02. While and until loops while, do, done

while loops

While-loops are much like for-loops, and continue to iterate until the exit status of the condition is false. While-loops consist of the following syntax:

while [ condition exits successfully ]
  # Commands here

Using shift to iterate through shell arguments

A common use of the while-loop is to iterate through shell arguments, to obtain all option arguments. Each call to the shift command helps us go down the list of options, with the arguments going down one by one. This causes the value of $1 to be replaced by $2, $2 to be replaced by $3, and so on.

Here's a script of how we can combine a while-loop with a case-statement to iterate through all options.

while [ $# -gt 0 ]
  case $1 in 
  # File option, followed by filename
  -f) filename=$2
  -v) verbose=true
  -q) quiet=true
  -l) long=true
  # Unrecognized option
  -*) echo "$0: $1: unrecognized option" >&2
  **) break
  # shift for next option

Getting option parameters with getopts

Another helpful command that is often seen with while-loops is getopts. This command follows standard POSIX option syntax (single letters preceded by a - and possibly one argument value).

For each option that contains an argument, we use the colon (:). For example, we can have a:bc if we plan to have the -a option contain an argument. An example call would look something like:

$ ./test -a argumentValue -b -b

Thus the argumentValue goes with the -a option.

In this example, we design a timer that has several options a user can specify.

if [ $# -lt 1 ]
  echo "Usage: $0 -t -m -a -r"
  echo "-t time (in seconds)"
  echo "-m message at countdown"
  echo "-a author"
  echo "-r repeat n number of times"
  exit 1
# Set default parameters
message="Time's up!"
# Need to have a colon (:) after each option that has     | an argument.
while getopts t:m:a:r: opt
  case $opt in 
    t) seconds=$OPTARG;;
    m) message=$OPTARG;;
    a) author=$OPTARG;;
    r) repeatNumber=$OPTARG;;
for iterationNumber in $(seq 1 $repeatNumber) 
  echo "Repeating $iterationNumber of $repeatNumber     | times!"
  for sec in $(seq $seconds 1)
    echo "Countdown: $sec"
    sleep 1
echo "$message by $author"

Take your Linux skills to the next level!

System Admin Handbook

Take your Linux skills to the next level! Try Linux & UNIX

This book approaches system administration in a practical way and is an invaluable reference for both new administrators and experienced professionals. It details best practices for every facet of system administration, including storage management, network design and administration, email, web hosting, scripting, and much more.

$ Check price
74.9974.99Amazon 4.5 logo(142+ reviews)

More Linux & UNIX resources

Aching back from coding all day?

Acupressure Mat & Pillow

Aching back from coding all day? Try Back Problems

Relieve your stress, back, neck and sciatic pain through 1,782 acupuncture points for immediate neck pain relief. Made for lower, upper and mid chronic back pain treatment, and improves circulation, sleep, digestion and quality of life.

$$ Check price
144.87144.87Amazon 4.5 logo(1,890+ reviews)

More Back Problems resources