Within a shell file the for structure without a specified list of values takes as its list of values the arguments specified on the command line when the shell was invoked. The syntax for "for" is as follows:
for variable
do
commands
done
The variable used in the for command is set automatically to each argument value in sequence. For example, if we had a file called "echofname" that contained this:
for fname
do
echo "$fname"
done
and we called it thusly:
$ echofname bob.txt frank.txt george.dat
we would get
bob.txt
frank.txt
george.dat
for - in
If in the for structure we specify a list of values, we can explicitly set the contents of the variable in the loop. For example, the following code
for grocery in milk cookies apples cheese
do
echo "Pick up $grocery"
done
would provide us with
Pick up milk
Pick up cookies
Pick up apples
Pick up cheese
while
The while loop repeats commands. A while loop begins with the keyword while and is followed by a Linux command. The keyword do follows on the next line. The end of the loop is specified by the keyword done. Thus, the syntax is
while Linux command
do
commands
done
The Linux command used in while structures is often a test command indicated by enclosing brackets. In the myname script in the next example, the user is asked to enter a name. The name is then printed out. The loop is controlled by testing the value of the variable again using the bracket form of the test command.
myname
again=yes
while [ "$again" = yes ]
do
echo -n "Please enter a name: "
read name
echo "The name you entered was $name"
echo -n "Do you wish to continue?"
read again
done
echo Good-bye
