Bash Scripting 2: Loops

Video Lecture

There are four types of loops in bash:

1. for VARIABLE in LIST
2. for (( ; ; ))
3. while EXPR
4. until EXPR

In every case, the actual body of the loop is contained within do … done keywords.

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 $( ) )

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

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 [[ command.

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.

Here are a couple of examples from the lecture:

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 "This is colour $C"
done
tput sgr0   # Reset to "normal" text mode

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-commend 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