strace echo 123 > /dev/ttyS4
execve("/bin/echo", ["echo", "123"], [/* 63 vars */]) = 0
brk(NULL)                               = 0x5597d6e5c000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=204193, ...}) = 0
mmap(NULL, 204193, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fc01e8c2000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\340\22\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1960656, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc01e8c0000
mmap(NULL, 4061792, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fc01e2ed000
mprotect(0x7fc01e4c3000, 2097152, PROT_NONE) = 0
mmap(0x7fc01e6c3000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1d6000) = 0x7fc01e6c3000
mmap(0x7fc01e6c9000, 14944, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fc01e6c9000
close(3)                                = 0
arch_prctl(ARCH_SET_FS, 0x7fc01e8c1500) = 0
mprotect(0x7fc01e6c3000, 16384, PROT_READ) = 0
mprotect(0x5597d5f55000, 4096, PROT_READ) = 0
mprotect(0x7fc01e8f4000, 4096, PROT_READ) = 0
munmap(0x7fc01e8c2000, 204193)          = 0
brk(NULL)                               = 0x5597d6e5c000
brk(0x5597d6e7d000)                     = 0x5597d6e7d000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=4245408, ...}) = 0
mmap(NULL, 4245408, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fc01dee0000
close(3)                                = 0
fstat(1, {st_mode=S_IFCHR|0660, st_rdev=makedev(4, 68), ...}) = 0
ioctl(1, TCGETS, {B9600 opost isig icanon echo ...}) = 0
write(1, "123\n", 4)                    = 4
close(1

тут висит до таймаута и отвисает

)                                = 0
close(2)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++