Good bash script style14.03.21

Some Bash coding conventions and good practices.

Coding conventions are... just conventions. They help to have a little fun with scripting, not to create new war/bias conversations.

Feel free to break the rules any time you can; it's important that you will always love what you would have written because scripts can be too fragile, too hard to maintain, or so many people hate them... And it's also important to have a consistent way in your scripts.

Naming and styles
        Tabs and Spaces
        Pipe
        Variable names
        Function names
    Error handling
        Sending instructions
        Pipe error handling
        Catch up with $?
        Automatic error handling
            Set -u
            Set -e
    Techniques
        Keep that in mind
        A little tracing
        Making your script a library
        Quick self-doc
        No excuse
        Meta programming
        Removing with care
        Shell or Python/Ruby/etc
    Contributions
        Variable names for arrays
    Good lessons
    Resources
    Author. License

Naming and Styles
Tabs and Spaces

Don't use (smart-)tabs. Replace a tab by two spaces. Do not accept any trailing spaces.

Many editors can't and/or aren't configured to display the differences between tabs and spaces. Another person editor is just not your editor. Having spaces does virtually help a strange reader of your script.
Pipe

There are inline pipe and display pipe. Unless your pipe is too short, please use display pipe to make things clear. For example,

# This is an inline pipe: "$(ls -la /foo/ | grep /bar/)"

# The following pipe is of display form: every command is on
# its own line.

_foobar="$( \
  ls -la /foo/ \
  | grep /bar/ \
  | awk '{print $NF}')"

_generate_long_lists \
| while IFS= read -r  _line; do
    _do_something_fun
  done

When using display form, put pipe symbol (|) at the beginning of of its statement. Don't put | at the end of a line, because it's the job of the line end (EOL) character and line continuation (\).

Here is another example

# List all public images found in k8s manifest files
# ignore some in-house image. 
list_public_images() {
  find . -type f -iname "*.yaml" -exec grep 'image: ' {} \; \
  | grep -v ecr. \
  | grep -v '#' \
  | sed -e "s#['\"]##g" \
  | awk '{print $NF}' \
  | sort -u \
  | grep -Eve '^(coredns|bflux|kube-proxy|logstash)$' \
}

Variable names

If you are going to have meanful variable name, please use them for the right purpose. The variable name _country_name should not be used to indicate a city name or a person, should they? So this is bad

_countries="australia germany berlin"
for _city in $_countries; do
  echo "city or country is: $_city
done

That's very bad example but that is to emphasize the idea. (FIXME: Add better examples)

A variable is named according to its scope.

If a variable can be changed from its parent environment, it should be in uppercase; e.g, THIS_IS_A_USER_VARIABLE.
    Other variables are in lowercase, started by an underscore; e.g, _this_is_a_variable. The primary purpose of the underscore (_) is to create a natural distance between the dollar ($) and the name when the variable is used (e.g, $_this_is_a_variable). This makes your code more readable, esp. when there isn't color support on your source code viewer.
    Any local variables inside a function definition should be declared with a local statement.

Example

# The following variable can be provided by user at run time.
D_ROOT="${D_ROOT:-}"

# All variables inside `_my_def` are declared with `local` statement.
_my_def() {
  local _d_tmp="/tmp/"
  local _f_a=
  local _f_b=

# This is good, but it's quite a mess
  local _f_x= _f_y=
}

Though local statement can declare multiple variables, that way makes your code unreadable. Put each local statement on its own line.

FIXME: Add flexibility support.
Function names

Name of internal functions should be started by an underscore (_). Use underscore (_) to glue verbs and nouns. Don't use camel form (ThisIsNotMyStyle; use this_is_my_style instead.)

Use two underscores (__) to indicate some very internal methods aka the ones should be used by other internal functions.
Error handling
Sending instructions

All errors should be sent to STDERR. Never send any error/warning message to aSTDOUT device. Never use echo directly to print your message; use a wrapper instead (warn, err, die,...). For example,

_warn() {
  echo >&2 ":: $*"
}

_die() {
  echo >&2 ":: $*"
  exit 1
}

Do not handle error of another function. Each function should handle error and/or error message by their own implementation, inside its own definition.

