path: root/doc/bash-style.cli

// file      : doc/bash-style.cli
// license   : MIT; see accompanying LICENSE file

"\title=Bash Style Guide"

// NOTES
//
// - Maximum <pre> line is 70 characters.
//

"\h1|Table of Contents|"
"\$TOC$"

"
\h1#intro|Introduction|

Bash works best for simple tasks. Needing arrays, arithmetic, and so on, is
usually a good indication that the task at hand may be too complex for Bash.

Most of the below rules can be broken if there is a good reason for it.
Besides making things consistent, rules free you from having to stop and think
every time you encounter a particular situation. But if it feels that the
prescribed way is clearly wrong, then it probably makes sense to break it.
You just need to be clear on why you are doing it.

See also \l{https://google.github.io/styleguide/shell.xml Google's Bash Style
Guide} as well as \l{https://github.com/progrium/bashstyle Let's do Bash
right!}; we agree with quite a few (but not all) items in there. In particular,
the former provides a lot more rationale compared to this guide.

\h1#style|Style|

Don't use any extensions for your scripts. That is, call it just \c{foo}
rather than \c{foo.sh} or \c{foo.bash} (though we do use the \c{.bash}
extension for
\l{https://build2.org/build2/doc/build2-build-system-manual.xhtml#module-bash
Bash modules}). Use lower-case letters and dash to separate words, for example
\c{foo-bar}.

Indentation is two spaces (not tabs). Maximum line length is 79 characters
(excluding newline). Use blank lines between logical blocks to improve
readability.

Variable and function names should use lower-case letters with underscores
separating words.

For \c{if}/\c{while} and \c{for}/\c{do} the corresponding \c{then} or \c{do}
is written on the same line after a semicolon, for example:

\
if [[ ... ]]; then
  ...
fi

for x in ...; do
  ...
done
\

Do use \c{elif} instead of nested \c{else} and \c{if} (and consider if
\c{case} can be used instead).

For \c{if}/\c{while} use \c{[[ ]]} since it results in cleaner code for
complex expressions, for example:

\
if [[ \"$foo\" && (\"$bar\" || \"$baz\") ]]; then
  ...
fi
\

\N|If for some reason you need the semantics of \c{[}, use \c{test} instead to
make it clear this is intentional.|

\h1#struct|Structure|

The overall structure of the script should be as follows:

\
#! /usr/bin/env bash

# <SUMMARY>
#
# [<FUNCTIONALITY-DESCRIPTION>]
#
# [<OPTIONS-DESCRIPTION>]
#
usage=\"usage: $0 <OPTIONS>\"

owd=\"$(pwd)\"
trap \"{ cd '$owd'; exit 1; }\" ERR
set -o errtrace   # Trap in functions and subshells.
set -o pipefail   # Fail if any pipeline command fails.
shopt -s lastpipe # Execute last pipeline command in the current shell.
shopt -s nullglob # Expand no-match globs to nothing rather than themselves.

function info () { echo \"$*\" 1>&2; }
function error () { info \"$*\"; exit 1; }

[<OPTIONS-ARGUMENTS-DEFAULTS>]

[<OPTIONS-ARGUMENTS-PARSING>]

[<OPTIONS-ARGUMENTS-VALIDATION>]

<FUNCTIONALITY>
\

\h#struct-summary|SUMMARY|

One-two sentences describing what the script does.

\h#struct-func-desc|FUNCTIONALITY-DESCRIPTION|

More detailed functionality description for more complex scripts.

\h#struct-opt-desc|OPTIONS-DESCRIPTION|

Description of command line options. For example:

\
# -q
#   Run quiet.
#
# -t <dir>
#   Specify the alternative toolchain installation directory.
\

\h#struct-opt|OPTIONS|

Command line options summary. For example:

\
usage=\"usage: $0 [-q] [-t <dir>] <file>\"
\

\h#struct-opt-arg-default|OPTIONS-ARGUMENTS-DEFAULTS|

Set defaults to variables that will contain option/argument values. For
example:

\
quiet=\"n\"
tools=/usr/local
file=
\

\h#struct-opt-arg-parse|OPTIONS-ARGUMENTS-PARSING|

Parse the command line options/arguments. For example:

\
while [[ \"$#\" -gt 0 ]]; do
  case \"$1\" in
    -q)
      quiet=\"y\"
      shift
      ;;
    -t)
      shift
      tools=\"${1%/}\"
      shift
      ;;
    *)
      if [[ -n \"$file\" ]]; then
        error \"$usage\"
      fi

      file=\"$1\"
      shift
      ;;
  esac
done
\

If the value you are expecting from the command line is a directory path,
then always strip the trailing slash (as shown above for the \c{-t} option).

