Accept some settings as environment variables.

Cargo.toml

@@ -25,7 +25,7 @@ debug = ['serde_json']
 [dependencies]
 anyhow = "1.0"
 bincode = "1.3.1"
-clap = { version = "3.0", default_features = false, features = ["std", "derive"] }
+clap = { version = "3.0", default_features = false, features = ["std", "derive", "env"] }
 humantime = "2.1.0"
 rand = "0.8"
 serde = { version = "1.0", features = ["derive"] }

README.md

@@ -155,7 +155,7 @@ frequently than the `--ttl` would suggest, which in turn can create unexpected
 load. If the backing command is failing due to an outage or bug (such as an
 overloaded website) triggering additional calls can exacerbate the issue and
 effectively DDoS the hampered system. It is generally safer *not* to set this
-flag and instead make the client robust to occasional failures. 
+flag and instead make the client robust to occasional failures.
 
 <a name="cache_dir"></a>
 ### Changing the Cache Directory
@@ -169,6 +169,16 @@ Note that the choice of directory can affect `bkt`'s performance: if the cache
 is stored under a [`tmpfs`](https://en.wikipedia.org/wiki/Tmpfs) or solid-state
 partition it will be significantly faster than caching to a spinning disk.
 
+### Environment Variables
+
+Some settings can be passed via the following environment variables:
+
+- `BKT_SCOPE` for [`--scope`](#setting-a-cache-scope)
+- `BKT_TMPDIR` for [`--cache-dir`](#changing-the-cache-directory)
+- `BKT_TTL` for [`--ttl`](#cache-lifespan)
+- `BKT_STALE` for [`--stale`](#cache-lifespan)
+
+
 ## Security and Privacy
 
 The default cache directory is potentially world-readable. On Unix the cache

src/main.rs

@@ -109,11 +109,11 @@ struct Cli {
     command: Vec<OsString>,
 
     /// Duration the cached result will be valid for
-    #[clap(long, default_value = "60s", visible_alias = "time-to-live")]
+    #[clap(long, default_value = "60s", visible_alias = "time-to-live", env = "BKT_TTL")]
     ttl: humantime::Duration,
 
     /// Duration after which the result will be asynchronously refreshed
-    #[clap(long, conflicts_with = "warm")]
+    #[clap(long, conflicts_with = "warm", env = "BKT_STALE")]
     stale: Option<humantime::Duration>,
 
     /// Asynchronously execute and cache the given command, even if it's already cached
@@ -142,14 +142,14 @@ struct Cli {
 
     /// If set, all cached data will be scoped to this value,
     /// preventing collisions with commands cached with different scopes
-    #[clap(long)]
+    #[clap(long, env = "BKT_SCOPE")]
     scope: Option<String>,
 
     /// The directory under which to persist cached invocations;
     /// defaults to the system's temp directory.
     /// Setting this to a directory backed by RAM or an SSD, such as a tmpfs partition,
     /// will significantly reduce caching overhead.
-    #[clap(long)]
+    #[clap(long, env = "BKT_TMPDIR")]
     cache_dir: Option<PathBuf>,
 }