_my_def() {
  _foobar_call

if [[ $? -ge 1 ]]; then
    echo >&2 "_foobar_call has some error"
    _error "_foobar_call has some error"
    return 1
  fi
}

In the above example, _my_def is trying to handle error for _foobar_call. That's not a good idea. Use the following code instead

_foobar_call() {
  # do something

if [[ $? -ge 1 ]]; then
    _error "${FUNCNAME[0]} has some internal error"
  fi
}

_my_def() {
  _foobar_call || return 1
}

Catch up with $?

$? is used to get the return code of the last statement. To use it, please make sure you are not too late. The best way is to save the variable to a local variable. For example,

_do_something_critical
local _ret="$?"
# from now on, $? is zero, because the latest statement (assignment)
# (always) returns zero.

_do_something_terrible
echo "done"
if [[ $? -ge 1 ]]; then
  # Bash will never reach here. Because "echo" has returned zero.
fi

$? is very useful. But don't trust it.

Please don't use $? with set -e ;)
Pipe error handling

Pipe stores its components' return codes in the PIPESTATUS array. This variable can be used only ONCE in the sub-{shell,process} followed the pipe. Be sure you catch it up!

echo test | fail_command | something_else
  local _ret_pipe=( "${PIPESTATUS[@]}" )
  # from here, `PIPESTATUS` is not available anymore

When this _ret_pipe array contains something other than zero, you should check if some pipe component has failed. For example,

# Note:
#   This function only works when it is invoked
#   immediately after a pipe statement.
_is_good_pipe() {
  echo "${PIPESTATUS[@]}" | grep -qE "^[0 ]+$"
}

_do_something | _do_something_else | _do_anything
_is_good_pipe \
|| {
  echo >&2 ":: Unable to do something"
}

Automatic error handling
Set -u

Always use set -u to make sure you won't use any undeclared variable. This saves you from a lot of headaches and critical bugs.

Because set -u can't help when a variable is declared and set to empty value, don't trust it twice.

It's recommended to emphasize the needs of your variables before your script actually starts. In the following example, the script just stops when SOME_VARIABLE or OTHER_VARIABLE is not defined; these checks are done just before any execution of the main routine(s).

: a lot of method definitions

set -u
: "${SOME_VARIABLE}"
: "${OTHER_VARIABLE}"

: your main routine

Set -e

Use set -e if your script is being used for your own business.

Be careful when shipping set -e script to the world. It can simply break a lot of games. And sometimes you will shoot yourself in the foot. If possible please have an option for user choice.

Let's see

set -e
_do_some_critical_check

if [[ $? -ge 1 ]]; then
  echo "Oh, you will never see this line."
fi

If _do_some_critical_check fails, the script just exits and the following code is just skipped without any notice. Too bad, right? The code above can be refactored as below

set +e
if _do_some_critical_check; then
  echo "Something has gone very well."
fi
echo "You will see this line."

Now, if you expect to stop the script when _do_some_critical_check fails (it's the purpose of set -e, right?), these lines don't help. Why? Because set -e doesn't work when being used with if. Confused? Okay, these lines are the correct one

set +e
if _do_some_critical_check; then
  echo "All check passed."
else
  echo "Something wrong we have to stop here"
  exit 1 # or return 1
fi

set -e doesn't help to improve your code: it just forces you to work hard, doesn't it?

Another example, in effect of set -e:

(false && true); echo not here

prints nothing, while:

{ false && true; }; echo here

prints here.

The result is varied with different shells or even different versions of the same shell.

In general, don't rely on set -e and do proper error handling instead.

For more details about set -e, please read

The correct answer to every exercise is actually "because set -e is crap".

http://mywiki.wooledge.org/BashFAQ/105/Answers
    When Bash scripts bite

Techniques
Keep that in mind

There are lot of shell scripts that don't come with (unit)tests. It's just not very easy to write tests. Please keep that in mind: Writing shell scripts is more about dealing with runtime and side effects.

