pgrep: compare --older with the process's age

src/uu/pgrep/src/process_matcher.rs

@@ -242,6 +242,23 @@ fn get_namespaces(_pid: usize, _list: Option<Vec<&str>>) -> UResult<Namespace> {
     Ok(Namespace::new())
 }
 
+/// Return the time since boot in clock ticks, the unit of a process's start
+/// time in /proc/<pid>/stat.
+#[cfg(target_os = "linux")]
+fn ticks_since_boot() -> u64 {
+    let uptime = std::fs::read_to_string("/proc/uptime")
+        .ok()
+        .and_then(|s| s.split_whitespace().next()?.parse::<f64>().ok())
+        .unwrap_or(0.0);
+    (uptime * clock_ticks_per_second() as f64) as u64
+}
+
+#[cfg(target_os = "linux")]
+fn clock_ticks_per_second() -> u64 {
+    // SAFETY: sysconf has no preconditions.
+    unsafe { uucore::libc::sysconf(uucore::libc::_SC_CLK_TCK) as u64 }
+}
+
 /// Collect pids with filter construct from command line arguments
 fn collect_matched_pids(settings: &Settings) -> UResult<Vec<ProcessInformation>> {
     // Filtration general parameters
@@ -260,6 +277,15 @@ fn collect_matched_pids(settings: &Settings) -> UResult<Vec<ProcessInformation>>
             HashSet::from([our_pid])
         };
 
+        // --older compares a process's age with the given seconds.
+        #[cfg(target_os = "linux")]
+        let older_than_ticks = settings.older.map(|secs| {
+            (
+                ticks_since_boot(),
+                secs.saturating_mul(clock_ticks_per_second()),
+            )
+        });
+
         let pid_from_pidfile = settings
             .pidfile
             .as_ref()
@@ -299,8 +325,13 @@ fn collect_matched_pids(settings: &Settings) -> UResult<Vec<ProcessInformation>>
 
             let tty_matched = any_matches(&settings.terminal, pid.tty());
 
-            let arg_older = settings.older.unwrap_or(0);
-            let older_matched = pid.start_time().unwrap() >= arg_older;
+            #[cfg(target_os = "linux")]
+            let older_matched = match older_than_ticks {
+                Some((now, min_age)) => now.saturating_sub(pid.start_time().unwrap()) >= min_age,
+                None => true,
+            };
+            #[cfg(not(target_os = "linux"))]
+            let older_matched = true;
 
             let parent_matched = any_matches(&settings.parent, pid.ppid().unwrap());
             let pgroup_matched = any_matches(&settings.pgroup, pid.pgid().unwrap());

tests/by-util/test_pgrep.rs

@@ -131,6 +131,24 @@ fn test_older_non_matching_pattern() {
         .no_stdout();
 }
 
+#[test]
+#[cfg(target_os = "linux")]
+fn test_older_compares_process_age() {
+    // A process started just now is younger than a minute.
+    let mut child = Command::new("sleep").arg("10").spawn().unwrap();
+    let pid = child.id().to_string();
+
+    let result = new_ucmd!()
+        .args(&["--older", "0", "-x", "sleep"])
+        .succeeds();
+    assert!(result.stdout_str().lines().any(|line| line == pid));
+    let result = new_ucmd!().args(&["--older", "60", "-x", "sleep"]).run();
+    assert!(!result.stdout_str().lines().any(|line| line == pid));
+
+    child.kill().unwrap();
+    child.wait().unwrap();
+}
+
 #[test]
 #[cfg(target_os = "linux")]
 fn test_full() {