Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

When I call telnet to connect to daytime server from interactive shell, it works fine. However, when the same telnet call is issued from a shell script run by cron or some bash daemon process, it fails to yield any otput. I straced the both calls, and here they are:

Successful one:

execve("/usr/bin/telnet", ["telnet", "192.168.0.11", "13"], [/* 24 vars */]) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|0x4000000, -1, 0) = 0x40005000
stat("/etc/ld.so.cache", {st_mode=S_IFREG|0644, st_size=3128, ...}) = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
mmap2(NULL, 3128, PROT_READ, MAP_SHARED, 3, 0) = 0x40006000
close(3)                                = 0
stat("/etc/ld.so.preload", 0xbea5d8d4)  = -1 ENOENT (No such file or directory)
open("/lib/libc.so.0", O_RDONLY)        = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=324423, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|0x4000000, -1, 0) = 0x40007000
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0\320\253\0\0004\0\0\0\0"..., 4096) = 4096
mmap2(NULL, 376832, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x4000e000
mmap2(0x4000e000, 319264, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 3, 0) = 0x4000e000
mmap2(0x40064000, 4935, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0x4e) = 0x40064000
mmap2(0x40066000, 14712, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x40066000
close(3)                                = 0
munmap(0x40007000, 4096)                = 0
munmap(0x40006000, 3128)                = 0
stat("/lib/ld-uClibc.so.0", {st_mode=S_IFREG|0755, st_size=20486, ...}) = 0
mprotect(0x40064000, 4096, PROT_READ)   = 0
mprotect(0x4000c000, 4096, PROT_READ)   = 0
ioctl(0, SNDCTL_TMR_TIMEBASE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(1, SNDCTL_TMR_TIMEBASE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
getuid32()                              = 0
ioctl(0, TIOCGWINSZ, {ws_row=55, ws_col=203, ws_xpixel=0, ws_ypixel=0}) = 0
ioctl(0, SNDCTL_TMR_TIMEBASE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
brk(0)                                  = 0x91000
brk(0x92000)                            = 0x92000
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 3
setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
connect(3, {sa_family=AF_INET, sin_port=htons(13), sin_addr=inet_addr("192.168.0.11")}, 16) = 0
setsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0
rt_sigaction(SIGINT, {0x667c0, [INT], SA_RESTART|0x4000000}, {SIG_DFL, [], 0}, 8) = 0
poll([{fd=0, events=POLLIN}, {fd=3, events=POLLIN}], 2, -1) = 1 ([{fd=3, revents=POLLIN}])
read(3, "Wed Apr 27 11:42:10 2011\r\n"..., 128) = 26
write(1, "Wed Apr 27 11:42:10 2011\r\n"..., 26) = 26
poll([{fd=0, events=POLLIN}, {fd=3, events=POLLIN}], 2, -1) = 1 ([{fd=3, revents=POLLIN}])
read(3, ""..., 128)                     = 0
write(1, "Connection closed by foreign host"..., 35) = 35
ioctl(0, SNDCTL_TMR_STOP or TCSETSW, {B38400 opost isig icanon echo ...}) = 0
exit(1)                                 = ?

Unsuccessful one:

execve("/usr/bin/telnet", ["/usr/bin/telnet", "192.168.0.11", "13"], [/* 22 vars */]) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|0x4000000, -1, 0) = 0x40005000
stat("/etc/ld.so.cache", {st_mode=S_IFREG|0644, st_size=3128, ...}) = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
mmap2(NULL, 3128, PROT_READ, MAP_SHARED, 3, 0) = 0x40006000
close(3)                                = 0
stat("/etc/ld.so.preload", 0xbe90a8f4)  = -1 ENOENT (No such file or directory)
open("/lib/libc.so.0", O_RDONLY)        = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=324423, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|0x4000000, -1, 0) = 0x40007000
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0(\0\1\0\0\0\320\253\0\0004\0\0\0\0"..., 4096) = 4096
mmap2(NULL, 376832, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x4000e000
mmap2(0x4000e000, 319264, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 3, 0) = 0x4000e000
mmap2(0x40064000, 4935, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0x4e) = 0x40064000
mmap2(0x40066000, 14712, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x40066000
close(3)                                = 0
munmap(0x40007000, 4096)                = 0
munmap(0x40006000, 3128)                = 0
stat("/lib/ld-uClibc.so.0", {st_mode=S_IFREG|0755, st_size=20486, ...}) = 0
mprotect(0x40064000, 4096, PROT_READ)   = 0
mprotect(0x4000c000, 4096, PROT_READ)   = 0
ioctl(0, SNDCTL_TMR_TIMEBASE or TCGETS, 0xbe90a714) = -1 ENOTTY (Inappropriate ioctl for device)
ioctl(1, SNDCTL_TMR_TIMEBASE or TCGETS, 0xbe90a714) = -1 ENOTTY (Inappropriate ioctl for device)
getuid32()                              = 0
ioctl(0, TIOCGWINSZ, 0xbe90a9cc)        = -1 ENOTTY (Inappropriate ioctl for device)
ioctl(0, SNDCTL_TMR_TIMEBASE or TCGETS, 0xbe90a9b4) = -1 ENOTTY (Inappropriate ioctl for device)
brk(0)                                  = 0x91000
brk(0x92000)                            = 0x92000
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 3
setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
connect(3, {sa_family=AF_INET, sin_port=htons(13), sin_addr=inet_addr("192.168.0.11")}, 16) = 0
setsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0
rt_sigaction(SIGINT, {0x667c0, [INT], SA_RESTART|0x4000000}, {SIG_DFL, [], 0}, 8) = 0
poll([{fd=0, events=POLLIN}, {fd=3, events=POLLIN}], 2, -1) = 2 ([{fd=0, revents=POLLIN}, {fd=3, revents=POLLIN}])
read(0, ""..., 128)                     = 0
exit(0)                                 = ?

Any help welcome.

share|improve this question
    
I do hope you are not using this to adjust the system clock, when a much better solution is already available. –  grawity Apr 27 '11 at 11:18

1 Answer 1

ioctl(0, SNDCTL_TMR_TIMEBASE or TCGETS, 0xbe90a714) = -1 ENOTTY (Inappropriate ioctl for device)
ioctl(1, SNDCTL_TMR_TIMEBASE or TCGETS, 0xbe90a714) = -1 ENOTTY (Inappropriate ioctl for device)

Trying to do an ioctl on STDIN or STDOUT when they don't exist (or are redirected like when running as a daemon) to get terminal details (TCGETS) won't work.

You would need to use something other than telnet, such as socat or nc, or write your own program to do it in some language with socket support (C, Perl, PHP, etc).


You can connect to simple TCP servers in pure bash:

# bash accepts symbolic service (port) names according to /etc/services
# (of course you can use "/dev/tcp/$server/13" too)
read -r line <"/dev/tcp/$server/daytime"
echo "Today is $line"

For more complex protocols than daytime, you could use exec {fd}<>"/dev/tcp/$host/$port", then use the resulting $fd as an ordinary file descriptor: read foo <&$fd, echo bar >&$fd, close with exec {fd}>&-.

share|improve this answer
    
I recommend socat instead of nc, since the latter ... varies. (I counted six different implementations once.) But bash itself has enough socket support; I added sample code to the answer. –  grawity Apr 27 '11 at 10:30
    
Wow! That's awesome stuff! Never knew you could do that sort of thing! –  Majenko Apr 27 '11 at 11:14
    
Before anybody asks, redirection to /dev/tcp is implemented entirely inside bash. At least Linux has no built-in OS-wide equivalent. –  grawity Apr 27 '11 at 11:17

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.