\h#struct-opt-arg-valid|OPTIONS-ARGUMENTS-VALIDATION|

Validate option/argument values. For example:

\
if [[ -z \"$file\" ]]; then
  error \"$usage\"
fi

if [[ ! -d \"$file\" ]]; then
  error \"'$file' does not exist or is not a directory\"
fi
\

\h#struct-func|FUNCTIONALITY|

Implement script logic. For diagnostics use the \c{info()} and \c{error()}
functions defined above (so that it goes to stderr, not stdout). If using
functions, then define them just before use.

\h1#quote|Quoting|

We quote every variable expansion, no exceptions. For example:

\
if [[ -n \"$foo\" ]]; then
  ...
fi
\

\N|While there is no word splitting in the \c{[[ ]]} context, we still quote
variable expansions for consistency.|

This also applies to command substitution (which we always write as
\c{$(foo arg)} rather than \c{`foo arg`}), for example:

\
list=\"$(cat foo)\"
\

Note that a command substitution creates a new quoting context, for example:

\
list=\"$(basename \"$1\")\"
\

We also quote values that are \i{strings} as opposed to options/file names,
paths, enum-like values, or integers. Prefer single quotes for \c{sed}
scripts, for example:

\
url=\"https://example.org\"  # String.
quiet=y                    # Enum-like.
verbosity=1                # Integer.
dir=/etc                   # Directory path.
out=/dev/null              # File path.
file=manifest              # File name.
option=--quiet             # Option name.
seds='s%^./%%'             # sed script.
\

Take care to quote globs that are not meant to be expanded, for example:

\
unset \"array[0]\"
\

And since quoting will inhibit globbing, you may end up with expansions along
these lines:

\
rm -f \"$dir/$name\".*
\

Note also that globbing is not performed in the \c{[[ ]]} context so this is
ok:

\
if [[ -v array[0] ]]; then
  ...
fi
\

\N|One exception to this quoting rule is arithmetic expansion (\c{$((\ ))}):
Bash treats it as if it was double-quoted and, as a result, any inner quoting
is treated literally. For example:

\
z=$(($x + $y))           # Ok.
z=$((\"$x\" + \"$y\"))       # Error.
z=$(($x + $(echo \"$y\"))) # Ok.
\

|


If you have multiple values (e.g., program arguments) that may contain spaces,
don't try to handle them with quoting and use arrays instead. Here is a
typical example of a space-aware argument handling:

\
files=()

while [[ \"$#\" -gt 0 ]]; do
  case \"$1\" in

    ...

    *)
      files+=(\"$1\")
      shift
      ;;
  esac
done

rm -f \"${files[@]}\"
\

In the same vein, never write:

\
cmd $*
\

Instead always write:

\
cmd \"$@\"
\

Also understand the difference between \c{@} and \c{*} expansion:

\
files=('one' '2 two' 'three')
echo \"files: ${files[@]}\"  # $1='files: one', $2='2 two', $3='three'
echo \"files: ${files[*]}\"  # $1='files: one 2 two three'
\


\h1#bool|Boolean|

For boolean values use empty for false and \c{true} for true. This way you
can have terse and natural looking conditions, for example:

\
first=true
while ...; do

  if [[ ! \"$first\" ]]; then
     ...
  fi

  if [[ \"$first\" ]]; then
     first=
  fi

done
\


\h1#subshell|Subshell|

Bush executes certain constructs in \i{subshells} and some of these constructs
may not be obvious:

\ul|

\li|Explicit subshell: \c{(...)}|

\li|Pipeline: \c{...|...}|

\li|Command substitution: \c{$(...)}|

\li|Process substitution: \c{<(...)}, \c{>(...)}|

\li|Background: \c{...&}, \c{coproc ...}|

|

Naturally, a subshell cannot modify any state in the parent shell, which
sometimes leads to counter-intuitive behavior, for example:

