Chapter 33. Scripting With Style

Get into the habit of writing shell scripts in a structured and systematic manner. Even "on−the−fly" and "written on the back of an envelope" scripts will benefit if you take a few minutes to plan and organize your thoughts before sitting down and coding.

Herewith are a few stylistic guidelines. This is not intended as an Official Shell Scripting Stylesheet.

33.1. Unofficial Shell Scripting Stylesheet

∙ Comment your code. This makes it easier for others to understand (and appreciate), and easier for you to maintain.

PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}"

#It made perfect sense when you wrote it last year, but now it's a complete mystery.

#(From Antek Sawicki's "pw.sh" script.)

Add descriptive headers to your scripts and functions.

#!/bin/bash

#************************************************#

# xyz.sh

#written by Bozo Bozeman

#	July 05, 2001
#	Clean up project files.
#************************************************#
BADDIR=65		# No such directory.
projectdir=/home/bozo/projects		# Directory to clean up.

#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−#

#cleanup_pfiles ()

#Removes all files in designated directory.

#Parameter: $target_directory

#Returns: 0 on success, $BADDIR if something went wrong. #−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−# cleanup_pfiles ()

{

if [ ! −d "$1" ] # Test if target directory exists. then

echo "$1 is not a directory." return $BADDIR

fi

rm −f "$1"/*

return 0 # Success.

}

cleanup_pfiles $projectdir

exit 0

Be sure to put the #!/bin/bash at the beginning of the first line of the script, preceding any comment headers.

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Advanced Bash−Scripting Guide

∙Avoid using "magic numbers", [61] that is, "hard−wired" literal constants. Use meaningful variable 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 "$MAXVAL" ]
...
E_NOTFOUND=75	# Uppercase for an errorcode,
	# and name begins with "E_".
if [ ! −e "$filename" ]
then
echo "File $filename not found."
exit $E_NOTFOUND
fi
MAIL_DIRECTORY=/var/spool/mail/bozo	# Uppercase for an environmental variable.
export MAIL_DIRECTORY
GetAnswer ()	# Mixed case works well for a function.
{
prompt=$1

echo −n $prompt read answer return $answer

}

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

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Advanced Bash−Scripting Guide

See also Appendix C.

∙Break complex scripts into simpler modules. Use functions where appropriate. See Example 35−3.

∙Don't use a complex construct where a simpler one will do.

COMMAND

if [ $? −eq 0 ]

...

# Redundant and non−intuitive.

if COMMAND

...

# More concise (if perhaps not quite as legible).

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