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https://github.com/dimo414/bash-cache

Transparent caching layer for bash functions; particularly useful for functions invoked as part of your prompt.
https://github.com/dimo414/bash-cache

bash cache decorator memoization profilegem prompt shell terminal

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Transparent caching layer for bash functions; particularly useful for functions invoked as part of your prompt.

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# Bash Cache



Bash Cache provides a transparent mechanism for caching, or memoizing, long-running Bash functions.

Although it can be used for scripting its motivating purpose is to cache the results of expensive

commands for display in your terminal prompt.



Originally part of [ProfileGem](http://git.mwdiamond.com/profilegem) and

[prompt.gem](http://git.mwdiamond.com/prompt.gem), this functionality has been pulled out

into a standalone utility.



This library has also inspired [`bkt`](http://git.mwdiamond.com/bkt), a standalone binary for

caching subprocess invocations. If bash-cache doesn't fit your use case see if `bkt` does.



## Installation



Simply `source bash-cache.sh` into your script or shell.



## Usage



```

bc::cache FUNCTION TTL REFRESH [ENV_VARS ...]

```



To cache a function pass its name to `bc::cache`, along with the amount of time the cached results

should persist. This function [decorates](https://en.wikipedia.org/wiki/Decorator_pattern) an

existing Bash function, wrapping it with a caching layer that temporarily retains the output and

exit status of the backing function.



By default the cache is keyed off the function arguments (meaning `some_func`, `some_func bar`, and

`some_func baz` are each cached separately).



Cached data **is shared across processes** by default; see below for ways to change this.



Some example usages can be seen in the

[prompt.gem project](https://github.com/dimo414/prompt.gem/blob/master/env_functions.sh).



### Fidelity



There are many existing command-caching utilities and patterns in the wild, however the cached

behavior is typically incomplete (often only caching stdout). bash-cache strives to provide

high-fidelity caching, such that cached results are as close to indiscernable as possible.



* stdout and stderr are both cached, and output separately to stdout and stderr respectively

* output is lossless; many implementations can't handle trailing whitespace or `nul` bytes

* exit status code is preserved

* positional arguments are respected; naive implementations may conflate `foo bar baz` (two args)

  and `foo 'bar baz'` (one arg with whitespace)



### Cache durations



Each cached result is associated with two durations; the *TTL* deadline and the *refresh* deadline.

Durations can be specified in (s)econds, (m)inutes, (h)ours, and (d)ays, for example `30s`, `1d`,

or `1h24m5s`. 



* Once a cached result exceeds its TTL it is eligible for cleanup, and will shortly be removed.

  Note that until it is cleaned up the cached data may still be returned from the cache.

  `1m` is a recommended TTL duration for functions that will be surfaced in a prompt.

* If a cached result exceeds its refresh deadline it will be asynchronously updated when the

  function is invoked. The cached data will continue to be used until the refresh completes.

  `10s` is a recommended refresh duration for functions that will be surfaced in a prompt.



### Customizing the cache key



If your function depends on additional state, such as the current working directory, you'll want to

ensure the cache is keyed off that state, in addition to the function's arguments. To do so pass

any relevant environment variable names to `bc::cache` after the function name.



* `PWD` is often used in order to cache a function based on the current working directory.

* `$` is less common, but can be used to isolate a function's cache to the current process. Note

  you'll need to single-quote this argument (`'$'`).



### Example usage



You can invoke `bc::cache` at any time, however you're encouraged to do so immediately following

the function definition as a form of self-documentation, similar to

[Python's `@decorator` notation](https://en.wikipedia.org/wiki/Python_syntax_and_semantics#Decorators):



```shell

my_expensive_function() {

  ...

} && bc::cache my_expensive_function 1m 10s PWD

```



Notice in this example `PWD` is specified, meaning the cache will key off the current working

directory in addition to any arguments to the function.



### Performance



Cached data is stored on-disk, which means accessing the cache will typically be *much* slower than

directly executing many simple commands. Generally speaking, operations which benefit from caching

are accessing the disk themselves or doing network I/O. You should benchmark your functions with and

without caching (see `bc::benchmark`) to ensure you see a meaningful improvement before deciding to

cache a particular function.



