Today I learned that bash has hashmaps (2024)

https://xeiaso.net/notes/2024/bash-hashmap/

by stefankuehnel • 2 years ago

177 134 2 years ago

Not only do they exist, but they have some fantastic foot guns! https://mywiki.wooledge.org/BashPitfalls#A.5B.5B_-v_hash.5B....

stefankuehnel• 2 years ago

Interesting, definitely need to keep that in mind.

forgotpwd162• 2 years ago

they have some fantastic foot guns!

Otherwise wouldn't be getting the full shell experience.

miohtama• 2 years ago

It's not like we would have 50 years of good programming language design and then stick to things that were created 50 years ago.

echelon• 2 years ago

Why hasn't shell scripting evolved? It's god-awful.

wruza3• 2 years ago

A comment worth more than a post, thanks!

chengiz• 2 years ago

Too many. I still write /bin/sh syntax (I know it's a symlink to bash now but I mean the old school sh). Anything that requires bash-that-isnt-sh is usually better written in perl or something else.

TristanBall• 2 years ago

Only on some distro's.

Debian variants tend to link dash, which self consciously limits itself to posix compatibility.

rqtwteye• 2 years ago

"fantastic foot guns"

It wouldn't be a worthy bash feature if after learning about it you wouldn't spend a few days figuring out why the damn thing doesn't work the way it works in any other language.

aunderscored2• 2 years ago

Indeed! It's delightful like all of bash. I've used them in a few places but I try not to

oniony2• 2 years ago

A couple of weeks ago I learnt that Bash on Mac does not have associative arrays. We worked around the issue by changing the script to run under Zsh, but beware.

ptdorf• 2 years ago

Sounds like a bash 3 issue.

$ bash --version

GNU bash, version 5.2.32(1)-release (aarch64-apple-darwin23.4.0)

$ declare -A aaa; aaa[a]=a; aaa[b]=bb; for i in ${!aaa[@]}; do echo "$i --> ${aaa[$i]}"; done

b --> bb

a --> a

forgotpwd162• 2 years ago

To elaborate on this, macOS default bash is still stuck (assuming due to licensing) in v3.2.x (released in 2007). Bash got associative arrays in v4 (released in 2009).

x3n0ph3n32• 2 years ago

You should be getting bash from homebrew anyways.

pletnes3• 2 years ago

Just use the included zsh.

eadmund• 2 years ago

Or using an OS which doesn’t ship ancient software. Linux exists. It’s pretty awesome.

teddyh7• 2 years ago

Many programs on macOS are stuck in the ancient past, due to Apple:

<https://web.archive.org/web/20240810094701/https://meta.ath0...>

agnishom2• 2 years ago

Q: How do I declare a Hashmap?
A: You use the command `declare -A HASHMAP_NAME`

This is why I think Bash is a horrible language

jolmg• 2 years ago

Because of the -A instead of an -H? -A is for "Associative array".

arter4• 2 years ago

what do you mean?

agnishom2• 2 years ago

Just that it is ugly and strange

teddyh7• 2 years ago

It’s amazing how much of a superpower merely reading the manual is nowadays.

<https://www.gnu.org/software/bash/manual/bash.html#Arrays>

latexr2• 2 years ago

While I agree with the larger sentiment I think you’re making (I also make it a habit to peruse the manual just to see what is available), how often do you reread the manual, especially for your shell? I knew about associative arrays in bash, but not by reading the manual as those were introduced in v4 and I’ve been using bash for longer than that.

teddyh7• 2 years ago

You should probably read the release notes for new major versions. Also, anytime you think “I wish there was <feature> available in this program”, you should probably double-check if such a feature has appeared since you last learned the program.

akvadrako2• 2 years ago

I do a fair bit of bash scripting and there is no way I can remember all the features and syntax I use, so of course I read parts of the manual regularly and often learn new things. You don't need to ever read the whole thing.

sgarland4• 2 years ago

I once worked with a dev (frontend, of all things) who reread the bash manual annually.

