Linux Pocket Guide, 2nd Edition

Loops

The while loop repeats a set of commands as long as a condition is true.

while command             While the exit status of command is 0
do
  body
done

For example, if this is the script myscript:

i=0
while [ $i -lt 3 ]
do
  echo "$i"
  i=`expr $i + 1`
done

$ ./myscript
0
1
2

The until loop repeats until a condition becomes true:

until command         While the exit status of command is nonzero
do
  body
done

For example:

i=0
until [ $i -ge 3 ]
do
  echo "$i"
  i=`expr $i + 1`
done

$ ./myscript
0
1
2

The for loop iterates over values from a list:

for variable in list
do
  body
done

For example:

for name in Tom Jack Harry
do
  echo "$name is my friend"
done
$ ./myscript
Tom is my friend
Jack is my friend
Harry is my friend

The for loop is particularly handy for processing lists of files; for example, all files of a certain type in the current directory:

for file in *.doc *.docx
do
  echo "$file is a stinky Microsoft Word file"
done

Be careful to avoid infinite loops, using while with the condition true, or until with the condition false:

while true           Beware: infinite loop!
do
  echo "forever"
done

until false          Beware: infinite loop!
do
  echo "forever again"
done

Use break or exit to terminate these loops based on some condition inside their bodies.