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.

  1 PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}" 2 # It made perfect sense when you wrote it last year, 3 #+ but now it's a complete mystery. 4 # (From Antek Sawicki's "pw.sh" script.)

  Add descriptive headers to your scripts and functions.

  1 #!/bin/bash 2  3 #************************************************# 4 # xyz.sh # 5 # written by Bozo Bozeman # 6 # July 05, 2001 # 7 # # 8 # Clean up project files. # 9 #************************************************# 10  11 E_BADDIR=65 # No such directory. 12 projectdir=/home/bozo/projects # Directory to clean up. 13  14 # --------------------------------------------------------- # 15 # cleanup_pfiles () # 16 # Removes all files in designated directory. # 17 # Parameter: $target_directory # 18 # Returns: 0 on success, $E_BADDIR if something went wrong. # 19 # --------------------------------------------------------- # 20 cleanup_pfiles () 21 { 22  if [ ! -d "$1" ] # Test if target directory exists. 23  then 24  echo "$1 is not a directory." 25  return $E_BADDIR 26  fi 27  28  rm -f "$1"/* 29  return 0 # Success. 30 } 31  32 cleanup_pfiles $projectdir 33  34 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.

  1 if [ -f /var/log/messages ] 2 then 3  ... 4 fi 5 # A year later, you decide to change the script to check /var/log/syslog. 6 # It is now necessary to manually change the script, instance by instance, 7 # and hope nothing breaks. 8  9 # A better way: 10 LOGFILE=/var/log/messages # Only line that needs to be changed. 11 if [ -f "$LOGFILE" ] 12 then 13  ... 14 fi

• Choose descriptive names for variables and functions.

  1 fl=`ls -al $dirname` # Cryptic. 2 file_listing=`ls -al $dirname` # Better. 3  4  5 MAXVAL=10 # All caps used for a script constant. 6 while [ "$index" -le "$MAXVAL" ] 7 ... 8  9  10 E_NOTFOUND=75 # Uppercase for an errorcode, 11  # +and name begins with "E_". 12 if [ ! -e "$filename" ] 13 then 14  echo "File $filename not found." 15  exit $E_NOTFOUND 16 fi 17  18  19 MAIL_DIRECTORY=/var/spool/mail/bozo # Uppercase for an environmental variable. 20 export MAIL_DIRECTORY 21  22  23 GetAnswer () # Mixed case works well for a function. 24 { 25  prompt=$1 26  echo -n $prompt 27  read answer 28  return $answer 29 } 30  31 GetAnswer "What is your favorite number? " 32 favorite_number=$? 33 echo $favorite_number 34  35  36 _uservariable=23 # Permissible, but not recommended. 37 # It's better for user-defined variables not to start with an underscore. 38 # Leave that for system variables.

• Use exit codes in a systematic and meaningful way.

  1 E_WRONG_ARGS=65 2 ... 3 ... 4 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.

  1 -a All: Return all information (including hidden file info). 2 -b Brief: Short version, usually for other scripts. 3 -c Copy, concatenate, etc. 4 -d Daily: Use information from the whole day, and not merely 5  information for a specific instance/user. 6 -e Extended/Elaborate: (often does not include hidden file info). 7 -h Help: Verbose usage w/descs, aux info, discussion, help. 8  See also -V. 9 -l Log output of script. 10 -m Manual: Launch man-page for base command. 11 -n Numbers: Numerical data only. 12 -r Recursive: All files in a directory (and/or all sub-dirs). 13 -s Setup & File Maintenance: Config files for this script. 14 -u Usage: List of invocation flags for the script. 15 -v Verbose: Human readable output, more or less formatted. 16 -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 34-4.

• Don't use a complex construct where a simpler one will do.

  1 COMMAND 2 if [ $? -eq 0 ] 3 ... 4 # Redundant and non-intuitive. 5  6 if COMMAND 7 ... 8 # More concise (if perhaps not quite as legible).