refactor: one helper each for the repeated blocks

src/cmd/shell.rs

@@ -1,3 +1,5 @@
+use anyhow::Result;
+
 use super::{Argv, Cmd};
 
 // sh -c, the only way to reach a shell feature inside a container
@@ -30,9 +32,9 @@ impl Pipeline {
         }
     }
 
-    pub fn pipe(mut self, next: &dyn Cmd) -> Self {
-        self.stages.push(next.shell());
-        self
+    pub fn pipe(mut self, next: &dyn Cmd) -> Result<Self> {
+        self.stages.push(next.shell()?);
+        Ok(self)
     }
 
     // for a pipeline whose last stage closes the pipe on purpose, where the
@@ -119,21 +121,26 @@ mod tests {
 
     #[test]
     fn a_pipeline_reports_a_stage_that_dies_mid_stream() {
-        let script = Gunzip.pipe(&Gzip).shell();
+        let script = Gunzip.pipe(&Gzip).unwrap().shell().unwrap();
 
         assert_eq!(script, "sh -c 'set -o pipefail; gunzip -c | gzip'");
     }
 
     #[test]
     fn a_pipeline_that_closes_the_pipe_on_purpose_keeps_the_default() {
-        let script = Gunzip.pipe(&Head::bytes(512)).allow_early_close().shell();
+        let script = Gunzip
+            .pipe(&Head::bytes(512))
+            .unwrap()
+            .allow_early_close()
+            .shell()
+            .unwrap();
 
         assert_eq!(script, "sh -c 'gunzip -c | head -c 512'");
     }
 
     #[test]
     fn a_pipeline_reaches_a_container_as_one_argument() {
-        let argv = Gunzip.pipe(&Gzip).in_container("abc123").argv();
+        let argv = Gunzip.pipe(&Gzip).unwrap().in_container("abc123").argv();
 
         assert_eq!(
             argv.words(),