Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
names instead. This makes the script easier to understand and permits making changes and updates
without breaking the application.
if [ −f /var/log/messages ]
then
...
fi
# A year later, you decide to change the script to check /var/log/syslog.
# It is now necessary to manually change the script, instance by instance,
# and hope nothing breaks.
# A better way:
LOGFILE=/var/log/messages # Only line that needs to be changed.
if [ −f "$LOGFILE" ]
then
...
fi
• Choose descriptive names for variables and functions.
fl=`ls −al $dirname`
# Cryptic.
file_listing=`ls −al $dirname` # Better.
MAXVAL=10 # All caps used for a script constant.
while [ "$index" −le "$MAXVAL" ]
...
E_NOTFOUND=75
if [ ! −e "$filename" ]
then
echo "File $filename not found."
exit $E_NOTFOUND
fi
# Uppercase for an errorcode,
# and name begins with "E_".
MAIL_DIRECTORY=/var/spool/mail/bozo # Uppercase for an environmental variable.
export MAIL_DIRECTORY
GetAnswer ()
{
prompt=$1
echo −n $prompt
read answer
return $answer
}
# Mixed case works well for a function.
GetAnswer "What is your favorite number? "
favorite_number=$?
echo $favorite_number
_uservariable=23
# Permissable, but not recommended.
# It's better for user−defined variables not to start with an underscore.
# Leave that for system variables.
• Use exit codes in a systematic and meaningful way.
E_WRONG_ARGS=65
...
...
exit $E_WRONG_ARGS
Chapter 33. Scripting With Style 334