Add missing file from the previous commit

Author: jpikl

src/env.rs

@@ -0,0 +1,76 @@
+use crate::args::GlobalArgs;
+use crate::stdbuf::StdBuf;
+use clap::crate_name;
+use std::env;
+
+// Publicly known env variables:
+pub const ENV_NULL: &str = "REW_NULL";
+pub const ENV_BUF_MODE: &str = "REW_BUF_MODE";
+pub const ENV_BUF_SIZE: &str = "REW_BUF_SIZE";
+
+// Internal env variables:
+//
+// When `rew` is spawned as a child of some parent `rew` process,
+// it recieves the parent's name through this environment variable.
+pub const ENV_SPAWNED_BY: &str = "_REW_SPAWNED_BY";
+
+pub struct Env {
+    pub command: &'static str,
+    pub args: GlobalArgs,
+    pub stdbuf: StdBuf,
+}
+
+impl Env {
+    pub fn new(command: &'static str, args: GlobalArgs) -> Self {
+        Self {
+            command,
+            args,
+            stdbuf: StdBuf::default(),
+        }
+    }
+
+    pub fn internal(&self) -> Vec<(String, String)> {
+        vec![
+            (ENV_NULL.to_owned(), self.args.null.to_string()),
+            (ENV_BUF_MODE.to_owned(), self.args.buf_mode.to_string()),
+            (ENV_BUF_SIZE.to_owned(), self.args.buf_size.to_string()),
+            (ENV_SPAWNED_BY.to_owned(), get_spawned_by(self.command)),
+        ]
+    }
+
+    pub fn external(&mut self) -> Vec<(String, String)> {
+        let mut env = Vec::new();
+
+        if self.args.buf_mode.is_line() {
+            env.extend(self.stdbuf.line_buf_env()); // libc based programs
+            env.push(("PYTHONUNBUFFERED".to_owned(), "1".to_owned())); // Python programs
+        }
+
+        env
+    }
+}
+
+pub fn get_spawned_by(cmd_name: &str) -> String {
+    format!("{} {cmd_name}", get_bin_name())
+}
+
+pub fn get_bin_name() -> String {
+    if let Some(spawned_by) = env::var_os(ENV_SPAWNED_BY) {
+        let mut spawned_by = spawned_by.to_string_lossy().to_string();
+
+        if let Some(bin_name_len) = spawned_by.rfind(' ') {
+            spawned_by.truncate(bin_name_len); // Trim rew subcommand name
+        }
+
+        // Parent `rew` process `argv[0]`
+        return spawned_by;
+    }
+
+    if let Some(bin_name) = env::args_os().next() {
+        // Current `rew` process `argv[0]`
+        return bin_name.to_string_lossy().to_string();
+    }
+
+    // exec syscall did not receive `argv0`?
+    crate_name!().to_owned()
+}