I work in ops, and I don’t even have that dedication. My hat is off to them.

dredmorbius4• 2 years ago

TBF, the Bash manpage now runs > 80 pages:

  man -Tps bash | ps2pdf - bash.pdf

That can be daunting. Though illuminating.

wruza3• 2 years ago

My one-shot memory worsened with years but grasping concepts from examples got much better due to experience. So I find walls of text much less useful than a continuous “synopsis” of snippets and best practices. One usage example is worth a hundred of words and few pages of it may be worth a whole manual, especially when “at hand” through editor means. For bash idk if that exists on the internet, so I maintain my own in my one big text file. I wish every manual had the synopsis-only section (maybe I should just LLM these).

farrelle25• 2 years ago

One usage example is worth a hundred words ... For bash idk if that exists on the internet, ...

Totally agree with that - I maintain a big txt file too.

Maybe this bash compendium is a bit similar:

https://www.commandlinefu.com/commands/browse

teddyh7• 2 years ago

You might like these:

• <https://learnxinyminutes.com/>

And some code search engines:

• Debian: <https://codesearch.debian.net/>

• Python: <https://www.programcreek.com/python/>

• Linux: <https://livegrep.com/search/linux>

• GitHub: <https://grep.app/>

• HTML, JS and CSS: <https://publicwww.com/>

johannes1234321• 2 years ago

Bash is low on the list of things to learn, especially as many greybeards suggest keeping on POSIX compatibility and/or using a "proper" language (Python, older: Perl) for anything longer than a few lines.

jeltz• 2 years ago

They suggest that based on their experience. Error handling in bash is awful so I advice against using it for anything complex.

superq2• 2 years ago

Give bash some credit - it's actually amazing for even very large, complex systems (of which there are many -- quietly doing their jobs for decades.)

With xargs or GNU parallel, you can have multi-processing, too. Combining with curl or ffmpeg, you can literally build a production-grade web scraper or video transcoding pipeline in a couple of minutes and a few dozen lines of code.

dolni• 2 years ago

I write a lot of shell and my advice is don't use plain POSIX shell. Write bash.

It is 2025. bash is present almost everywhere. If, by chance, your script is run somewhere that doesn't have bash then guess what?

Your POSIX script probably won't work anyway. It will be a different system, with different utilities and conventions.

Line count is not a good reason to choose or avoid bash. Bash is quite reliably the lowest common denominator.

I dare say it is even the best choice when your goal is accomplished by just running a bunch of commands.

All that said, bash is gross. I wish we had something as pervasive that wasn't so yucky.

crabbone2• 2 years ago

To me, this is a development in the wrong direction. Shell is great precisely because it's so minimal. Everything is string rule is one that calms all of your type-induced fears.

Having to implement hash-tables, while still keeping the appearances of everything being a string is the anti-pattern known as "string programming" (i.e. when application develops a convention about storing type / structure information inside unstructured data, often strings).

I would love for there to be a different system shell with a small number of built-in types that include some type for constructing complex types. There are many languages that have that kind of type system: Erlang, for example. But, extending Unix Shell in this way is not going to accomplish this.

latexr2• 2 years ago

To me, this is a development in the wrong direction.

That ship has sailed. These were introduced in Bash 4, released in 2009, and Bash is already on version 5.

mhd• 2 years ago

Isn't this basically the same feature that existed in ksh93?

crabbone2• 2 years ago

Oh, but we live with a lot of consequences of bad decisions, some of which have been made thousands of years ago :) A decade or two is nothing compared to it.

biorach• 2 years ago

Every few years I rediscover this fact and every few years I do my best to forget it

j45• 2 years ago

For some tasks, if as much as possible was coded in bash, it would work being called anywhere from any programming language.

Now to add hashtables to that.

somat• 2 years ago

I love shell, I think it's killer feature are pipes and whoever figured out how to design so you can pipe in and out of control structures(Doug Mcilroy?) is a goddamn genius. however, after writing one to many overly clever shell scripts, I have a very clearly delineated point in which the script has become too complex and it is time to rewrite in a language better suited for the task. and that point is when I need associative arrays.

A lot of the sins of shell are due to it's primary focus as an interactive language. Many of those features that make it so nice interactively really hurt it as a scripting language.

DSpinellis2• 2 years ago

Pipe-related concepts in various restricted forms were floating around for years. Doug McIlroy indeed proposed them in 1964 and was heading the Bell Labs team when they were implemented in the Third Research Edition of Unix (1973).

