Loops in Bash

Traditional for loop

for (( i=0; i<=5;i++  )); do
  echo $i
done

# 1 2 3 4 5

for..in: loop through an array

for element in "${arr[@]}"
do
    echo "$element"
done

While loop

while loops execute while a condition is true (0)

We can use a while loop as a condition in two senses:

  • execute while a given condition obtains
  • expand and execute a given command as long as the final command in the `while’ for the command has an exit status of zero (i.e. truthy)

Here is an exampe of using while in the former case:

declare -i n=0
while (( n<10 ))
do
  echo "n:$n"
  (( n++ ))
done

Here is an example of using while in the latter case:

while read line;
do
    # something
done < "$file_to_read"

Until loop

until loops execute until a condition is false (1)

declare -i m=0
until (( m==10 )): do
  echo "m:$m"
  (( m++ ))
done

This gives us the same output as n with the while loop but here it runs so long as m==10 is false. As soon as m is equal to 100, the condition becomes true and hence the loop stops.

logo