Fedora Linux

Since loops that count through a range of numbers are often needed, bash also supports the C-style for loop. Inside double parentheses, specify an initial expression, a conditional expression, and a per-loop expression, separated by semicolons:

# Initial value of A is 1
# Keep looping as long as A<=20
# Each time you loop, increment A by 1
for ((A=1; A<=20; A++))
do
 echo $A
done

Note that the conditional expression uses normal comparison symbols ( <= ) instead of the alphabetic options ( -le ) used by test .

Don't confuse the C-style for loop with the for...in loop! 

4.12.1.5. Making your scripts available to users of other shells

So far we have been assuming that the user is using the bash shell; if the user of another shell (such as tcsh ) tries to execute one of your scripts, it will be interpreted according to the language rules of that shell and will probably fail.

To make your scripts more robust, add a shebang line at the beginning a pound-sign character followed by an exclamation mark, followed by the full path of the shell to be used to interpret the script ( /bin/bash ):

#!/bin/bash
# script to count from 1 to 20
for ((A=1; A<=20; A++))
do
 echo $A
done

I also added a comment line (starting with # ) after the shebang line to describe the function of the script.

The shebang line gets its name from sharp and bang, common nicknames for the #! characters.

4.12.1.6. An example

Here is an example of a longer script, taking advantage of some of the scripting features in bash :

#!/bin/bash
#
# number-guessing game
#
# If the user entered an argument on the command
# line, use it as the upper limit of the number
# range.
if [ '$#' -eq 1 ]
then
 MAX=$1
else
 MAX=100
fi
# Set up other variables
SECRET=$(( (RANDOM % MAX) + 1 )) # Random number 1-100
TRIES=0
GUESS=-1
# Display initial messages
clear
echo 'Number-guessing Game'
echo '--------------------'
echo
echo 'I have a secret number between 1 and $MAX.'
# Loop until the user guesses the right number
while [ '$GUESS' -ne '$SECRET' ]
do
 # Prompt the user and get her input
 ((TRIES++))
 echo -n 'Enter guess #$TRIES: '
 read GUESS
# Display low/high messages
 if [ '$GUESS' -lt '$SECRET' ]
 then
  echo 'Too low!'
 fi
 if [ '$GUESS' -gt '$SECRET' ]
 then
  echo 'Too high!'
 fi
done
# Display final messages