dd: support seek=N when destination is stdout

Add support for the `seek=N` argument when the destination is stdout
and not a file. Previously, the argument was ignored when writing to
stdout.

src/uu/dd/src/dd.rs

@@ -5,7 +5,7 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 
-// spell-checker:ignore fname, tname, fpath, specfile, testfile, unspec, ifile, ofile, outfile, fullblock, urand, fileio, atoe, atoibm, behaviour, bmax, bremain, btotal, cflags, creat, ctable, ctty, datastructures, doesnt, etoa, fileout, fname, gnudd, iconvflags, nocache, noctty, noerror, nofollow, nolinks, nonblock, oconvflags, outfile, parseargs, rlen, rmax, rposition, rremain, rsofar, rstat, sigusr, sigval, wlen, wstat
+// spell-checker:ignore fname, tname, fpath, specfile, testfile, unspec, ifile, ofile, outfile, fullblock, urand, fileio, atoe, atoibm, behaviour, bmax, bremain, btotal, cflags, creat, ctable, ctty, datastructures, doesnt, etoa, fileout, fname, gnudd, iconvflags, nocache, noctty, noerror, nofollow, nolinks, nonblock, oconvflags, outfile, parseargs, rlen, rmax, rposition, rremain, rsofar, rstat, sigusr, sigval, wlen, wstat seekable
 
 #[cfg(test)]
 mod dd_unit_tests;
@@ -294,8 +294,17 @@ impl OutputTrait for Output<io::Stdout> {
     fn new(matches: &Matches) -> UResult<Self> {
         let obs = parseargs::parse_obs(matches)?;
         let cflags = parseargs::parse_conv_flag_output(matches)?;
+        let oflags = parseargs::parse_oflags(matches)?;
+        let seek = parseargs::parse_seek_amt(&obs, &oflags, matches)?;
 
-        let dst = io::stdout();
+        let mut dst = io::stdout();
+
+        // stdout is not seekable, so we just write null bytes.
+        if let Some(amt) = seek {
+            let bytes = vec![b'\0'; amt];
+            dst.write_all(&bytes)
+                .map_err_context(|| String::from("write error"))?;
+        }
 
         Ok(Output { dst, obs, cflags })
     }

tests/by-util/test_dd.rs

@@ -1,4 +1,4 @@
-// spell-checker:ignore fname, tname, fpath, specfile, testfile, unspec, ifile, ofile, outfile, fullblock, urand, fileio, atoe, atoibm, availible, behaviour, bmax, bremain, btotal, cflags, creat, ctable, ctty, datastructures, doesnt, etoa, fileout, fname, gnudd, iconvflags, nocache, noctty, noerror, nofollow, nolinks, nonblock, oconvflags, outfile, parseargs, rlen, rmax, rposition, rremain, rsofar, rstat, sigusr, sigval, wlen, wstat
+// spell-checker:ignore fname, tname, fpath, specfile, testfile, unspec, ifile, ofile, outfile, fullblock, urand, fileio, atoe, atoibm, availible, behaviour, bmax, bremain, btotal, cflags, creat, ctable, ctty, datastructures, doesnt, etoa, fileout, fname, gnudd, iconvflags, nocache, noctty, noerror, nofollow, nolinks, nonblock, oconvflags, outfile, parseargs, rlen, rmax, rposition, rremain, rsofar, rstat, sigusr, sigval, wlen, wstat abcdefghijklm
 
 use crate::common::util::*;
 
@@ -592,5 +592,17 @@ fn test_conv_ebcdic_implies_block() {
         .stdout_is_bytes(b"\x40\xc1\x40\xc1\x40\xc1\x40\x40");
 }
 
+/// Test for seeking forward N bytes in the output file before copying.
+#[test]
+fn test_seek_bytes() {
+    // Since the output file is stdout, seeking forward by eight bytes
+    // results in a prefix of eight null bytes.
+    new_ucmd!()
+        .args(&["seek=8", "oflag=seek_bytes"])
+        .pipe_in("abcdefghijklm\n")
+        .succeeds()
+        .stdout_is("\0\0\0\0\0\0\0\0abcdefghijklm\n");
+}
+
 // conv=[ascii,ebcdic,ibm], conv=[ucase,lcase], conv=[block,unblock], conv=sync
 // TODO: Move conv tests from unit test module