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.

`until` Loops¶

The until loop will continue to run until a specific condition is met.
This is essentially the opposite of a while loop.

The basic syntax for this is like other loops.

until CONDITION; do
    code here
done

The CONDITION can be either a comparative condition or a command. Either way, it will continue to run until the CONDITION evaluates to 0 (true).

This type of loop is particularly useful if you need to continuously run a command until it is successful.

`until` Examples¶

If we are waiting for a directory to exist, we could use until to continuously check the condition of that directory.

until [[ -d /home/kolkhis/wait ]]; do
    printf "Directory /home/kolkhis/wait does not exist...\n"
    sleep 0.5
done
printf "Found the directory /home/kolkhis/wait\n"

This will check the condition given, the existence of the directory /home/kolkhis/wait, and while that condition is false, it will run sleep 0.5.
This means the condition will be checked every 0.5 seconds until it evaluates to true (zero).

If we're waiting for a netcat listener to be set up on the localhost, serving a string of text over port 1234, but we don't know the exact moment it will be set up:

# This will set up the listener to serve the string
nc -lp 1234 <<< "This will be served to the first one to connect to port 1234"

Then we can use an until loop to try to connect.

# Run command until successful
until nc localhost 1234; do sleep 0.2; done

This will continue to run the command, nc localhost 1234, until the exit code of that command is zero (successful).

Loops in Bash¶

Table of Contents¶

for Loops¶

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

Using seq,¶