It's very hard to refactor shell scripts. Be prepared, and don't hate bash/shell scripts too much ;)
A little tracing

It would be very helpful if you can show in your script logs some tracing information of the being-invoked function/method.

Bash has two jiffy variables LINENO and FUNCNAME that can help. While it's easy to understand LINENO, FUNCNAME is a little complex. It is an array of chained functions. Let's look at the following example

funcA() {
  log "This is A"
}

funcB() {
  log "This is B"
  funcA
}

funcC() {
  log "This is C"
  funcB
}

: Now, we call funcC

funcC

In this example, we have a chain: funcC -> funcB -> funcA. Inside funcA, the runtime expands FUNCNAME to

FUNCNAME=(funcA funcB funcC)

The first item of the array is the method per-se (funcA), and the next one is the one who instructs funcA (it is funcB).

So, how can this help? Let's define a powerful log function

log() {
  echo "(LOGGING) ${FUNCNAME[1]:-unknown}: *"
}

You can use this little log method everywhere, for example, when funcB is invoked, it will print

LOGGING funcB: This is B

Making your script a library

First thing first: Use function if possible. Instead of writting some direct instructions in your script, you have a wrapper for them. This is not good

: do something cool
: do something great

Having them in a function is better

_default_tasks() {
  : do something cool
  : do something great
}

Now in the very last lines of you script, you can execute them

case "${@:-}" in
":")  echo "File included." ;;
"")   _default_tasks        ;;
esac

From other script you can include the script easily without executing any code:

# from other script
source "/path/to_the_previous_script.sh" ":"

(When being invoked without any argument the _default_tasks is called.)

By advancing this simple technique, you have more options to debug your script and/or change your script behavior.
Quick self-doc

It's possible to generate beautiful self documentation by using grep, as in the following example. You define a strict format and grep them:

_func_1() { #public: Some quick introduction
  :
}

_func_2() { #public: Some other tasks
  :
}

_quick_help() {
  LANG=en_US.UTF_8
  grep -E '^_.+ #public' "$0" \
  | sed -e 's|() { #public: |☠|g' \
  | column -s"☠" -t \
  | sort
}

When you execute _quick_help, the output is as below

_func_1    Some quick introduction
_func_2    Some other tasks

No excuse

When someone tells you to do something, you may blindly do as said, or you would think twice then raise your white flag.

Similarly, you should give your script a white flag. A backup script can't be executed on any workstation. A clean up job can't silently send rm commands in any directory. Critical mission script should

exit immediately without doing anything if argument list is empty;
    exit if basic constraints are not established.

Keep this in mind. Always.
Meta programming

Bash has a very powerful feature that you may have known: It's very trivial to get definition of a defined method. For example,

my_func() {
  echo "This is my function`"
}

echo "The definition of my_func"
declare -f my_func

# <snip>

Why is this important? Your program manipulates them. It's up to your imagination.

For example, send a local function to remote and excute them via ssh

{
  declare -f my_func    # send function definition
  echo "my_func"        # execution instruction
} \
| ssh some_server

This will help your program and script readable especially when you have to send a lot of instructions via ssh. Please note ssh session will miss interactive input stream though.
Removing with care

It's hard to remove files and directories correctly. Please consider to use rm with backup options. If you use some variables in your rm arguments, you may want to make them immutable.

export _temporary_file=/path/to/some/file/
readonly _temporary_file
# <snip>
rm -fv "$_temporary_file"

Shell or Python/Ruby/etc

In many situations you may have to answer to yourself whether you have to use Bash and/or Ruby/Python/Go/etc.

One significant factor is that Bash doesn't have a good memory. That means if you have a bunch of data (in any format) you probably reload them every time you want to extract some portion from them. This really makes your script slow and buggy. When your script needs to interpret any kind of data, it's a good idea to move forward and rewrite the script in another language, Ruby/Python/Golang/....

Anyway, probably you can't deny to ignore Bash: it's still very popular and many services are woken up by some shell things. Keep learning some basic things and you will never have to say sorry. Before thinking of switching to Python/Ruby/Golang, please consider to write better Bash scripts first ;)

https://github.com/icy/bash-coding-style#variable-names

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Good bash script style14.03.21

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