Scripting with style

continue

These are some coding guidelines that helped me to read and understand my own code over the years. They also will help to produce code that will be a bit more robust than “if something breaks, I know how to fix it”.

This is not a bible, of course. But I have seen so much ugly and terrible code (not only in shell) during all the years, that I'm 100% convinced there needs to be some code layout and style. No matter which one you use, use it throughout your code (at least don't change it within the same shellscript file); don't change your code layout with your mood.

Some good code layout helps you to read your own code after a while. And of course it helps others to read the code.

Indentation is nothing that technically influences a script, it's only for us humans.

I'm used to seeing/using indentation of two space characters (though many may prefer 4 spaces, see below in the discussion section):

• it's easy and fast to type
• it's not a hard-tab that's displayed differently in different environments
• it's wide enough to give a visual break and small enough to not waste too much space on the line

Speaking of hard-tabs: Avoid them if possible. They only make trouble. I can imagine one case where they're useful: Indenting here-documents.

Whenever you need to break lines of long code, you should follow one of these two rules:

Indention using command width:

activate some_very_long_option \
         some_other_option

Indention using two spaces:

activate some_very_long_option \
  some_other_option

Personally, with some exceptions, I prefer the first form because it supports the visual impression of “these belong together”.

Compound commands form the structures that make a shell script different from a stupid enumeration of commands. Usually they contain a kind of “head” and a “body” that contains command lists. This type of compound command is relatively easy to indent.

I'm used to (not all points apply to all compound commands, just pick the basic idea):

• put the introducing keyword and the initial command list or parameters on one line (“head”)
• put the “body-introducing” keyword on the same line
• the command list of the “body” on separate lines, indented by two spaces
• put the closing keyword on a separated line, indented like the initial introducing keyword

What?! Well, here again:

HEAD_KEYWORD parameters; BODY_BEGIN
  BODY_COMMANDS
BODY_END

This construct is a bit special, because it has keywords (elif, else) “in the middle”. The visually appealing way is to indent them like this:

if ...; then
  ...
elif ...; then
  ...
else
  ...
fi

for f in /etc/*; do
  ...
done

while [[ $answer != [YyNn] ]]; do
  ...
done

The case construct might need a bit more discussion here, since its structure is a bit more complex.

In general, every new “layer” gets a new indentation level:

case $input in
  hello)
    echo "You said hello"
  ;;
  bye)
    echo "You said bye"
    if foo; then
      bar
    fi
  ;;
  *)
    echo "You said something weird..."
  ;;
esac

Some notes:

• if not 100% needed, the optional left parenthesis on the pattern is not used
• the patterns (hello)) and the corresponding action terminator (;;) are indented at the same level
• the action command lists are indented one more level (and continue to have their own indentation, if needed)
• though optional, the very last action terminator is given

Cryptic constructs, we all know them, we all love them. If they are not 100% needed, avoid them, since nobody except you may be able to decipher them.

It's - just like in C - the middle ground between smart, efficient and readable.

If you need to use a cryptic construct, include a comment that explains what your “monster” does.

Since all reserved variables are UPPERCASE, the safest way is to only use lowercase variable names. This is true for reading user input, loop counting variables, etc., … (in the example: file)

• prefer lowercase variables
• if you use UPPERCASE names, do not use reserved variable names (see SUS for an incomplete list)
• if you use UPPERCASE names, prepend the name with a unique prefix (MY_ in the example below)

#!/bin/bash

# the prefix 'MY_'
MY_LOG_DIRECTORY=/var/adm/

for file in "$MY_LOG_DIRECTORY"/*; do
  echo "Found Logfile: $file"
done

As in C, it's always a good idea to initialize your variables, though, the shell will initialize fresh variables itself (better: Unset variables will generally behave like variables containing a null string).

It's no problem to pass an environment variable to the script. If you blindly assume that all variables you use for the first time are empty, anybody can inject content into a variable by passing it via the environment.

The solution is simple and effective: Initialize them

my_input=""
my_array=()
my_number=0

If you do that for every variable you use, then you also have some in-code documentation for them.

Unless you are really sure what you're doing, quote every parameter expansion.

There are some cases where this isn't needed from a technical point of view, e.g.

• inside [[ ... ]] (other than the RHS of the ==, !=, and =~ operators)
• the parameter (WORD) in case $WORD in ….
• variable assignment: VAR=$WORD

But quoting these is never a mistake. If you quote every parameter expansion, you'll be safe.

