Loops in Bash¶

Table of Contents¶

for Loops
- Create 200 files named file<number> skipping every even number 001-199
while Loops

`for` Loops¶

Create 200 files named `file<number>` skipping every even number 001-199¶

Using `seq`,¶

# Using `seq`
for i in $(seq 1 2 200); do  
    touch file${i}
done

Using Brace Expansion (Parameter Expansion)¶

# Bash 4+ Exclusive: Brace expansion  
for i in {1..200..2}; do  
    touch file${i}
done

Using and C-style For-Loops¶

# C-style loop  
for (( i=0; i<200; i+2 )); do  
    touch file${i}
done

`while` Loops¶

Loop with a Counter¶

counter=0
while [[ $counter < 10 ]]; do
    printf "Counter: %d\n" "$counter"
    counter++
done

Loop over lines from a file in bash¶

while read -r line; do  
    echo "$line"  
done < file.txt  # Reading from `file.txt`

Loop over lines from a command (process substitution)¶

while read -r line; do  
    echo "$line"  
done < <(find . -maxdepth 1 -name '*.txt')  # All .txt files in current dir

Forever (Infinite Loop)¶

while true; do  
    echo "This will run forever."  
done  

while :; do
    echo "This will also run forever."
done

`select` Loops¶

select is a bash builtin that's used to handle user input.

The select builtin is used to create interactive menus for the user to select a specific thing.

It creates a loop that continues to prompt the user for input.

select word in {one,two,three}; do
    echo "Input: $word"
    break
done

break: Breaks out of the loop. The prompt will keep running unless we use break.

It prompts the user with a menu containing the items given:

1) one
2) two
3) three
#?

The #? is what the user's prompt will be by default.
- This prompt can be changed by modifying the PS3 variable.
The user must select one of the numbers on the itemized list.

This loop will run indefinitely unless you put in a break.
You'd want to handle the user input anyway, so that's where you'd break.

select word in {apple,orange,banana}; do
    printf "You chose: %s\n" "$word"
    case $REPLY in
        1) break ;;
        2) break ;;
        3) break ;;

        *)
            printf "Bad selection: %d\n" "$REPLY" # Don't break here
            ;;
    esac
done

The word variable will contain the actual value of the string that corresponds to the number that the user selected.

If you want the number that the user selected, that is stored in the REPLY variable.

select word in {apple,orange,banana}; do
    printf "You chose number %d: %s\n" "$REPLY" "$word"
    break
done

We can also use a single case to check if the number that was inputted is within a specific range.

select word in {apple,orange,banana}; do
    printf "You chose number %d: %s\n" "$REPLY" "$word"
    case $REPLY in
        [1-3])
            printf "Valid selection: %d\n" "$REPLY"
            break
            ;;

        *)
            printf "Invalid selection: %d\n" "$REPLY"
            ;;
    esac
done

This is yet another method you can use to interactively get user input.