Hi!
While tracking an issue with some scripts (described at [I]) we noticed
that dash is not POSIX compliant when parsing the script to invoke
from stdin. In contradiction with POSIX, it does block reads instead
of character-per-character reads to only get the current command,
quoting from POSIX [P]:
,---
| STDIN
|
| […]
|
| When the shell is using standard input and it invokes a command that
| also uses standard input, the shell shall ensure that the standard
| input file pointer points directly after the command it has read
| when the command begins execution. It shall not read ahead in such a
| manner that any characters intended to be read by the invoked
| command are consumed by the shell (whether interpreted by the shell
| or not) or that characters that are not read by the invoked command
| are not seen by the shell. When the command expecting to read
| standard input is started asynchronously by an interactive shell, it
| is unspecified whether characters are read by the command or
| interpreted by the shell.
`---
[I] <https://michael-prokop.at/blog/2017/05/18/debugging-a-mystery-ssh-causing-strange-exit-codes/>
[P] <http://pubs.opengroup.org/onlinepubs/9699919799/utilities/sh.html>
The following test demonstrates the problem:
,--- sh-stdin.sh ---
#!/bin/sh
: ${TEST_SH:=dash}
$TEST_SH <<"EOF"
echo "Inner: pre"
while read line; do echo "Eat: $line"; done
echo "Inner: post"
exit 3
EOF
echo "Outer: exit code = $?"
`---
,--- test session ---
$ TEST_SH=bash ./sh-stdin
Inner: pre
Eat: echo "Inner: post"
Eat: exit 3
Outer: exit code = 0
$ TEST_SH=dash ./stdin.sh
Inner: pre
Inner: post
Outer: exit code = 3
`---
All other shells that we tested are POSIX compliant, this includes:
bash, ksh, mksh, posh and zsh
Thanks,
Guillem
Looks like a trivial one-line fix to, quite literally, read with count=1 if reading from the standard input stream; patch posted to dash@, archived at forwarded-to. наб
Turns out it was /not/ a one-line fix, since it exposed a lot of other latent bugs; either way, fixed upstream in this series: https://git.kernel.org/pub/scm/utils/dash/dash.git/commit/?id=07cad228ae4d1567aeb6d13d2af83d0a28d13b67 https://git.kernel.org/pub/scm/utils/dash/dash.git/commit/?id=91a375576d37bb4db1eca48e6bf5bac0db6cc3fa https://git.kernel.org/pub/scm/utils/dash/dash.git/commit/?id=5f094d08c5bcee876191404a4f3dd2d075571215 https://git.kernel.org/pub/scm/utils/dash/dash.git/commit/?id=44ae22beedf8a3d68bbfa1d065ad677182372de2 (I think that's the smallest viable part of the fix, anyway). All test cases work on trunk, including your original reproducer: $ src/dash <<EOF id cat good! EOF uid=1000(nabijaczleweli) gid=100(users) groups=100(users),4(adm),20(dialout),24(cdrom),25(floppy),27(sudo),29(audio),30(dip),44(video),46(plugdev),108(netdev),109(ssh),113(debian-transmission),116(kvm),120(minidlna),1000(nabijaczleweli),1002(smb-share),1006(smb-spool) good! $ src/dash <<"EOF" id read Q good! echo Q$Q EOF uid=1000(nabijaczleweli) gid=100(users) groups=100(users),4(adm),20(dialout),24(cdrom),25(floppy),27(sudo),29(audio),30(dip),44(video),46(plugdev),108(netdev),109(ssh),113(debian-transmission),116(kvm),120(minidlna),1000(nabijaczleweli),1002(smb-share),1006(smb-spool) Qgood! $ echo "|$(printf %10000s)echo bug" | src/dash -i $ src/dash: 1: Syntax error: "|" unexpected $ src/dash: 1: Cannot set tty process group (No such process) $ TEST_SH=src/dash ./sh-stdin.sh Inner: pre Eat: echo "Inner: post" Eat: exit 3 Outer: exit code = 0 наб
The issue has been raised another time on the debian-user mailing list, so I have decided to look closer at it. Even more commits are required, see Herbert Xu. [v3 PATCH 0/3] Improve performance when reading stdin. Sun, 02 Jun 2024 09:31:30 +0800 https://lore.kernel.org/dash/cover.1717291846.git.herbert@gondor.apana.org.au/t/#u Accordingly to earlier commit messages, impact on performance was considered unimportant. I have not tried to find commits adding unit tests covering the issue. Initially I had a hope that it may be safe to add patches to Debian package before next dash release, but now I am less sure.