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Update documentation for environment variable

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senhuang42
2020-09-08 08:42:46 -04:00
parent 136a0673f6
commit cc29492c5f
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programs/README.md
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@@ -204,14 +204,19 @@ Benchmark arguments :
### Passing parameters through Environment Variables
`ZSTD_CLEVEL` can be used to modify the default compression level of `zstd`
(usually set to `3`) to another value between 1 and 19 (the "normal" range).
This can be useful when `zstd` CLI is invoked in a way that doesn't allow passing arguments.
`ZSTD_NUMTHREADS` can be used to specify number of threads that `zstd` will use during compression, which by default is `1`.
This functionality only exists when `zstd` is compiled with multithread support.
The max # of threads is capped at: `ZSTDMT_NBWORKERS_MAX==200`.
This functionality can be useful when `zstd` CLI is invoked in a way that doesn't allow passing arguments.
One such scenario is `tar --zstd`.
As `ZSTD_CLEVEL` only replaces the default compression level,
it can then be overridden by corresponding command line arguments.
As `ZSTD_CLEVEL` and `ZSTD_NUMTHREADS` only replace the default compression level
and number of threads, respectively, they can both be overridden by corresponding command line arguments:
`-#` for compression level and `-T#` for number of threads.
There is no "generic" way to pass "any kind of parameter" to `zstd` in a pass-through manner.
Using environment variables for this purpose has security implications.
Therefore, this avenue is intentionally restricted and only supports `ZSTD_CLEVEL`.
Therefore, this avenue is intentionally restricted and only supports `ZSTD_CLEVEL` and `ZSTD_NUMTHREADS`.
### Long distance matching mode
The long distance matching mode, enabled with `--long`, is designed to improve