Caching performance can differ drastically across machines. Notably, if the cache directory (under

`/tmp` or `TMPDIR` by default) is on a [`tmpfs`](https://en.wikipedia.org/wiki/Tmpfs) partition or a

solid-state drive performance will be significantly better than caching to a spinning disk.



### Calling the original function



The original function is renamed to `bc::orig::[FUNCTION_NAME]` (e.g.

`bc::orig::my_expensive_function`). This can be used to bypass caching if needed.



### Manually refreshing the cache



Two other functions, `bc::warm::[FUNCTION_NAME]` and `bc::force::[FUNCTION_NAME]`, are provided to

update the cache on demand. Both unconditionally execute and cache the backing function, but

differ in how they are intended to be used.



`bc::warm::[FUNCTION_NAME]` refreshes a cached invocation asynchronously and silently, returning

control to the caller immediately. This can be used to ensure invocations are freshly cached

before the output is needed. For example, prompt.gem can

[warm cached functions displayed in the prompt](https://github.com/dimo414/prompt.gem/blob/7e6b5d2a7d773d5aa44880c5425a4b12c1dc066f/callback_functions.sh#L61)

before the prompt is actually constructed, allowing the functions to be refreshed concurrently

rather than one at a time.



By contrast `bc::force::[FUNCTION_NAME]` forces a synchronous cache refresh, blocking until the

backing function completes and outputing the cached contents exactly like calling

`[FUNCTION_NAME]`.



### Cleanup



A cleanup task is run regularly to remove stale cache data, however no attempt is made to clean up

the cache directory on exit since by design the cache can be shared by multiple processes. By

default, cached data is stored in a temp directory that the OS will clean up from time to time

(generally on reboot), but if you override the cache directory via `BC_CACHE_DIR` you may want to

clean up the directory yourself.



**Note:** cached data is cleaned up asynchronously, therefore data may persist longer than the

specified TTL duration.



### Locking



By design the caching provided by bash-cache is racy - concurrent invocations may or may not end up

reusing the same cached value. For most cases (idempotent functions, to be precise) this should be

sufficient.



For cases where concurrent calls to the backing function are problematic, use `bc::locked_cache`

instead of `bc::cache`. This behaves identically to `bc::cache` but uses an advisory mutex lock to

prevent concurrent invocations of the backing function.



Note that needing mutual-exclusion is a **strong** signal that you should be using a more powerful

language than Bash, and that the locking bash-cache provides is

[advisory](https://en.wikipedia.org/wiki/File_locking#In_Unix-like_systems) and best-effort only.



## Other Functions



### `bc::benchmark`



Benchmarks a function without caching enabled, and with a cold and warm cache. This allows you to

see the overhead introduced by Bash Cache and decide if it's beneficial for your function.



This function runs in a subshell against a clean cache directory, and works for any function - you

do not need to have previously called `bc::cache`.



`bc::benchmark_memoize` provides the same basic benchmarking for `bc::memoize`.



### `bc::copy_function`



This helper function copies an existing function to a new name. This can be used to decorate or

replace a function by first copying the function and then defining a new function with the original

name. This is how `bc::cache` overwrites the function being decorated.



If desired you can stop caching a particular function by copying the `bc::orig::...` function back

to its original name:



```shell

bc::copy_function bc::orig::my_expensive_function my_expensive_function

```



### `bc::on` and `bc::off`



Enables or disables caching process-wide. If `bc::off` is called all cached functions will delegate

immediately to the original function they decorate and will not attempt to use cached data or

cache new data. Call `bc::on` to re-enable caching.



## Configuration



### Use an isolated cache directory



By default bash-cache stores cached output in a user-specific directory under `/tmp` or the path

specified by `TMPDIR`. To use a different path as the cache root set `BC_CACHE_DIR` before sourcing

`bash-cache.sh`. This is useful if you're using Bash Cache across multiple scripts, as you could

otherwise run into namespace collisions (e.g. two scripts caching different functions with the same

name).



## Copyright and License



Copyright 2012-2020 Michael Diamond



This program is free software: you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation, either version 3 of the License, or

(at your option) any later version.



This program is distributed in the hope that it will be useful,

but WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

GNU General Public License for more details.



You should have received a copy of the GNU General Public License

along with this program.  If not, see .