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Some tangential musings:

I can't stop but see here parallels between cmdline arguments vs. environment variables for programs, and keyword arguments vs. dynamic binding for functions in a program (particularly in a Lisp one).

That is, $ program --foo=bar vs. $ FOO=bar program

seems analogous to:

  (function :foo "bar")
  ;; vs.
  (let ((*foo* "bar"))
    (function))

When writing code (particularly Lisp), keyword arguments are preferred to dynamic binding because the function signature is then explicitly listing arguments it uses, and dynamic binding (the in-process phenomenon analogous to inheriting environment from a parent process) is seen as dangerous, and a source of external state that may be difficult to trace/spot in the code.

I suppose the latter argument applies to env vars as well - you can accidentally pass values differing from expectation because you didn't know a parent process changed them. The former doesn't, because processes don't have a "signature" specifying its options, at least not in a machine-readable form. Which is somewhat surprising - pretty much all software written over the past decades follows the pattern of accepting argc, argv[] (and env[]), and yet the format of arguments is entirely hidden inside the actual program. I wonder why there isn't a way to specify accepted arguments as e.g. metadata of the executable file?

Interesting that you bring up Lisp and dynamic scope, since I've previously combined env vars with Racket's "parameters" (AKA dynamic bindings): https://lobste.rs/s/gnniei/setenv_fiasco_2015#c_owz2pp

Yeah, I just can't stop thinking about envs / dynamic binding when someone brings up the other.

Speaking of using both direct and dynamic arguments, a common pattern in (Common) Lisp is using dynamically-bound variables (so-called "special" variables) to provide default values. Since default values in (Common) Lisp can be given as any expression, which gets evaluated at function-call time, I can write this:

  (defun function (&key (some-keyword-arg *some-special*))
    ...)

And then, if I call this function with no arguments, i.e (some-function), the some-keyword-arg will pull its value from current dynamic value of some-special. In process land, this would be equivalent to having command line parser use values in environment variables as defaults for arguments that were not provided.

Lisp will at least warn you if you bind a variable with earmuffs that isn't declared special though. Biggest downside to environment variables is the lack of warning if you misspell something.

In my head there's been a hierarchy for a long time.

when I build command line utilities and I think about the way that they'll be used, I tend to use configuration files for things that will change very slowly over time, and environment variables as a way to override default behaviors, and command line arguments to specify things that will often vary from invocation to invocation. In fact, most of the time, I use the environment variables either for development/testing features that I don't really intend to expose to most users, or for credentials that don't get persisted.

it's never occurred to me to use environment variables as a primary way to configure an application. I'll have to noodle on that for a while. My gut says that it's enough of a deviation from the Unix convention that I probably won't use that.

> My gut says that it's enough of a deviation from the Unix convention that I probably won't use that.

It's not completely unheard of. Checking the man pages for a few I remember being affected:

* `ls` looks at LS_COLORS

* `grep` looks at GREP_OPTIONS, GREP_COLOR, GREP_COLORS, as well as the not-grep-specific LC_ALL, LC_COLLATE, LANG, etc, and POSIXLY_CORRECT

* `find` also looks at LC_%, LANG, POSIXLY_CORRECT as well, plus TZ and PATH for some of its other options

Looks like the program-specific ones LS_% and GREP_% can be overridden by options directly on the command, but the others don't have such options.

(Makefile wildcards because asterisk keeps doing italics)

These are all GNU/Linuxisms and are nowhere to be found in a real UNIX®️ like illumos, Solaris, HP-UX or IRIX64.

This is also a good example of why having GNU/Linux as one's first OS instead of a real UNIX®️ is so toxic.

Is this a joke?

No, it's a judgment on GNU, GNU/Linux and the horrid incompetence of contemporary information technology industry.

I feel like I didn't make "primary" clear enough - it would be like using an environment variable for the path for ls. That's gonna be a big no from me, dog.

> it would be like using an environment variable for the path for ls. That's gonna be a big no from me, dog

Me too. My point about env vars was limited to key/value options. I'm happy to use positional arguments when names are irrelevant, e.g. `ls foo/` is better than `DIR=foo/ ls` or `ls --dir foo/` or `ls --dir=foo/` or whatever, since the latter just adds noise and increases the chance we'll get something wrong.

I'm also happy to use arguments for flags which are either present or absent, where values are irrelevant, e.g. `ls --all` rather than e.g. `ALL=1 ls` or `ls --all=yes` or whatever. In this case the presence of a value is even more hamful, since the program might only be checking for the variable's presence, in which case `ALL=0 ls` would be misleading; plus we'd have to guess whether the value should be 0/1, yes/no, y/n, on/off, enable/disable, etc.