See https://youtu.be/FbDebSinSQo?si=xbWIqES80hI2S802&t=901

emmelaich• 2 years ago

Sure, but in and out of control structures? I think that's the major point your parent was making.

orbisvicis• 2 years ago

Control structures? Do you mean something like '{ cmd1 || cmd2; } | cmd3', where the control structures are the braces?

layer85• 2 years ago

“Control structure” normally refers to conditionals and loops. You can pipe in and out of a for loop in the shell, maybe that’s what they mean.

dghf2• 2 years ago

Picky and probably pointless question: are they actually hashmaps? If I understand correctly, a hashmap isn’t the only way to implement an associative array.

zabzonk• 2 years ago

That is certainly true - associative arrays in c++ (std::map) for example, are implemented as red-black trees.

tgv3• 2 years ago

I think many devs don't know the difference and simply call any dictionary/associative array a hash map. It might be one of the concepts that "the internet" promotes: it sounds fancy, more technical, makes you seem more knowledgeable, so it gets repeated. Then newcomers think this is the way it's always been called, and that gives it enough visibility to become the preferred name.

meltyness• 2 years ago

This seems pertinent, NEWS > bash-5.0 to bash-5.1 > mm

Seems to suggest it's indeed a hashmap in implementation, but I can't be bothered to look any closer.

layer85• 2 years ago

…and if so, do they mitigate against hash collision attacks?

wwoessi• 2 years ago

bashmaps?

DSpinellis2• 2 years ago

I advocate the following rules for when to write and when not to write a shell script.

# Write a shell script:

* Heavy lifting done by powerful tool (sort, grep, curl, git, sed, find, …)

* Script will glue diverse tools

* Workflow resembles a pipeline

* Steps can be interactively developed as shell commands

* Portability

* Avoid dependency hell

* One-off job

# Avoid shell scripting:

* Difficult to see the preceding patterns

* Hot loops

* Complex arithmetic / data structures / parameters / error handling

* Mostly binary data

* Large code body (> 500 LoC)

* Need a user interface

A need for associative arrays (implemented in Bash as via hashmaps) moves the task to the second category (avoid shell scripting).

nativeit3• 2 years ago

I’ve been a sysadmin for nearly a decade, a frontend web designer for much longer (I still have a book on all the exciting changes in HTML4), and while I can very easily learn, compose, and use many markup and scripting languages, I have always struggled with full-on programming languages, and I’m not exactly sure why. Part of it, I think, is that most tutorials and online learning resources are focused on novices who don’t have any existing grasp on general programming concepts and syntax, but I’m already pretty deep into Bash. To the extent that I am sure I have crossed the thresholds you list, and used quite long and complex Bash scripts for tasks that would almost certainly be easier in Python. I’d love to find A Bash Scripter’s Guide To Python or something similar—an intermediate course that assumes that I already know about variables, operators, functions, Boolean expressions, et al. I have searched for this a few times, but it’s full of keywords that makes searching Google difficult.

So this has inspired me to Ask HN, I’m getting ready to post it with a reference to this discussion, but thought I’d start here: does anyone know of a good resource for learning Python (or Go, Perl…any good small project/scripting/solo hacking languages) that’s tailored for folks who already have a good foundation in shell scripting, markup, config/infra automation, build tools, etc.? I’m open to books, web-based tutorials, apps, or just good examples for studying.

I’m open to the notion that I simply haven’t put in the work, and powered through the doldrums of novice tutorials far enough to get to where the meaty stuff is. Worst case, I’m a big fan of taking courses at the local community college for my own edification, absent any specific degree program or professional certification. It would still necessitate a lot of remedial/redundant steps, but I can always help others through those chapters while filling in any of my own gaps. Day-to-day, I generally work alone, but I find such things are easier with others to help motivate and focus my efforts. Perhaps I have answered my own question, but even still, I appreciate any advice/recommendations folks might have.

nativeit3• 2 years ago

My search through existing Ask HN has shown that this, unsurprisingly, has been asked before by several people in other specific contexts (side note, Ask HN is such an incredibly useful resource, thanks to everyone who engages with these questions). I don’t want to add more noise, so I’m going to work through the existing answers before posting.

