ps: Implement --pid flag

src/uu/ps/src/process_selection.rs

@@ -4,6 +4,7 @@
 // file that was distributed with this source code.
 
 use clap::ArgMatches;
+use std::collections::HashSet;
 use uu_pgrep::process::{walk_process, ProcessInformation, RunState, Teletype};
 use uucore::error::UResult;
 
@@ -46,6 +47,9 @@ pub struct ProcessSelectionSettings {
     /// - '-x' Lift "must have a tty" restriction.
     pub dont_require_tty: bool,
 
+    /// Select specific process IDs (-p, --pid)
+    pub pids: Option<HashSet<usize>>,
+
     /// - `-r` Restrict the selection to only running processes.
     pub only_running: bool,
 
@@ -60,6 +64,9 @@ impl ProcessSelectionSettings {
             select_non_session_leaders_with_tty: matches.get_flag("a"),
             select_non_session_leaders: matches.get_flag("d"),
             dont_require_tty: matches.get_flag("x"),
+            pids: matches
+                .get_many::<Vec<usize>>("pid")
+                .map(|xs| xs.flatten().copied().collect()),
             only_running: matches.get_flag("r"),
             negate_selection: matches.get_flag("deselect"),
         }
@@ -75,6 +82,10 @@ impl ProcessSelectionSettings {
                 return Ok(false);
             }
 
+            if let Some(ref pids) = self.pids {
+                return Ok(pids.contains(&process.pid));
+            }
+
             if self.select_all {
                 return Ok(true);
             }

src/uu/ps/src/ps.rs

@@ -138,6 +138,15 @@ fn collect_format(
     Ok(collect)
 }
 
+fn parse_numeric_list(s: &str) -> Result<Vec<usize>, String> {
+    s.split(|c: char| c.is_whitespace() || c == ',')
+        .map(|word| {
+            word.parse::<usize>()
+                .map_err(|_| format!("invalid number: '{}'", word))
+        })
+        .collect()
+}
+
 #[allow(clippy::cognitive_complexity)]
 pub fn uu_app() -> Command {
     Command::new(uucore::util_name())
@@ -262,6 +271,14 @@ pub fn uu_app() -> Command {
                 .action(ArgAction::SetTrue)
                 .help("do not print header at all"),
         )
+        .arg(
+            Arg::new("pid")
+                .short('p')
+                .long("pid")
+                .action(ArgAction::Append)
+                .value_parser(parse_numeric_list)
+                .help("select by process ID"),
+        )
     // .args([
     //     Arg::new("command").short('c').help("command name"),
     //     Arg::new("GID")
@@ -272,7 +289,6 @@ pub fn uu_app() -> Command {
     //         .short('g')
     //         .long("group")
     //         .help("session or effective group name"),
-    //     Arg::new("PID").short('p').long("pid").help("process id"),
     //     Arg::new("pPID").long("ppid").help("parent process id"),
     //     Arg::new("qPID")
     //         .short('q')

tests/by-util/test_ps.rs

@@ -202,3 +202,38 @@ fn test_deselect() {
         .succeeds()
         .stdout_matches(&Regex::new("\n *1 ").unwrap());
 }
+
+#[test]
+#[cfg(target_os = "linux")]
+fn test_pid_selection() {
+    let our_pid = std::process::id();
+    // Test that only pid 1 and pid of the test runner is present
+    let test = |pid_args: &[&str]| {
+        let match_regex = Regex::new(&format!("^ *1 *\n *{our_pid} *\n$")).unwrap();
+        let mut args = vec!["--no-headers", "-o", "pid"];
+        args.extend_from_slice(pid_args);
+        new_ucmd!()
+            .args(&args)
+            .succeeds()
+            .stdout_matches(&match_regex);
+    };
+
+    for flag in ["-p", "--pid"] {
+        test(&[flag, &format!("1 {our_pid}")]);
+        test(&[flag, &format!("1,{our_pid}")]);
+        test(&[flag, "1", flag, &our_pid.to_string()]);
+    }
+
+    // Test nonexistent PID (should show no output)
+    new_ucmd!()
+        .args(&["-p", "0", "--no-headers"])
+        .fails()
+        .code_is(1)
+        .stdout_is("");
+
+    // Test invalid PID
+    new_ucmd!()
+        .args(&["-p", "invalid"])
+        .fails()
+        .stderr_contains("invalid number");
+}