There's a few reason I don't like environment variables for this: the first is that a random env variable can influence the operation of a program. Do "export foo=bar" and then maybe 30 minutes later you unexpectedly pass "foo". It's also hard to inspect; the output of "env" tends to be somewhat long, and you don't know which the program will pick up on. Flags are much clearer and more explicit.

There's also the issue that typo'ing "foo" as "fooo" will silently fail. Okay, that's a simple example, but some tools have "PROGRAMNAME_LONGDESCRIPTION". Being in all-caps also makes it hard to spot typos for me. You generally want your env vars to be long, so ensure they're unique.

> It's also hard to inspect; the output of "env" tends to be somewhat long, and you don't know which the program will pick up on. Flags are much clearer and more explicit.

I don't think I disagree with your post, but this particular point seems to be trivially solved by prefixing the variable name with the program name?

IOW: `env | grep '^GIT_'` to see what you're passing to `git`.

Yeah, fair enough; that's what I do as well.

I’m curious if your preference for environment variables is only for writing programs or for using programs as well. From a user’s perspective, would you prefer that common commands use environment variables to get options? For example, would you prefer a “find” command that uses environment variables instead of command line options?

That would depend on the target, the Grand Parent talks about Automation thus the target for their utilities is most likely the system not a user.

When writing automation routines env vars are often better form my experience

Tools that will be manually run by a user or admin then cli options are better

> For example, would you prefer a “find” command that uses environment variables instead of command line options?

I use 'find' so much that my muscle memory would hurt if it changed, but it's actually a really interesting example. Its use of arguments to build a domain-specific language is a cool hack, but pretty horrendous; e.g.

    find . -type f -a -not \( -name \*.htm -o -name \*.html \)

We can compare this to tools like 'jq', which use a DSL but keep it inside a single string argument:

    key=foo val=bar jq -n '{(env.key): env.val}'

Note that 'jq' also accepts key/value pairs via commandline arguments, but that requires triplets of arguments, e.g.

    jq -n --arg key foo --arg val bar '{(env.key): env.val}'

The second example didn't work for jq-1.6 ---I had to write it as

  jq -n --arg key foo --arg val bar '{($key): $val}'

Yes you're right. I spotted it myself, but only after I couldn't edit anymore :(

I think there's a place for both, but env vars can make things really annoying to troubleshoot. Already, I often print all env vars before running automated commands and it can be a mess to dig through. Culling down environment variables when spawning a subprocess is difficult. Bad flags can error immediately if you made a typo. I've often misspelled an envvar and its hard to tell it did nothing (I think I saw a recent bug where trailing white space like "FOO " was the source of a years long bug). `FOO=BAR cmd` is also weird for history (although that's mostly a tooling issue).

It seems equivalent to long options requiring a value. Also, if you mistype an environment variable, you won't get warned about it.

Another nice thing about env vars is that they don't appear in the process table, unlike argv. For many applications, that makes them a suitable place to inject secrets, and makes them convenient to inject via wrapper tools.

For instance, the fact that the aws(1) cli tool and the AWS SDKs by default take IAM creds from AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, etc. means that you can write programs like aws-vault (https://github.com/99designs/aws-vault) which can acquire a temporary AWS session and then execute another command with the creds injected in the standard AWS_... env vars. For instance:

    aws-vault exec prod-read -- aws s3 ls
    aws-vault exec stage-admin -- ./my-script-that-uses-the-aws-sdk-internally

Also, passing arguments as env vars is part of the "12-factor app" design (https://12factor.net/config). That page has some good guidance.

It's protected by at least uid, but `/proc/$PID/environ` does exist, and it might be exposed other ways, too.

The environment does appear in the process table: see `ps e` on Debian at https://manpages.debian.org/buster/procps/ps.1.en.html or ps -e on BSD at https://www.freebsd.org/cgi/man.cgi?query=ps&apropos=0&sekti... or pargs -e on Solaris at https://www.unix.com/man-page/opensolaris/1/pargs

That's actually a bad thing for non-secure arguments, and environment variables aren't much better for secure arguments.

Quick comment, not an expert, but environ vars keep their state after the program is called. From a functional programming perspective, or just for my own sanity, wouldn’t it be more interesting to keep the states into minimum?

You're right that mutable state should be kept to a minimum, but immutable state is fine. There usually isn't much need to mutate env vars.

