  done