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.

• Avoid using "magic numbers", [1] 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

  See also Appendix C.

• Break complex scripts into simpler modules. Use functions where appropriate. See Example 35-4.

• 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).