When will Bash give HOME an initial value?

Today, I have no intention of discovering the following:

$ env-i bash-c CD Bash:line 0:cd:home Not set $ env-i bash-c ' echo $HOME '

This shows that Bash only inherits the HOME variable from the environment variable and never initializes it itself? In order to confirm this idea, I went through the source code, and found that in fact, in a situation, Bash will actively initialize the HOME variable:

if 1 0 )  Set_home_var ();

From the source, Bash is only possible to assign an initial value to the HOME when the shell is a login shell and not in Posix mode. Then I tried it out:

$ env-i Bash--login-c ' echo $HOME '

Hold the grass, why don't you have a value? Then Google a bit, found 14 years someone reported a bug https://lists.gnu.org/archive/html/bug-bash/2014-01/msg00063.html. After looking at this bug I understand that in Bash, there are three different cases of landing Shell:

/* Non-zero means that this shell is a login shell.   Specifically:   0 = not login shell.   1 = Login Shell from Getty (or equivalent fake off)  -1 = login shell from "--login" (or-l) flag.   */ int 0;

One is that the first character of the No. 0 parameter (specified by the exec* function) that is passed in bash when the parent process starts bash is the case where the--login or-l option is used when Bash starts, and the last is a mixture of the above two expressions login_s The value of the hell variable is 1,-1,-2, but from the source just seen, Bash will initialize the HOME variable only if Login_shell is 1. So let's check again:

$ env-i bash-c ' exec-a-whatever bash-c "echo \ $HOME" ' /home/admin

This command is a bit complicated, with the exec-a option used to specify ARGV[0]. We can also write a C program to verify that:

$ cat A.C #include <unistd.h> int main () { Char *argv[] = {"-whatever", "-C", "Echo $HOME", NULL}; Char *envp[] = {NULL}; Execve ("/bin/bash", argv, ENVP); } $ gcc A.c-o A $./A /home/admin

When will Bash give HOME an initial value?