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34.1. 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=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 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 Section F.1.

  • Break complex scripts into simpler modules. Use functions where appropriate. See Example 36-4.

  • 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

Notes

[1]

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

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