\
lines=()

... | while read l; do
  lines+=(\"$l\")
done
\

At the end of the loop, \c{lines} will remain empty since the loop body is
executed in a subshell. One way to resolve this is to use the program
substitution instead of the pipeline:

\
lines=()

while read l; do
  lines+=(\"$l\")
done < <(...)
\

This, however, results in an unnatural, backwards-looking (compared to the
pipeline) code. Instead, we can request the last command of the pipeline to be
executed in the parent shell with the \c{lastpipe} shell option, for example:

\
shopt -s lastpipe

lines=()

... | while read l; do
  lines+=(\"$l\")
done
\

\N|The \c{lastpipe} shell option is inherited by functions and subshells.|


\h1#function|Functions|

If a function takes arguments, provide a brief usage after the function
header, for example:

\
function dist() # <pkg> <dir>
{
  ...
}
\

For non-trivial/obvious functions also provide a short description of its
functionality/purpose, for example:

\
# Prepare a distribution of the specified packages and place it
# into the specified directory.
#
function dist() # <pkg> <dir>
{
  ...
}
\

Inside functions use local variables, for example:

\
function dist()
{
  local x=\"foo\"
}
\

If the evaluation of the value may fail (e.g., it contains a program
substitution), then place the assignment on a separate line since \c{local}
will cause the error to be ignored. For example:

\
function dist()
{
  local b
  b=\"$(basename \"$2\")\"
}
\

A function can return data in two primary ways: exit code and stdout.
Normally, exit code 0 means success and exit code 1 means failure though
additional codes can be used to distinguish between different kinds of
failures (for example, \"hard\" and \"soft\" failures), signify special
conditions, etc., see \l{#error-handing Error Handling} for details.

A function can also write to stdout with the result available to the caller in
the same way as from programs (command substitution, pipeline, etc). If a
function needs to return multiple values, then it can print them separated
with newlines with the caller using the \c{readarray} builtin to read them
into an indexed array, for example:

\
function func ()
{
  echo one
  echo two
  echo three
}

func | readarray -t r
\

\N|The use of the newline as a separator means that values may not contain
newlines. While \c{readarray} supports specifying a custom separator with the
\c{-d} option, including a \c{NUL} separator, this support is only available
since Bash 4.4.|

This technique can also be extended to return an associative array by first
returning the values as an indexed array and then converting them to
an associative array with \c{eval}, for example:

\
function func ()
{
  echo \"[a]=one\"
  echo \"[b]=two\"
  echo \"[c]=three\"
}

func | readarray -t ia

eval declare -A aa=(\"${ia[@]}\")
\

Note that if a key or a value contains whitespaces, then it must be quoted.
The recommendation is to always quote both, for example:

\
function func ()
{
  echo \"['a']='one ONE'\"
  echo \"['b']='two'\"
  echo \"['c']='three'\"
}
\

Or, if returning a local array:

\
function func ()
{
  declare -A a=([a]='one ONE' [b]=two [c]=three)

  for k in \"${!a[@]}\"; do
    echo \"['$k']='${a[$k]}'\"
  done
}
\

For more information on returning data from functions, see
\l{https://mywiki.wooledge.org/BashFAQ/084 BashFAQ#084}.


\h1#error-handing|Error Handling|

Our scripts use the \c{ERR} trap to automatically terminate the script in case
any command fail. This semantics is also propagated to functions and subshells
by specifying the \c{errtrace} shell option and to all the commands of a
pipeline by specifying the \c{pipefail} option.

\N|Without \c{pipefail}, a non-zero exit of any command in the pipeline except
the last is ignored. The \c{pipefail} shell option is inherited by functions
and subshells.|

\N|While the \c{nounset} options may also seem like a good idea, it has
subtle, often latent pitfalls that make it more trouble than it's worth (see
\l{https://mywiki.wooledge.org/BashPitfalls#nounset \c{nounset} pitfalls}).|

The \c{pipefail} semantics is not without pitfalls which should be kept in
mind. In particular, if a command in a pipeline exits before reading the
preceding command's output in its entirety, such a command may exit with a
non-zero exit status (see \l{https://mywiki.wooledge.org/BashPitfalls#pipefail
\c{pipefail} pitfalls} for details).

