I observe that, on Ubuntu 24.04.2 with coreutils version 9.4-3ubuntu6, running:
$ tail -c 4097 /dev/zero
$ echo $?
0
exits immediately with a status code of 0. I expected the command to block indefinitely since /dev/zero is an endless stream.
In contrast, the following commands behave as expected (i.e., they block until interrupted):
$ tail -c 4096 /dev/zero
^C
$ echo $?
130
$ cat /dev/zero | tail -c 4097
^C
$ echo $?
130
Debug attempt
The strace output shows differences between the two invocations:
| strace tail -c 4096 /dev/zero | strace tail -c 4097 /dev/zero |
|---|---|
| … | … |
| close(3) = 0 | close(3) = 0 |
| openat(AT_FDCWD, "/dev/zero", O_RDONLY) = 3 | openat(AT_FDCWD, "/dev/zero", O_RDONLY) = 3 |
| fstat(3, {st_mode=S_IFCHR|0666, st_rdev=makedev(0x1, 0x5), …}) = 0 | fstat(3, {st_mode=S_IFCHR|0666, st_rdev=makedev(0x1, 0x5), …}) = 0 |
| lseek(3, -4096, SEEK_END) = 0 | lseek(3, -4097, SEEK_END) = 0 |
| read(3, "\0\0\0\0\0\0\0\0\00"…, 8192) = 8192 | read(3, "\0\0\0\0\0\0\0\0\0\…, 4097) = 4097 |
| read(3, "\0\0\0\0\0\0\0\0\00"…, 8192) = 8192 | fstat(1, {st_mode=S_IFIFO|0600, st_size=0, …}) = 0 |
| read(3, "\0\0\0\0\0\0\0\0\00"…, 8192) = 8192 | write(1, "\0\0\0\0\0\0\0\0\0\…, 4096 |
| read(3, "\0\0\0\0\0\0\0\0\00"…, 8192) = 8192 | close(3) = 0 |
| read(3, "\0\0\0\0\0\0\0\0\00"…, 8192) = 8192 | write(1, "\0", 1) = 1 |
| read(3, "\0\0\0\0\0\0\0\0\00"…, 8192) = 8192 | close(1) = 0 |
| read(3, "\0\0\0\0\0\0\0\0\00"…, 8192) = 8192 | close(2) = 0 |
| read(3, "\0\0\0\0\0\0\0\0\00"…, 8192) = 8192 | exit_group(0) = ? |
| read(3, "\0\0\0\0\0\0\0\0\00"…, 8192) = 8192 | ~~+~~ exited with 0 ~~+~~ |
| read(3, "\0\0\0\0\0\0\0\0\00"…, 8192) = 8192 | |
| read(3, "\0\0\0\0\0\0\0\0\00"…, 8192) = 8192 | |
| … |