Some thoughts/remarks:

- If we don't change a variable/value then it's immutable. Some languages let us enforce this (e.g. 'const'), which is nice, but we shouldn't worry too much if our language doesn't.

- We're violating 'single source of truth' if we have some parts of our code reading the env var, and some reading from a normal language variable. This also applies to arguments through.

- Reading from an env var is an I/O effect, which we should minimise.

- We can solve the last 2 problems by reading each env var once, up-front, then passing the result around as a normal value (i.e. functional core, imperative shell)

- Env vars are easy to shadow, e.g. if our script uses variables FOO and BAR, we can use a different value for BAR within a sub-command, e.g. in bash:

    BAR=abc someCommand

This will inherit 'FOO' but use a different value of 'BAR'. This isn't mutable state, it's a nested context, more like:

    let foo  = 123
        bar  = "hello"
        baz  = otherCommand
        quux = let bar = "abc"
                in someCommand

As TeMPOraL notes, env vars are more like dynamically scoped variables, whilst most language variables are lexically scoped.

That's only if you `export` the vars. You can do `FOO=1 BAR=2 cmd` and FOO and BAR will only have those values for the process and children. This is isomorphic to `cmd --foo=1 --bar=2`.

Absolutely, this is one of the reasons that environment variables are a nightmare to work with

The beautiful thing of environment variables is that you can read them whenever you actually need them in your program. They pierce a hole through your call stack to the point that you need them. On the contrary, for command line arguments, you need to pass their values from the main function through whatever deep of the call stack you need them.

I think that's an advantage. You know what goes in and out, you can see where it's passed and used. It provides transparency and fewer accidental behavioral changes.

E.g. I had a really weird bug in a script, git seemed to break for no reason, until i figured out that some other script had exported GIT_DIR which pointed to the git top level directory. The name of the variable isn't bad if want to save that directory. But git uses GIT_DIR as the location of the .git directory it should look at when it is defined.

Using environment variables is kind of like using global state, it can often cause weird behavior at a distance without a clear path of influence if you don't know exactly which environment variables are used by all your scripts.

In many languages you can always get at the argv list; e.g. sys.argv in Python, os.Args() in Go, ARGV in Ruby, etc.

It's not universal though; you can't in C or shell scripts for example (although, if you really want to, it's not hard to assign a global). I never had the need to inspect commandline arguments beyond the main() function though.

Same thing can be said about global variables. I think any sane developer would agree it's a anti-pattern.

Mutating global variables is certainly an anti-pattern, but I'm not so convinced that global constants are a problem; i.e. if we treat env vars like immutable variables (in fact I treat most variables as immutable).

I'd say that's an example of "shotgun parsing", which is a bad idea since we might have already performed some irrevocable actions by the time we notice part of the input is missing/invalid ( https://stackoverflow.com/questions/50852226/what-does-shotg... ). It's better to check for and parse all input up-front https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-va...

I write a lot of tools in Haskell, where 'getEnv' is an "IO action" whose type prevents us using it inside most code. The simplest way to handle this ends up with a "functional core, imperative shell" model, e.g. most of my CLIs have the form (where 'interact' takes a pure String -> String function and uses it to transform stdio):

    main = do
      foo <- either error id . parseFoo <$> getEnv "FOO"
      interact (doTheThing foo)

    doTheThing :: Foo -> String -> String
    doTheThing = ...

This is also trivial to test. Some languages make it easy to replace env vars inside a single expression (e.g. bash), but others require us to set and reset the global environment, which is painful and error prone (especially if tests are run concurrently).

If the command line arguments are getting to0 much, the next step is to use an config file, like an INI file and parse that.

Very simple and removes all the extensive command line clutter, especially if many settings stay the same.

I think environment variables are not very user friendly and not that visible.

Config files introduce a whole slew of complexity: race conditions, a whole bunch of new error conditions (permission denied, parse errors, read-only filesystems, disk full, etc.); system- or user-wide config files are spooky action-at-a-distance, which can make problems hard to track down, makes commands non-deterministic and may impact reproducibility (e.g. on other people's machines).

If commands insist on taking options via a config file, my preferred way to call them is to generate the config in-place via process substitution, e.g.

    someCommand -c <(printf 'foo\n\tbar="%s"\n\tbaz="%s"\n' "$BAR" "$BAZ")

Although this isn't always possible, e.g. if 'someCommand' tries seeking around the config file rather than just parsing it in one pass.

Libraries can provide validation of key:value options. For example in python-land ‘click’ provides ‘nargs’ and ‘type’ parameters for the common case, and if those don’t suffice a custom coerce/validate callback.