Unofficial Shell Scripting Stylesheet

Advanced Bash-Scripting Guide:
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# 35.1. 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 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 G.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

### 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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