If anyone else is interested, this thread from 2020 is where I am starting, it seems to align with my own quest pretty well: https://news.ycombinator.com/item?id=22932794

wellshapedwords• 2 years ago

assumes that I already know about variables, operators, functions, Boolean expressions, et al.

Learning Go by Jon Bodner is a good choice. It seems to assume that Go is the reader's second (or tenth) language.

brianpan• 2 years ago

One of the differences between "general programming" and the kinds of coding that you have done is the /approach/. I'm not sure if that's even the right word, but there's a different set of concerns that are important.

My guess is that you can easily learn the syntax and that you have the logical and analytical skills, but the part you have to learn is how to THINK about programming and how to DESIGN a program. If you have or haven't taking coding classes, I think reviewing topics like data structures, algorithms, encapsulation/object oriented programming, functional programming, etc. is the way to learn how to think about general programming. I don't think the language matters, but there might be resources about these topics in the language you're interested in.

An example of what you DON'T want is something like Effective Go (not just because it's out-of-date now): https://go.dev/doc/effective_go. This page can give you a really good base of information about what Go is, but I think you'll get much more bang for your buck with a resource that is more about the WHYs of programming rather than the WHATs.

kmstout• 2 years ago

A couple things:

- Between using Bash's functions and breaking large scripts into multiple, separate scripts, one can keep things reasonably tidy. Also, functions and scripts can be combined in all the ways (e.g., piping and process substition) that other programs can.

- If I run into a case where Bash is a poor fit for a job, I ask myself, "Self, what program would make this easy to do in a shell script?" If I can respond with a good answer, I write that, then continue with the shell scripting. If not, I write in something else (what the kids would call a "proper" language).

pie_flavor• 2 years ago

I'm guilty of this. I knew zsh had them but since I can never remember the exact things zsh has that bash doesn't, I just assume anything remotely useful isn't compatible.

This policy comes from a six hour debugging session involving (somewhere) a script that manipulated a binary file - bash can't store zero bytes in variables and zsh can, but it's not like it'll tell you that it's pointlessly truncating your data and I never thought to look. So now every step in that script gets converted back and forth through xxd, and I don't trust Bash anymore.

rednafi• 2 years ago

I discover stuff like this every day, and it’s delightful. Sure, reading the manual would’ve saved me from the surprise, but it’s incredibly difficult for me to read them unless I have a specific goal in hand.

I found out about hashmaps in Bash a year ago[1], and it came as a surprise. This morning, I came across dynamic shell variables and the indirection syntax[2]. Each time I wrote about them and learned more than I would have if I had just passively grokked the manual.

[1] https://rednafi.com/misc/associative_arrays_in_bash/

[2] https://rednafi.com/misc/dynamic_shell_variables/

lervag• 2 years ago

Notice that other shells also has associative arrays, or at least zsh. I've found hyperpolyglot [0] to be a nice Rosetta stone for translating syntax between e.g. bash and zsh.

[0] https://hyperpolyglot.org/unix-shells#associative-arrays

nickjj2• 2 years ago

One thing the article doesn't mention is how you can use indirect expansion to get a list of all keys.

For example: `${!myvar[@]}` would list all of the keys.

I've written about associative arrays in Bash a bit here: https://nickjanetakis.com/blog/associative-arrays-in-bash-ak...

singularity2001• 2 years ago

doesn't work in zsh?

nickjj2• 2 years ago

It doesn't but if you're putting this into a script that's ok. You can set the script's shebang to bash so even if your user's shell is using zsh, the script will run with bash.

PeterWhittaker• 2 years ago

bash associative arrays are fantastic, I've made heavy use of them, but be warned that there have been several memory leaks in the implementation. (Sorry, no version notes, once we realized this, we rewrote a key component in C.)

IIRC, the latest bash addresses all of these, but that doesn't help too much if you are stuck with an older, stable OS, e.g., RHEL 7 or 8; even 9 likely has a few remaining.

These leaks become an issue if you have a long running bash script with frequent adds and updates. The update leak can be mitigated somewhat by calling unset, e.g., unset a[b], before updating, but only partially (again, apologies, no notes, just the memory of the discovery and the need to drop bash).

