32.1. 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. #
  #************************************************#
  E_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, $E_BADDIR if something went wrong. #
  # --------------------------------------------------------- #
  cleanup_pfiles ()
  {
   if [ ! -d "1ドル" ] # Test if target directory exists.
   then
   echo "1ドル is not a directory."
   return $E_BADDIR
   fi
   rm -f "1ドル"/*
   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ドル
   echo -n $prompt
   read answer
   return $answer
  } 
  GetAnswer "What is your favorite number? "
  favorite_number=$?
  echo $favorite_number
  _uservariable=23 # Permissible, 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 D.

  Ender suggests using the exit codes in /usr/include/sysexits.h in shell scripts, though these are primarily intended for C and C++ programming.

• Use standardized parameter flags for script invocation. Ender proposes the following set of flags.

  -a All: Return all information (including hidden file info).
  -b Brief: Short version, usually for other scripts.
  -c Copy, concatenate, etc.
  -d Daily: Use information from the whole day, and not merely
   information for a specific instance/user.
  -e Extended/Elaborate: (often does not include hidden file info).
  -h Help: Verbose usage w/descs, aux info, discussion, help.
   See also -V.
  -l Log output of script.
  -m Manual: Launch man-page for base command.
  -n Numbers: Numerical data only.
  -r Recursive: All files in a directory (and/or all sub-dirs).
  -s Setup & File Maintenance: Config files for this script.
  -u Usage: List of invocation flags for the script.
  -v Verbose: Human readable output, more or less formatted.
  -V Version / License / Copy(right|left) / Contribs (email too).

  See also Appendix F.

• Break complex scripts into simpler modules. Use functions where appropriate. See Example 34-4.

• 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).