"hongy...@gmail.com" <hongyi.zhao@gmail.com>:

And strace can't trace the system calls and signals of this script:

$ strace ./test.sh
execve("./test.sh", ["./test.sh"], 0x7ffc7d8136c0 /* 102 vars */) = -1 ENOEXEC (Exec format error)
strace: exec: Exec format error
+++ exited with 1 +++

An interpreter (here: shell) script is not a runable program
image.  That's why the system call execve, that is told to execute
that script, returns an error.

See the execve(2) and errno(3) manual pages and the POSIX error
numbers (<https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_03>)
for what the ENOEXEC error is.

Give that script a Shebang in the first line of it, then make it
executable:

chmod -- u+x ./test.sh

Try again tracing it.  Does ist work now?


Let me append my ceterum censeo:

You really should get a book about the unix kernel, not just about
the shell.  Learn the purpose and behavior of the system calls – in
this case: the execve system call and the various wrappers for that
system call in the C language runtime library: the exec family of
functions.

Understand, what the wrapper function execlp does, when given a file
with read and execute permissions, starting with a Shebang line: 
It invokes the named interpreter, giving it the pathname of the
given (script) file as a parameter.

Then, get a book about the shell (or just read the shell manual page
and the POSIX specification <https://pubs.opengroup.org/onlinepubs/9699919799/utilities/toc.html>),
to know, what the shell does when reading and executing a shell
script, especially:  Understand, how the shell transforms a shell
command line invoking a simple command into the corresponding execve
argument vector.

Finally, understand, why the seen "strace" behavior above follows
from the lessions learned.

As long as you don't understand the unix system calls provided by
the kernel, you'll neither understand the strace error message above
nor make progress in shell programming.

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