The while And until Loops

The general form of the while loop is this:
while control-command
do
commands
done

Each cycle of the while loop, the shell attempts to execute the control command. If it returns a zero exit status (success), the commands between do and done are executed. This process continues until the control command yields a nonzero exit status (failure).

The general form of the until loop is this:
until control-command
do
commands
done

Each cycle of the until loop, the shell attempts to execute the control command. If it returns a nonzero exit status (failure), the commands between do and done are executed. This process continues until the control command yields a zero exit status (success).

• The while and until loops are termed indefinite loops because in many cases it is not known in advance how many cycles they will loop through.
• Instead of taking a list to control looping, their behaviour depends upon the success or failure of the control-command.
• The while loop runs until the control-command fails, the until loop runs until the control-command succeeds.
• The words while, until, do and done MUST come after a command separator.