If you need to parse a parameter as a list of words, you can't quote, of course, e.g.

list="one two three"

# you MUST NOT quote $list here
for word in $list; do
  ...
done

Function names should be all lowercase and meaningful. The function names should be human readable. A function named f1 may be easy and quick to write down, but for debugging and especially for other people, it reveals nothing. Good names help document your code without using extra comments.

do not use command names for your functions. e.g. naming a script or function test, will collide with the UNIX test command.

Unless absolutely necessary, only use alphanumeric characters and the underscore for function names. /bin/ls is a valid function name in Bash, but is not a good idea.

As noted in the article about command substitution, you should use the $( … ) form.

If portability is a concern, use the backquoted form ` … `.

In any case, if other expansions and word splitting are not wanted, you should quote the command substitution!

Well, like Greg says: “If eval is the answer, surely you are asking the wrong question.”

Avoid it, unless absolutely neccesary:

• eval can be your neckshot
• there are most likely other ways to achieve what you want
• if possible, re-think the way your script works, if it seems you can't avoid eval with your current method
• if you really, really, have to use it, then take care, and be sure about what you're doing

The basic structure of a script simply reads:

#!SHEBANG

CONFIGURATION_VARIABLES

FUNCTION_DEFINITIONS

MAIN_CODE

If possible (I know it's not always possible!), use a shebang.

Be careful with /bin/sh: The argument that “on Linux /bin/sh is Bash” is a lie (and technically irrelevant)

The shebang serves two purposes for me:

• it specifies the interpreter to be used when the script file is called directly: If you code for Bash, specify bash!
• it documents the desired interpreter (so: use bash when you write a Bash-script, use sh when you write a general Bourne/POSIX script, …)

I call variables that are meant to be changed by the user “configuration variables” here.

Make them easy to find (directly at the top of the script), give them meaningful names and maybe a short comment. As noted above, use UPPERCASE for them only when you're sure about what you're doing. lowercase will be the safest.

Unless there are reasons not to, all function definitions should be declared before the main script code runs. This gives a far better overview and ensures that all function names are known before they are used.

Since a function isn't parsed before it is executed, you usually don't have to ensure they're in a specific order.

The portable form of the function definition should be used, without the function keyword (here using the grouping compound command):

getargs() {
  ...
}

Speaking about the command grouping in function definitions using { …; }: If you don't have a good reason to use another compound command directly, you should always use this one.

Fail early, this sounds bad, but usually is good. Failing early means to error out as early as possible when checks indicate an error or unmet condition. Failing early means to error out before your script begins its work in a potentially broken state.

If you use external commands that may not be present on the path, or not installed, check for their availability, then tell the user they're missing.

Example:

my_needed_commands="sed awk lsof who"

missing_counter=0
for needed_command in $my_needed_commands; do
  if ! hash "$needed_command" >/dev/null 2>&1; then
    printf "Command not found in PATH: %s\n" "$needed_command" >&2
    ((missing_counter++))
  fi
done

if ((missing_counter > 0)); then
  printf "Minimum %d commands are missing in PATH, aborting\n" "$missing_counter" >&2
  exit 1
fi

The exit code is your only way to directly communicate with the calling process without any special provisions.

If your script exits, provide a meaningful exit code. That minimally means:

• exit 0 (zero) if everything is okay
• exit 1 - in general non-zero - if there was an error

This, and only this, will enable the calling component to check the operation status of your script.

You know: “One of the main causes of the fall of the Roman Empire was that, lacking zero, they had no way to indicate successful termination of their C programs.” – Robert Firth

• if the script is interactive, if it works for you and if you think this is a nice feature, you can try to save the terminal content and restore it after execution
• output clean and understandable screen messages
• if applicable, you can use colors or specific prefixes to tag error and warning messages
  • make it easy for the user to identify those messages
• write normal output to STDOUT. write error, warning and diagnostic messages to STDERR
  • enables message filtering
  • keeps the script from mixing output data with diagnostic, or error messages
  • if the script gives syntax help (-? or -h or –help arguments), it should go to STDOUT
• if applicable, write error/diagnostic messages to a logfile
  • avoids screen clutter
  • messages are available for diagnostic use

• never blindly assume anything. If you want the user to input a number, check for numeric input, leading zeros, etc. If you have specific format or content needs, always validate the input!

• some of these guidelines, such as indentation, positioning of “body-introducing” keywords, and portable function declarations, can be enforced by shfmt