35.1. Unofficial Shell Scripting Stylesheet (Revision #16)

• 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=85                       # 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" ]  # Test if target directory exists.
    then
      echo "$1 is not a directory."
      return $E_BADDIR
    fi
  
    rm -f "$1"/*
    return 0   # Success.
  } 
  
  cleanup_pfiles $projectdir
  
  exit $?

• 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=95                        #  Uppercase for an errorcode,
                                       #+ and name prefixed 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
  export MAIL_DIRECTORY                #+ variable.
  
  GetAnswer ()                         #  Mixed case works well for a
  {                                    #+ function name, especially
    prompt=$1                          #+ when it improves legibility.
    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=95
  ...
  ...
  exit $E_WRONG_ARGS

         See also Appendix E.

         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 Section F.1 .

• Break complex scripts into simpler modules. Use functions where appropriate. See Example 37-4.
   
• 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).

... reading the UNIX source code to the Bourne shell (/bin/sh). I was shocked at how much simple
algorithms could be made cryptic, and therefore useless, by a poor choice of code style. I asked myself,
"Could someone be proud of this code?"
--Landon Noll

Note

1. ^ In this context, "magic numbers" have an entirely different meaning than the magic numbers used to designate file types.