\N|Note that in such a situation the preceding command may exit with zero
status not only because it gracefully handled \c{SIGPIPE} but also because all
of its output happened to fit into the pipe buffer.|

For example, these are the two common pipelines that may exhibit this issue:

\
prog | head -n 1
prog | grep -q foo
\

In these two cases, the simplest (though not the most efficient) way to work
around this issue is to reimplement \c{head} with \c{sed} and to get rid of
\c{-q} in \c{grep}, for example:

\
prog | sed -n -e '1p'
prog | grep foo >/dev/null
\

If you need to check the exit status of a command, use \c{if}, for example:

\
if grep -q \"foo\" /tmp/bar; then
  info \"found\"
fi

if ! grep -q \"foo\" /tmp/bar; then
  info \"not found\"
fi
\

Note that the \c{if}-condition can be combined with capturing the output, for
example:

\
if v=\"$(...)\"; then
  ...
fi
\

But keep in mind that in Bash a failure is often indistinguishable from a
true/false result. For example, in the above \c{grep} command, the result will
be the same whether there is no match or if the file does not exist.

Furthermore, in certain contexts, the above-mentioned \c{ERR} trap is ignored.
Quoting from the Bash manual:

\i{The \c{ERR} trap is not executed if the failed command is part of the
command list immediately following an \c{until} or \c{while} keyword, part of
the test following the \c{if} or \c{elif} reserved words, part of a command
executed in a \c{&&} or \c{||} list except the command following the final
\c{&&} or \c{||}, any command in a pipeline but the last, or if the command’s
return status is being inverted using \c{!}. These are the same conditions
obeyed by the \c{errexit} (\c{-e}) option.}

To illustrate the gravity of this point, consider the following example:

\
function cleanup()
{
  cd \"$1\"
  rm -f *
}

if ! cleanup /no/such/dir; then
  ...
fi
\

Here, the \c{cleanup()} function will continue executing (and may succeed)
even if the \c{cd} command has failed.

Note, however, that notwithstanding the above statement from the Bash manual,
the \c{ERR} trap is executed inside all the subshell commands of a pipeline
provided the \c{errtrace} option is specified. As a result, the above code can
be made to work by temporarily disabling \c{pipefail} and reimplementing it as
a pipeline:

\
set +o pipefail
cleanup /no/such/dir | cat
r=\"${PIPESTATUS[0]}\"
set -o pipefail

if [[ \"$r\" -ne 0 ]]; then
  ...
fi
\

\N|Here, if \c{cleanup}'s \c{cd} fails, the \c{ERR} trap will be executed in
the subshell, causing it to exit with an error status, which the parent shell
then makes available in \c{PIPESTATUS}.|

The recommendation is then to avoid calling functions in contexts where the
\c{ERR} trap is ignored resorting to the above pipe trick where that's not
possible.  And to be mindful of the potential ambiguity between the true/false
result and failure for other commands. The use of the \c{&&} and \c{||}
command expressions is best left to the interactive shell.

\N|The pipe trick cannot be used if the function needs to modify the global
state. Such a function, however, might as well return the exit status also as
part of the global state. The pipe trick can also be used to ignore the exit
status of a command.|

The pipe trick can also be used to distinguish between different exit codes,
for example:

\
function func()
{
  bar  # If this command fails, the function returns 1.

  if ... ; then
    return 2
  fi
}

set +o pipefail
func | cat
r=\"${PIPESTATUS[0]}\"
set -o pipefail

case \"$r\" in
  0)
    ;;
  1)
    exit 1
    ;;
  2)
    ...
    ;;
esac
\

\N|In such functions it makes sense to keep exit code 1 to mean failure so
that the inherited \c{ERR} trap can be re-used.|

This technique can be further extended to implement functions that both
return multiple exit codes and produce output, for example:

\
function func()
{
  bar  # If this command fails, the function returns 1.

  if ... ; then
    return 2
  fi

  echo result
}

set +o pipefail
func | readarray -t ro
r=\"${PIPESTATUS[0]}\"
set -o pipefail

case \"$r\" in
  0)
    echo \"${ro[0]}\"
    ;;
  1)
    exit 1
    ;;
  2)
    ...
    ;;
esac
\

\N|We use \c{readarray} instead of \c{read} since the latter fails if the left
hand side of the pipeline does not produce anything.|

"