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====== Bash Scripting 2: Loops ======


===== Looping in Bash ======

There are four types of loops in bash:
  - ''for //VARIABLE// in //LIST//''
  - ''for <nowiki>(( ; ; ))</nowiki>''
  - ''while //EXPR//''
  - ''until //EXPR//''
  
In every case, the actual body of the loop is contained within ''do'' ... ''done'' keywords.

==== for VARIABLE in LIST ====

This type of loop iterates through a list of values; the //VARIABLE// is sequentially set to the each of the items in the list of values and the body of the loop is executed for each value. The list of values may be constants:

  for X in 1 2 3
  do
    ... body of the loop ...
  done

  for COLOUR in red green blue
  do
    ... body of the loop ...
  done

Or the list may be the list of parameters (arguments to the script), written as ''"$@"'':

  for A in "$@"
  do
    ... body of the loop ...
  done

The most common use of this type of loop may be one or more globbing patterns (filename patterns with wildcard characters), which will be replaced with a list of all matching filenames:

  for SOURCEFILE in *.c
  do
    ... body of the loop ...
  done

One thing to remember with this last form of the loop is that //if the pattern does not match any file// then the pattern itself will be assigned to the variable. For example, in the example above, if there are no files that match the pattern ''*.c.'' then the variable SOURCEFILE will be set to the actual pattern ''SOURCEFILE="*.c"''

This can be used in various ways. For example, this loop will compile all of the C source files found in the current directory:

  for SOURCEFILE in *.c
  do
    echo "=== Compiling $SOURCEFILE ==="
    
    # This next line removes the extension from the filename
    BINARY="$(echo $SOURCEFILE | cut -d. -f1)"
    
    gcc SOURCEFILE -o $BINARY
  done

(It's also possible to generate the //LIST// in other ways -- for example, from the output of another command, captured with ''$( )'' )

==== The "C-style" for loop: for (( ; ; )) ====

This loop is very similar to the "for" loop available in the C language. The C parenthesis are changed to bash double-parenthesis, to invoke the bash arithmetic syntax, and the curly-braces ''{ }'' used in C are replaced by the bash ''do'' and ''done'' keywords. The three arguments in the double-parenthesis are the initial condition (variable initialization), the control condition (an expression that, while true, causes the loop to continue), and the increment/decrement (an expression which is executed at the end of each loop, which typically increments or decrements a counter).

For example, this loop counts from 1 to 10:

  for (( i=0; i<=10; i++ ))
  do
    echo $i
  done

==== while EXPR ====

This loop continues as long as the expression //EXPR// is true:

  while [[ "$A" == "$B" ]]
  do
    ... body of the loop ...
  done

//EXPR// may be any single bash command, or a list of bash commands separated by newline characters (ENTER key) or semicolons, or a pipeline of commands. Most commonly it is a ''test'' ''<nowiki>[[</nowiki>'' command.

==== until EXPR ====

This loop continues as long as the expression //EXPR// is false:

  until [[ "$X" -gt "$Y" ]]
  do
    ... body of the loop ...
  done

This loop will continue as long as ''$X'' is less than or equal to ''$Y'' -- as soon as the expression becomes true (when ''$X'' is greater than ''$Y''), the loop will stop.

===== Examples =====

Here are a couple of examples from the lecture:

==== tput Colour Codes ====

The ''tput'' command outputs terminal codes to stdout to perform actions such as setting the text colour, clearing the screen, and so forth. 

The ''tput setaf //n//'' and ''tput setab //n//'' commands set the foreground and background text colours, respectively. To see the available colours, this script can be used:

  #!/usr/bin/bash
  
  for ((C=0; C<16; C++))
  do
  	tput setaf $C   # Set foreground to colour $C
  	echo "Colour $C"
  done
  tput sgr0   # Reset to "normal" text mode

==== Number-Guessing Game ====

This is the main example from the lecture:

  #!/usr/bin/bash
  
  MAX=100		# Maximum value of secret
  MAX_TRIES=7	# Maximum number of tries (guesses)
  GAMES=0		# How many games have been played
  WINS=0		# How many games the user has won
  
  clear		# Clear the screen
  tput setaf 5	# Select purple text
  echo "=== Number-Guessing Game ==="
  echo
  echo "I have a secret number between 1 and $MAX"
  echo "Your mission is to guess it in as few tries as possible."
  echo "You have a maximum of $MAX_TRIES guesses to succeed."
  echo
  
  PLAY="Y"
  # This next loop continues until the user says
  # they don't want to play again
  while [[ "$PLAY" == "Y" || "$PLAY" == "y" || "$PLAY" == "YES" ||
       "$PLAY" == "Yes" || "$PLAY" == "yes" ]]
  do
        # Generate a random number from 1-MAX
  	SECRET=$(( RANDOM % MAX + 1 ))	
  
        # Un-comment this line when debugging!
        # echo "NOTE: the secret number is $SECRET"
  
  	GUESS=0
  	TRIES=0
  	((GAMES++))
  
  	# This next loop continues until the user guess correctly
  	# or the maximum number of guesses is exhausted
  	until [[ $GUESS -eq $SECRET || $TRIES -ge $MAX_TRIES ]]
  	do
  		tput setaf 15	# White text
  		read -p "Enter your guess (#$((++TRIES))): " GUESS
  		if [[ $GUESS -gt $SECRET ]]
  		then
  			tput setaf 1	# Red text
  			echo "Too high!"
  		elif [[ $GUESS -lt $SECRET ]]
  		then
  			tput setaf 1	# Red text
  			echo "Too low!"
  		else
  			tput setaf 10	# Green text
  			echo "You got it in $((TRIES)) tries!"
  			((WINS++))
  		fi
  		echo
  	done
  	if [[ ! $GUESS -eq $SECRET ]]
  	then
  		tput setaf 11	# Yellow text
  		echo "You lose after $TRIES attempts. The number was $SECRET. "
  	fi
  
  	tput setaf 15	# White texxt
  	echo 
  	read -p "Do you want to play again (Y/N)? " PLAY
  	echo
  
  done
  tput setaf 5	# Purple text
  
  # In the next line, the multiplication 
  # must preceed the division because 
  # this is integer math
  echo "You executed $GAMES missions and succeeded $WINS times ($((WINS*100/GAMES))% success)."
  
  echo
  tput sgr0	# Reset to normal text