date: implement --resolution flag

src/uu/date/locales/en-US.ftl

@@ -82,6 +82,8 @@ date-help-iso-8601 = output date/time in ISO 8601 format.
   'hours', 'minutes', 'seconds', or 'ns'
   for date and time to the indicated precision.
   Example: 2006-08-14T02:34:56-06:00
+date-help-resolution = output the available resolution of timestamps
+  Example: 0.000000001
 date-help-rfc-email = output date and time in RFC 5322 format.
   Example: Mon, 14 Aug 2006 02:34:56 -0600
 date-help-rfc-3339 = output date/time in RFC 3339 format.

src/uu/date/src/date.rs

@@ -3,14 +3,16 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 
-// spell-checker:ignore strtime ; (format) DATEFILE MMDDhhmm ; (vars) datetime datetimes
+// spell-checker:ignore strtime ; (format) DATEFILE MMDDhhmm ; (vars) datetime datetimes getres
 
 use clap::{Arg, ArgAction, Command};
 use jiff::fmt::strtime;
 use jiff::tz::TimeZone;
 use jiff::{Timestamp, Zoned};
 #[cfg(all(unix, not(target_os = "macos"), not(target_os = "redox")))]
-use libc::{CLOCK_REALTIME, clock_settime, timespec};
+use libc::clock_settime;
+#[cfg(all(unix, not(target_os = "redox")))]
+use libc::{CLOCK_REALTIME, clock_getres, timespec};
 use std::fs::File;
 use std::io::{BufRead, BufReader};
 use std::path::PathBuf;
@@ -35,6 +37,7 @@ const OPT_FORMAT: &str = "format";
 const OPT_FILE: &str = "file";
 const OPT_DEBUG: &str = "debug";
 const OPT_ISO_8601: &str = "iso-8601";
+const OPT_RESOLUTION: &str = "resolution";
 const OPT_RFC_EMAIL: &str = "rfc-email";
 const OPT_RFC_822: &str = "rfc-822";
 const OPT_RFC_2822: &str = "rfc-2822";
@@ -57,6 +60,7 @@ enum Format {
     Iso8601(Iso8601Format),
     Rfc5322,
     Rfc3339(Rfc3339Format),
+    Resolution,
     Custom(String),
     Default,
 }
@@ -68,6 +72,7 @@ enum DateSource {
     FileMtime(PathBuf),
     Stdin,
     Human(String),
+    Resolution,
 }
 
 enum Iso8601Format {
@@ -136,6 +141,8 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         .map(|s| s.as_str().into())
     {
         Format::Rfc3339(fmt)
+    } else if matches.get_flag(OPT_RESOLUTION) {
+        Format::Resolution
     } else {
         Format::Default
     };
@@ -149,6 +156,8 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         }
     } else if let Some(file) = matches.get_one::<String>(OPT_REFERENCE) {
         DateSource::FileMtime(file.into())
+    } else if matches.get_flag(OPT_RESOLUTION) {
+        DateSource::Resolution
     } else {
         DateSource::Now
     };
@@ -230,6 +239,12 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
             let iter = std::iter::once(Ok(date));
             Box::new(iter)
         }
+        DateSource::Resolution => {
+            let resolution = get_clock_resolution();
+            let date = resolution.to_zoned(TimeZone::system());
+            let iter = std::iter::once(Ok(date));
+            Box::new(iter)
+        }
         DateSource::Now => {
             let iter = std::iter::once(Ok(now));
             Box::new(iter)
@@ -283,6 +298,7 @@ pub fn uu_app() -> Command {
                 .long(OPT_FILE)
                 .value_name("DATEFILE")
                 .value_hint(clap::ValueHint::FilePath)
+                .conflicts_with(OPT_DATE)
                 .help(translate!("date-help-file")),
         )
         .arg(
@@ -297,6 +313,14 @@ pub fn uu_app() -> Command {
                 .default_missing_value(OPT_DATE)
                 .help(translate!("date-help-iso-8601")),
         )
+        .arg(
+            Arg::new(OPT_RESOLUTION)
+                .long(OPT_RESOLUTION)
+                .conflicts_with_all([OPT_DATE, OPT_FILE])
+                .overrides_with(OPT_RESOLUTION)
+                .help(translate!("date-help-resolution"))
+                .action(ArgAction::SetTrue),
+        )
         .arg(
             Arg::new(OPT_RFC_EMAIL)
                 .short('R')
@@ -325,6 +349,7 @@ pub fn uu_app() -> Command {
                 .long(OPT_REFERENCE)
                 .value_name("FILE")
                 .value_hint(clap::ValueHint::AnyPath)
+                .conflicts_with_all([OPT_DATE, OPT_FILE, OPT_RESOLUTION])
                 .help(translate!("date-help-reference")),
         )
         .arg(
@@ -374,6 +399,7 @@ fn make_format_string(settings: &Settings) -> &str {
             Rfc3339Format::Seconds => "%F %T%:z",
             Rfc3339Format::Ns => "%F %T.%N%:z",
         },
+        Format::Resolution => "%s.%N",
         Format::Custom(ref fmt) => fmt,
         Format::Default => "%a %b %e %X %Z %Y",
     }
@@ -398,6 +424,50 @@ fn parse_date<S: AsRef<str> + Clone>(
     }
 }
 
