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author | Luke Shumaker <lukeshu@sbcglobal.net> | 2016-03-01 19:15:29 -0500 |
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committer | Luke Shumaker <lukeshu@sbcglobal.net> | 2016-03-01 19:15:29 -0500 |
commit | c83d2b58e1776982a7ce45009fb373ec5702c521 (patch) | |
tree | 5e6cae25f11846392936895f2c2a8ca019c21bbb /HACKING/code-quality.md | |
parent | 47004c9601147b4c5c55c2bd6401bda8b6be0406 (diff) |
improve HACKING documentation
Diffstat (limited to 'HACKING/code-quality.md')
-rw-r--r-- | HACKING/code-quality.md | 134 |
1 files changed, 134 insertions, 0 deletions
diff --git a/HACKING/code-quality.md b/HACKING/code-quality.md new file mode 100644 index 0000000..a520ce5 --- /dev/null +++ b/HACKING/code-quality.md @@ -0,0 +1,134 @@ +Code content +============ + +Be aware of the `librelib(7)` library suite, which lives `src/lib`. +It is a suite of Bash libraries that will help you out. Most of the +people looking at the libretools code are familiar with the `messages` +part of it, which actually contains a bunch of utility routines, not +just message printing. There is also a library for dealing with +`blacklist.txt`, and one for loading configuration files and +PKGBUILDs. These are common tasks, but are tricky to handle +consistently--the libraries are there to make things easier. Take a +look at the man pages. + +Message printing: All of the message printing routines, except for +`term_title` and `flag`, take printf-type arguments. Take advantage +of that; don't use string interpolation (don't do `"foo ${var} +bar"`). The reason for this is that if you don't do string +interpolation, messages can be automatically internationalized. +(Internationalization is incomplete at the momement) + +Message printing: The in `--help`/`-h` text, use `print` to print +lines that should not wrap, `echo` to print blank lines, `prose` to +print paragraphs, `bullet` to print bullet points, and `flag` to print +option flags. The text should follow this general format: + + print "Usage: %s [OPTIONS] VARS_ARE_UNDERSCORE_AND_CAPITAL" "${program_name}" + print "One line description of program, no period" + echo + prose "More details. This is a paragraph." + echo + print "Options:" + flag "-h" "Show this message" + +In the "Usage:" line, use printf `%s` and the value `"${0##*/}"` to +determine the program name at runtime. + +There used to be guidelines for how to align the option flags and +descriptions, but now the `flag` command exists takes care of it for +you. Yay for things being easier! + +When using `set -u`, `set -e`, or `trap`, you should also use `set -E` +to have the error handling be passed down to subshells. + +Feel free to use `set -e` (fail on error), but be careful of the +caveats (there are a bunch of them); don't assume all errors are +checked because of it. + +Use `set -u` if you can; it makes using an unset variable an error. + - If a variable not being set is valid (perhaps a configuration + option), use `${var:-}` when accessing it to suppress the error. + - An empty array counts as unset, so if you have an array that may be + empty, use `set +u` before accessing it. + - The reason for this is that a normal string variable is basically + an array with length=1; an unset variable looks like an array + with length=0. Weird stuff. + +In the shebang, use `#!/usr/bin/env bash`. This allows us to not +hardcode the location of bash (I'm not sure why this is useful for +something distro-dependent like libretools, but fauno seems to have a +use-case for it). + +In the shebang, don't pass flags to bash, besides breaking `env` +(above), it means people will make mistakes when debugging, and +running things with `bash FILENAME`. Instead, use `set` to adjust the +flags inside of the program. + +Obey `$TMPDIR`. It's usually as easy as passing `--tmpdir` to +`mktemp`. + +Use `trap` to clean up your temporary files. This way, even if your +program terminates early, things will be cleaned up. + +Bash best practices +=================== + +Basically, know what you are doing, and be safe with it. The problem +is that most people don't know about safe bash scripting. + +A lot of people look at the "Advanced Bash Scripting" ebook--DO NOT do +that, it is trash... though it contains a "reference card" page that +may be useful and isn't trash. + +Take a look at Gentoo's Bash guidelines +<http://devmanual.gentoo.org/tools-reference/bash/index.html>. +They're pretty good, and cover most of the "gotcha's" about Bash +syntax. It mentions but discourages the use of Bash 3 +features... why? Who still uses Bash 2? Feel free to use Bash 4 +features! + +I wrote an article on Bash arrays +<https://lukeshu.com/blog/bash-arrays.html>. A lot of people think +they're tricky, but they're simple once you know how they work. It's +short enough that you should read the whole thing. Know the +difference between `"${array[@]}"` and `"${array[*]}"`. And I'll say +it again here, ALWAYS wrap those in double quotes; there is no reason +I can think of that the unquoted behavior would ever be the correct +thing. + +My brief rules of thumb: + + - Quote every variable. + - That includes arrays: `"${array[@]}"` and `"${array[*]}"`. + - In most (but not all!) cases inside of `[[ ... ]]` conditions, + variables don't need to be quoted. When in doubt, quote them. + - When assigning one variable to another, you don't need quotes; + you don't need quotes for `foo=$bar` + - Try to avoid global variables; declare all variables in functions + with `local`. + - Or `declare`; inside of a function, unless you pass the `-g` + flag, `declare` makes the variable local. + - Use `local VAR` before a `for VAR in LIST` loop--the variable is created in the + current scope, not the scope of the loop. + - Feeding input to `while` loops is weird because of how subshells + work: + + # Input from a file + # BAD + cat file | while read line; do + ... + done + # GOOD + while read line; do + ... + done <file + + # Input from a program + # BAD + prog | while read line; do + ... + done + # GOOD + while read line; do + ... + done < <(prog) |