I'd agree with the idea that bash wasn't the best choice for this purpose in the first place, but there was history and other technical debt involved.

oweiler• 2 years ago

They are also slow AF because a lookup takes linear time.

Izkata2• 2 years ago

Which is because they're not hashmaps, they're associative arrays. Article treats them as the same thing, but they're not - that's why the "declare -A" is A and not H - it stands for "associative".

xena• 2 years ago

Author of the article here. As far as I care, if it quacks like a hashmap, it's better to describe it as a hashmap. The fact that it's an associative array under the hood is irrelevant.

DiabloD3• 2 years ago

No one else seems to have mentioned this, but POSIX sh does not include this feature.

Until it does, and major POSIX shs have shipped with it for a decade, then the feature will actually exist in a way that the average shell coder cares about. You're better off just shipping Rust/Go/etc binaries and leave sh for your simple glue tasks.

Even I've eventually switched to this, and I've written 10k+ long Bash scripts that followed best practices and passed shellcheck's more paranoid optional tests.

Use the right language for the right job.

abathur• 2 years ago

then the feature will actually exist in a way that the average shell coder cares about

I think it's worth picking at this a bit. At least IME, a fairly small fraction of the Shell I write needs to run literally (any|every)where.

I don't mean to suggest there aren't people/projects for whom the opposite is true--just that it's worth chunking those cases and thinking about them differently.

It obviously isn't a global solution, but in the Nix ecosystem we have decent idioms for asserting control over the shell and tools any given script runs with.

Reliable environment provisioning can spare hundreds of lines of code real-world Shell tends to accumulate to sanity-check the runtime environment and adapt to the presence/absence of various utilities. It also enables us to use newer shells/features (and shell libraries) with confidence.

It's enough leverage that I only give it up when I must.

nmz5• 2 years ago

POSIX sh doesn't even have arrays. I remember having to use awk for that functionality.

nielsbot• 2 years ago

Nice that these exist—but does anyone else absolutely abhor shell programming? The syntax is impossible to memorize, it’s incredibly easy to make mistakes, and debugging is a pain. I hate it more than C++ and AppleScript.

ryapric• 2 years ago

I enjoy it quite a lot :shrug:

Bash itself isn't a very big language, so I wouldn't call it "impossible to memorize".

irunmyownemail2• 2 years ago

I have around a billion short bash scripts, it's rare I've used an array but it's cool it has it as long as the script doesn't go beyond a few lines.

nmz5• 2 years ago

Have you tried plan9's rc? easy to memorize syntax thing. It's still easy to make mistakes though and nobody uses it.

layer85• 2 years ago

I find the semantics (e.g. the order in which elements are replaced/parsed/processed) more of a problem than the syntax.

akvadrako2• 2 years ago

It has some rough edges but bash has the world's best REPL and along with -x that makes debugging quite easy.

lars512• 2 years ago

Unfortunately, MacOS ships an earlier version of bash that does not include associative arrays, so they’re not as portable as you might like.

fastily• 2 years ago

You can use homebrew to install the latest version of bash and then chsh to set it as your default shell. That’s what I do anyways

syntacticbs2• 2 years ago

Oh dear. I've been trying to get people to not use this feature for a while.

One thing that has bitten me in the past is that, if you declare your associative arrays within a function, that associative array is ALWAYS global. Even if you declare it with `local -A` it will still be global. Despite that, you cannot pass an associative array to a function by value. I say "by value" because while you can't call `foo ${associative_array}` and pick it up in foo with `local arg=$1, you can pass it "by reference" with `foo associative_array` and pick it up in foo with `local -n arg=$1`, but if you give the passed in dereferenced variable a name that is already used in the global scope, it will blow up, eg `local -n associative_array=$1`.