+#[cfg(not(any(unix, windows)))]
+fn get_clock_resolution() -> Timestamp {
+    unimplemented!("getting clock resolution not implemented (unsupported target)");
+}
+
+#[cfg(all(unix, not(target_os = "redox")))]
+fn get_clock_resolution() -> Timestamp {
+    let mut timespec = timespec {
+        tv_sec: 0,
+        tv_nsec: 0,
+    };
+    unsafe {
+        // SAFETY: the timespec struct lives for the full duration of this function call.
+        //
+        // The clock_getres function can only fail if the passed clock_id is not
+        // a known clock. All compliant posix implementors must support
+        // CLOCK_REALTIME, therefore this function call cannot fail on any
+        // compliant posix implementation.
+        //
+        // See more here:
+        // https://pubs.opengroup.org/onlinepubs/9799919799/functions/clock_getres.html
+        clock_getres(CLOCK_REALTIME, &raw mut timespec);
+    }
+    #[allow(clippy::unnecessary_cast)] // Cast required on 32-bit platforms
+    Timestamp::constant(timespec.tv_sec as i64, timespec.tv_nsec as i32)
+}
+
+#[cfg(all(unix, target_os = "redox"))]
+fn get_clock_resolution() -> Timestamp {
+    // Redox OS does not support the posix clock_getres function, however
+    // internally it uses a resolution of 1ns to represent timestamps.
+    // https://gitlab.redox-os.org/redox-os/kernel/-/blob/master/src/time.rs
+    Timestamp::constant(0, 1)
+}
+
+#[cfg(windows)]
+fn get_clock_resolution() -> Timestamp {
+    // Windows does not expose a system call for getting the resolution of the
+    // clock, however the FILETIME struct returned by GetSystemTimeAsFileTime,
+    // and GetSystemTimePreciseAsFileTime has a resolution of 100ns.
+    // https://learn.microsoft.com/en-us/windows/win32/api/minwinbase/ns-minwinbase-filetime
+    Timestamp::constant(0, 100)
+}
+
 #[cfg(not(any(unix, windows)))]
 fn set_system_datetime(_date: Zoned) -> UResult<()> {
     unimplemented!("setting date not implemented (unsupported target)");

tests/by-util/test_date.rs

@@ -3,7 +3,7 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 //
-// spell-checker: ignore: AEDT AEST EEST NZDT NZST Kolkata
+// spell-checker: ignore: AEDT AEST EEST NZDT NZST Kolkata Iseconds
 
 use chrono::{DateTime, Datelike, Duration, NaiveTime, Utc}; // spell-checker:disable-line
 use regex::Regex;
@@ -745,3 +745,36 @@ fn test_date_empty_tz_time() {
         .succeeds()
         .stdout_only("Thu Jan  1 00:00:00 UTC 1970\n");
 }
+
+#[test]
+fn test_date_resolution() {
+    // Test that --resolution flag returns a floating point number by default
+    new_ucmd!()
+        .arg("--resolution")
+        .succeeds()
+        .stdout_str_check(|s| s.trim().parse::<f64>().is_ok());
+
+    // Test that --resolution flag can be passed twice to match gnu
+    new_ucmd!()
+        .arg("--resolution")
+        .arg("--resolution")
+        .succeeds()
+        .stdout_str_check(|s| s.trim().parse::<f64>().is_ok());
+
+    // Test that can --resolution output can be formatted as a date
+    new_ucmd!()
+        .arg("--resolution")
+        .arg("-Iseconds")
+        .succeeds()
+        .stdout_only("1970-01-01T00:00:00+00:00\n");
+}
+
+#[test]
+fn test_date_resolution_no_combine() {
+    // Test that date fails when --resolution flag is passed with date flag
+    new_ucmd!()
+        .arg("--resolution")
+        .arg("-d")
+        .arg("2025-01-01")
+        .fails();
+}