Chris Tyler - Fedora Linux

echo $(( ++A ))

done

The expression $(( ++A )) returns the value of A after it is incremented. You could also use $(( A++ )) , which returns the value of A before it is incremented:

A=1

while [ "$A" -le 20 ]

do

echo $(( A++ ))

done

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!

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.

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

echo

echo "You guessed it!"

echo "It took you $TRIES tries."

echo

This script could be saved as /usr/local/bin/guess-it and then made executable:

# chmod a+rx /usr/local/bin/guess-it

Here's a test run of the script:

$ guess-it

Number-guessing Game

--------------------

I have a secret number between 1 and 100.

Enter guess #1:

50

Too low!

Enter guess #2:

75

Too low!

Enter guess #3:

83

Too low!

Enter guess #4:

92

Too high!

Enter guess #5:

87

Too high!

Enter guess #6:

85

Too low!

Enter guess #7:

86

You guessed it!

It took you 7 tries.

Another test, using an alternate upper limit:

$ guess-it 50

Number-guessing Game

--------------------

I have a secret number between 1 and 50.

Enter guess #1:

25

Too low!

Enter guess #2:

37

Too low!

Enter guess #3:

44

Too high!

Enter guess #4:

40

You guessed it!

It took you 4 tries.

When a user logs in, the system-wide script /etc/profile and the per-user script ~/.bash_profile are both executed. This is the default /etc/profile :

# /etc/profile

# System wide environment and startup programs, for login setup

# Functions and aliases go in /etc/bashrc

pathmunge ( ) {

if ! echo $PATH | /bin/egrep -q "(^|:)$1($|:)" ; then

if [ "$2" = "after" ] ; then

PATH=$PATH:$1

else

PATH=$1:$PATH

fi

fi

}

# ksh workaround

if [ -z "$EUID" -a -x /usr/bin/id ]; then

EUID=\Qid -u\Q

UID=\Qid -ru\Q

fi

# Path manipulation

if [ "$EUID" = "0" ]; then

pathmunge /sbin

pathmunge /usr/sbin

pathmunge /usr/local/sbin

fi

# No core files by default

ulimit -S -c 0 > /dev/null 2>&1

if [ -x /usr/bin/id ]; then

USER="\Qid -un\Q"

LOGNAME=$USER

MAIL="/var/spool/mail/$USER"

fi

HOSTNAME=\Q/bin/hostname\Q

HISTSIZE=1000

if [ -z "$INPUTRC" -a ! -f "$HOME/.inputrc" ]; then

INPUTRC=/etc/inputrc

fi

export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE INPUTRC

for i in /etc/profile.d/*.sh ; do

if [ -r "$i" ]; then

. $i

fi

done

unset i

unset pathmunge

This script adds /sbin , /usr/sbin , and /usr/local/sbin to the PATH if the user is the root user. It then creates and exports the USER , LOGNAME , MAIL , HOSTNAME , and HISTSIZE variables, and executes any files in /etc/profile.d that end in .sh .