Bash Scripting: Loops

Introduction

Bash scripting allows for automation and execution of complex workflows.
Key feature of bash scripting: loops.
Four types of loops in bash:
- for VARIABLE in LIST
- for (( ; ; ))
- while EXPR
- until EXPR

`for VARIABLE in LIST`

Iterates through a list of values, executing the loop body for each value.
Example: for X in 1 2 3 do ... body of the loop ... done for COLOUR in red green blue do ... body of the loop ... done
Can use script parameters as list values with $@.
Can match filenames using globbing patterns, e.g., *.c.
If no match is found, the pattern is assigned to the variable.

Example: Compile C Source Files

for SOURCEFILE in *.c
  do
    echo "=== Compiling $SOURCEFILE ==="
    BINARY="$(echo $SOURCEFILE | cut -d. -f1)"
    gcc SOURCEFILE -o $BINARY
  done

C-style `for` Loop: `for (( ; ; ))`

Similar to C language for loops.
Uses bash arithmetic syntax (( )) and do/done for body.
Contains initial condition, control condition, and increment/decrement.

Example: Count from 1 to 10

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

`while EXPR`

Continues as long as the expression is true.
Expression can be a single command, list of commands, or pipeline.
Commonly uses the test command [[ ]].

Example

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

`until EXPR`

Continues as long as the expression is false.
Stops when expression becomes true.

Example

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

Examples from Lecture

tput Colour Codes

tput command uses terminal codes for actions like setting text color.
tput setaf _n_ and tput setab _n_ set foreground and background colors.

#!/usr/bin/bash

for ((C=0; C<16; C++))
do
  tput setaf $C
  echo "This is colour $C"
done
tput sgr0

Number-Guessing Game

Interactive script where the player guesses a secret number.

#!/usr/bin/bash

MAX=100
MAX_TRIES=7
GAMES=0
WINS=0

clear
tput setaf 5

while [[ "$PLAY" == "Y" || "$PLAY" == "y" || "$PLAY" == "YES" ||
         "$PLAY" == "Yes" || "$PLAY" == "yes" ]]
do
  SECRET=$(( RANDOM % MAX + 1 ))
  GUESS=0
  TRIES=0
  ((GAMES++))

  until [[ $GUESS -eq $SECRET || $TRIES -ge $MAX_TRIES ]]
  do
    tput setaf 15
    read -p "Enter your guess (#$((++TRIES))): " GUESS
    if [[ $GUESS -gt $SECRET ]]
    then
      tput setaf 1
      echo "Too high!"
    elif [[ $GUESS -lt $SECRET ]]
    then
      tput setaf 1
      echo "Too low!"
    else
      tput setaf 10
      echo "You got it in $((TRIES)) tries!"
      ((WINS++))
    fi
    echo
  done

  if [[ ! $GUESS -eq $SECRET ]]
  then
    tput setaf 11
    echo "You lose after $TRIES attempts. The number was $SECRET."
  fi

  tput setaf 15
  read -p "Do you want to play again (Y/N)? " PLAY
  echo
done

tput setaf 5
echo "You executed $GAMES missions and succeeded $WINS times ($((WINS*100/GAMES))% success)."
echo
tput sgr0

Bash Scripting with Loops Overview

Bash Scripting: Loops

Introduction

for VARIABLE in LIST

Example: Compile C Source Files

C-style for Loop: for (( ; ; ))

Example: Count from 1 to 10

while EXPR

Example

until EXPR

Example

Examples from Lecture

tput Colour Codes

Number-Guessing Game

`for VARIABLE in LIST`

C-style `for` Loop: `for (( ; ; ))`

`while EXPR`

`until EXPR`