Guardando il codice sorgente - si può vedere che esso controlla per questo caso (il file è già aperto per la lettura da parte di grep
) e le relazioni, vedi la verifica SAME_INODE
di seguito:
/* If there is a regular file on stdout and the current file refers
to the same i-node, we have to report the problem and skip it.
Otherwise when matching lines from some other input reach the
disk before we open this file, we can end up reading and matching
those lines and appending them to the file from which we're reading.
Then we'd have what appears to be an infinite loop that'd terminate
only upon filling the output file system or reaching a quota.
However, there is no risk of an infinite loop if grep is generating
no output, i.e., with --silent, --quiet, -q.
Similarly, with any of these:
--max-count=N (-m) (for N >= 2)
--files-with-matches (-l)
--files-without-match (-L)
there is no risk of trouble.
For --max-count=1, grep stops after printing the first match,
so there is no risk of malfunction. But even --max-count=2, with
input==output, while there is no risk of infloop, there is a race
condition that could result in "alternate" output. */
if (!out_quiet && list_files == 0 && 1 < max_count
&& S_ISREG (out_stat.st_mode) && out_stat.st_ino
&& SAME_INODE (st, out_stat))
{
if (! suppress_errors)
error (0, 0, _("input file %s is also the output"), quote (filename));
errseen = true;
goto closeout;
}
Grazie per la risposta, ho dimenticato di come il guscio in realtà imposta descrittori di file del processo in anticipo e che grep potrebbe verificare informazioni su questi. – iobender
@iobender, prego. – falsetru