...

 - recurring pattern: the file descriptor to the tmpfile is dup'ed to fd N, where N is equal
   to the one in /dev/fd/N, the device file specified to be opened by the child.

@@ strace -ff -e open,close,unlink,read,write,execve,dup2 -- bash ./here_doc_disx.sh                                                        
# here_doc_disx.sh:
# cat /dev/fd/0 <<\EOF
# Hello, world!
# EOF

execve("/usr/bin/bash", ["bash", "./here_doc_disx.sh"], 0x7ffcaa1b6388 /* 63 vars */) = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
close(3)                                = 0
open("/lib64/libtinfo.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\300\331\0\0\0\0\0\0"..., 832) = 832
close(3)                                = 0
open("/lib64/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\240\r\0\0\0\0\0\0"..., 832) = 832
close(3)                                = 0
open("/lib64/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\0P\10\2\0\0\0\0\0"..., 832) = 832
close(3)                                = 0
open("/dev/tty", O_RDWR|O_NONBLOCK)     = 3
close(3)                                = 0
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
close(3)                                = 0
open("/usr/lib64/gconv/gconv-modules.cache", O_RDONLY) = 3
close(3)                                = 0
open("./here_doc_disx.sh", O_RDONLY)    = 3
read(3, "cat /dev/fd/0 <<\\EOF\nHello, worl"..., 80) = 39
dup2(3, 255)                            = 255
close(3)                                = 0
read(255, "cat /dev/fd/0 <<\\EOF\nHello, worl"..., 39) = 39
strace: Process 1681 attached
[pid  1681] close(255)                  = 0
[pid  1681] open("/tmp/sh-thd-932991243", O_WRONLY|O_CREAT|O_EXCL|O_TRUNC, 0600) = 3
[pid  1681] write(3, "Hello, world!\n", 14) = 14
[pid  1681] open("/tmp/sh-thd-932991243", O_RDONLY) = 4
[pid  1681] close(3)                    = 0
[pid  1681] unlink("/tmp/sh-thd-932991243") = 0
[pid  1681] dup2(4, 0)                  = 0
[pid  1681] close(4)                    = 0
[pid  1681] execve("/usr/bin/cat", ["cat", "/dev/fd/0"], 0x75ebd70e90 /* 62 vars */) = 0
[pid  1681] open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
[pid  1681] close(3)                    = 0
[pid  1681] open("/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
[pid  1681] read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0P\10\2\0\0\0\0\0"..., 832) = 832
[pid  1681] close(3)                    = 0
[pid  1681] open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
[pid  1681] close(3)                    = 0
[pid  1681] open("/dev/fd/0", O_RDONLY) = 3
[pid  1681] read(3, "Hello, world!\n", 131072) = 14
[pid  1681] write(1, "Hello, world!\n", 14) = 14
Hello, world!
[pid  1681] read(3, "", 131072)         = 0
[pid  1681] close(3)                    = 0
[pid  1681] close(1)                    = 0
[pid  1681] close(2)                    = 0
[pid  1681] +++ exited with 0 +++
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=1681, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
read(255, "", 39)                       = 0
+++ exited with 0 +++
@@ 

#=====