guide2.md

installimage - Style Guide

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Contents

• Indentation Guidelines
• Multiline Output to File
• Functions
• Escaping
• Preferred Usage of Bash Builtins
• Multiple Parameter Validation
• Brackets Notation
• Comments in Files
• Variable Convention
• Inspiration

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Indentation Guidelines

we use two whitespaces for Indentation, and no hard tabs. This results in the following vim settings:

set tabstop=2
set shiftwidth=2
set expandtab
set softtabstop=2

Multiline Output to File

Group the output of multiple commands with braces and redirect this once into a file. Also do not redict STDERR to a debugfile, this is useless for echos (} > "$NETWORKFILE" 2>> "$DEBUGFILE"). Here is a bad example:

echo "### $COMPANY - installimage" > "$CONFIGFILE"
echo "# Loopback device:" >> "$CONFIGFILE"
echo "auto lo" >> "$CONFIGFILE"
echo "iface lo inet loopback" >> "$CONFIGFILE"
echo "" >> "$CONFIGFILE" 2>> "$DEBUGFILE"

The { and the } have to be in own lines and the content between them indented by two spaces. Here is another bad example:

{ echo "### $COMPANY - installimage"
echo "# Loopback device:"
echo "auto lo"
echo "iface lo inet loopback"
echo "" } > "$CONFIGFILE" 2>> "$DEBUGFILE"

Besides the formatting, this also redirects STDERR to $DEBUGFILE, this is useless because the brackets only encapsulate echos, you only need the redirect if you do something else that could actually fail.

A good example for this is:

{
  echo "### $COMPANY - installimage"
  echo "# Loopback device:"
  echo "auto lo"
  echo "iface lo inet loopback"
  echo ""
} > "$CONFIGFILE"

Functions

Functions should be pure if possible, e.g. the same input produces the same output and they should not access global variables. This makes reasoning about correctness much easier.

Escaping

We don't want dirty escaping for variables in echo, we should prefer printf in these cases, here is a bad example:

echo -e "SUBSYSTEM==\"net\", ACTION==\"add\", DRIVERS==\"?*\", ATTR{address}==\"$2\", ATTR{dev_id}==\"0x0\", ATTR{type}==\"1\", KERNEL==\"eth*\", NAME=\"$1\"" >> $UDEVFILE

and here a good one:

printf 'SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="%s", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="%s"\n' "$2" "$1" >> "$UDEVFILE"

Preferred Usage of bash builtins

For security and performance reasons we should use bash builtins wherever possible. Bad example for iterations:

for i in $(seq 1 $COUNT_DRIVES) ; do
  if [ $SWRAID -eq 1 -o $i -eq 1 ] ;  then
    local disk="$(eval echo "\$DRIVE"$i)"
    execute_chroot_command "grub-install --no-floppy --recheck $disk 2>&1"
  fi
done

and a good example:

for ((i=1; i<="$COUNT_DRIVES"; i++)); do
  if [ "$SWRAID" -eq 1 ] || [ "$i" -eq 1 ] ;  then
    local disk; disk="$(eval echo "\$DRIVE"$i)"
    execute_chroot_command "grub-install --no-floppy --recheck $disk 2>&1"
  fi
done

Multiple Parameter Validation

Always use seperate testcases for params, bad example:

if [ "$1" -a "$2" ]; then

good example:

if [ -n "$1" ] && [ -n "$2" ]; then

Brackets Notation

We want to avoid useless whitespace in general, for example in brackets. here is a bad awk example:

awk '{ print $2 }'

and the correct one:

awk '{print $2}'

Comments in Files

There are two kinds of comments, those that contain higher level descriptions should be easily and clearly visible --> Empty comment line before and after. Commented code lines are without any empty comment lines.

Variable Convention

we've got two types of varibles:

• global ones

  • are uppercase
  • explictly exported

• local variables

  • are lowercase
  • used in functions
  • defined with local

Try to use local vars whereever possible. Complex variable names (consisting of multiple names) are always connected with a _, for example COUNT_DRIVES as a global one or count_drives as a local one.

Variables that contain an array: Arrays should be indicated by name and the loop variable should resamble this. Good example (take a look at the singular/plural here):

declare -a harddrives
declare -i harddrives_number="${#harddrives[@]}"
if [[ $harddrives_number -gt 0 ]]; then
  for harddrive in harddrives; do
    echo "$harddrive"
  done
fi

Inspiration

This is loosely based on:

• Bash Hackers Style Guide
• Googles Shell Style Guide