As a general rule for myself when writing bash, if I think one of my colleagues who has an passable knowledge of bash will need to get man pages out to figure out what my code is doing, the bash foo is too strong and it needs to be dumbed down or re-written in another language. Associative arrays almost always hit this bar.

jpavel2• 2 years ago

I'm not seeing this local scope leak with bash 5.2.15. The below script works as I'd expect:

  #!/bin/bash
  
  declare -A map1=([x]=2)
  echo "1. Global scope map1[x]: ${map1[x]}"
  
  func1() {
      echo " * Enter func1"
      local -A map1
      map1[x]=3
      echo "   Local scope map1[x]: ${map1[x]}"
  }
  
  func1
  
  echo "2. Global scope map1[x]: ${map1[x]}"

outputting

  1. Global scope map1[x]: 2
   * Enter func1
     Local scope map1[x]: 3
  2. Global scope map1[x]: 2

syntacticbs2• 2 years ago

The local scope leak seems to only happen when you drop down the call stack. See below how I can call func2 from the top level and it's fine, but if I call it from within func1, it will leak.

  #!/bin/bash
  
  declare -A map1=([x]=2)
  echo "1. Global scope map1[x]: ${map1[x]}"
  func1() {
      echo " * Enter func1"
      local -A map1
      map1[x]=3
      echo "   Local scope map1[x]: ${map1[x]}"
      func2
  }
  func2() {
      echo "  * Enter func2"
      echo "      Local scope map1[x]: ${map1[x]}"
  }
  func1
  func2
  echo "2. Global scope map1[x]: ${map1[x]}"

outputing: 1. Global scope map1[x]: 2 * Enter func1 Local scope map1[x]: 3 * Enter func2 Local scope map1[x]: 3 * Enter func2 Local scope map1[x]: 2 2. Global scope map1[x]: 2

UPDATE: I did a bit of exploration and it turns out ANY variable declared `local` is in the scope of a function lower down in the call stack. But if you declare a variable as `local` in a called function that shadows the name of a variable in a callee function, it will shadow the callee's name and reset the variable back to the vallee's value when the function returns. I have been writing bash for years and did not realise this is the case. It is even described in the man page: When local is used within a function, it causes the variable name to have a visible scope restricted to that function and its children.

Thank you. You have taught me two things today. One is a bash feature I did not know existed. The second is a new reason to avoid writing complex bash.

faragon• 2 years ago

LZ77 compression/decompression in pure Bash using hashmaps:

https://github.com/faragon/lzb/blob/master/lzb

evgpbfhnr• 2 years ago

Don't loop on values with `*`; the difference key/value is the lack of `!` at the start of the expression; `*` and `@` rules are the sames a $@ and $* and you almost never want *.

ssahoo• 2 years ago

With that kind of syntax, Wouldn't you just use perl or python instead?

bluedino• 2 years ago

Got burned on this on an interview question before.

In a related question I said something about switching to a language like Python when a script started to get "complicated"

Then the interviewer explained how his favorite language was bash, how he uses it for everything...

I did not get the job. Ironically my next job I did a bunch of Perl to Bash conversion.

layer85• 2 years ago

Perl to Bash conversion

That sounds like the wrong direction. ;)

harel• 2 years ago

Before I learned about SSH identities, I used to have a shell script sourced into my bashrc with a hashmap containing all my hosts and full connect string, so that I could `ssh hostname` like one does with SSH identities.

rixed• 2 years ago

When you feel the need for a hash in shell, or even an array, is also probably when you should rewrite your shell in python.

(I feel obliged to add that when you feel the need to add type annotations a bit later is when to abandon python)

globular-toast2• 2 years ago

Where are people learning that "hashmap" means associative array? It's obviously an easier word so I can see the natural language preferring that. Is this common? I can see it causing some communication problems when the details are important.

adityaathalye• 2 years ago

Bash Associative Arrays [1] are handy! Some examples of how I've used them:

- my site builder (for evalapply.org): inject metadata into page templates. e.g. https://github.com/adityaathalye/shite/blob/b4163b566f0708fd...

- oxo game (tic-tac-toe): reverse index lookup table for board positions: https://github.com/adityaathalye/oxo/blob/7681e75edaeec5aa1f...

- personal machine setup: associate name of installed application to its apt source name, so we can check for the app, and then install package https://github.com/adityaathalye/bash-toolkit/blob/f856edd30...

[1] I'd say "hashmap" is acceptable, colloquially. However, I don't think Bash computes hashes of the keys.

(edit: fix formatting snafu)

Today I learned that bash has hashmaps (2024)

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