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

src/uu/pgrep/Cargo.toml

@@ -17,7 +17,7 @@ workspace = true
 clap = { workspace = true }
 regex = { workspace = true }
 rustix = { workspace = true, features = ["fs", "process", "thread"] }
-uucore = { workspace = true, features = ["entries", "signals"] }
+uucore = { workspace = true, features = ["entries", "signals", "uptime"] }
 walkdir = { workspace = true }
 
 [lib]

src/uu/pgrep/src/process_matcher.rs

@@ -18,6 +18,7 @@ use uucore::{
 };
 
 use uucore::error::{UResult, USimpleError};
+use uucore::uptime::get_uptime;
 
 use crate::process::{walk_process, walk_threads, Namespace, ProcessInformation, Teletype};
 
@@ -242,6 +243,19 @@ fn get_namespaces(_pid: usize, _list: Option<Vec<&str>>) -> UResult<Namespace> {
     Ok(Namespace::new())
 }
 
+/// The unit of a process's start time in /proc/<pid>/stat.
+#[cfg(unix)]
+fn clock_ticks_per_second() -> u64 {
+    // SAFETY: sysconf has no preconditions.
+    unsafe { uucore::libc::sysconf(uucore::libc::_SC_CLK_TCK) as u64 }
+}
+
+// No process is listed on these platforms, so no start time gets compared.
+#[cfg(not(unix))]
+fn clock_ticks_per_second() -> u64 {
+    1
+}
+
 /// Collect pids with filter construct from command line arguments
 fn collect_matched_pids(settings: &Settings) -> UResult<Vec<ProcessInformation>> {
     // Filtration general parameters
@@ -260,6 +274,16 @@ fn collect_matched_pids(settings: &Settings) -> UResult<Vec<ProcessInformation>>
             HashSet::from([our_pid])
         };
 
+        // --older compares a process's age with the given seconds.
+        let older_than_ticks = settings
+            .older
+            .map(|secs| -> UResult<(u64, u64)> {
+                let ticks = clock_ticks_per_second();
+                let uptime = (get_uptime(None)? as u64).saturating_mul(ticks);
+                Ok((uptime, secs.saturating_mul(ticks)))
+            })
+            .transpose()?;
+
         let pid_from_pidfile = settings
             .pidfile
             .as_ref()
@@ -299,8 +323,10 @@ 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;
+            let older_matched = match older_than_ticks {
+                Some((now, min_age)) => now.saturating_sub(pid.start_time().unwrap()) >= min_age,
+                None => 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() {