The C-style for-loop

for (( <EXPR1> ; <EXPR2> ; <EXPR3> )); do
  <LIST>
done

The C-style for-loop is, as the name says, derived from the C-keyword "for", but in Bash, it only allows arithmetic expressions as header contents, not arbitrary commands.

It operates as follows: The arithmetic expression <EXPR1> is evaluated, then <EXPR2> is checked, if it's true, then the loop body is executed. After the first iteration, the <EXPR3> is evaluated, <EXPR2> is checked again, if it's true, the loop body is executed, etc...

• <EXPR1> is to initialize before the first run
• <EXPR2> is to check if the loop should be ran
• <EXPR3> is to change conditions after every loop run

It's very simple. Another equivalent loop using a while-construct with arithmetic expressions compound command would be:

(( <EXPR1> ))
while (( <EXPR2> )); do
  <LIST>
  (( <EXPR3> ))
done

If one of these arithmetic expressions in the for-loop is empty, it behaves as if it would be 1 (TRUE in arithmetic context).

Like all loops (both for-loops, while and until), this loop can be

• terminated (broken) by the break command, optionally as break N to break N levels of nested loops
• forced to immediately do the next iteration using the continue command, optionally as continue N analog to break N

The return status is the one of the last command executed from <LIST> or FALSE if any of the arithmetic expressions fail.

A simple counter, the loop iterates 101 times ("0" to "100" are 101 numbers → 101 runs!), and everytime the variable x is set to the current value.

• it initializes x = 0
• before every iteration it checks if x ⇐ 100
• after every iteration it changes x++

for ((x = 0 ; x <= 100 ; x++)); do
  echo "Counter: $x"
done

This is the very same counter (compare it to the simple counter example above), but the change that is made is a x = x + 10. That means, it will count from 0 to 100, but with a step of 10.

for ((x = 0 ; x <= 100 ; x = x + 10)); do
  echo "Counter: $x"
done

This is a bit more complex, but really just a little bit.

It loops through the bit-values of a Byte, beginning from 128, ending at 1. If that bit is set in the testbyte, it prints "1", else "0" ⇒ it prints the binary representation of the testbyte value (8 bits).

testbyte=123
for (( n = 128 ; n >= 1 ; n = n / 2 )); do
  if (( testbyte & n )); then
    echo -n "1"
  else
    echo -n "0"
  fi
done
echo # final linefeed

• Internal: Arithmetic expressions
• Internal: The classic for-loop
• Internal: The while-loop