Run command as other user

-u username option can run command as other user, and this option only take effect when executing as root. Check following example:

# cat getuid.c
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>

int main(void)
{
    printf("uid is %d\n", getuid());
    return 0;
}

Build and run it as root:

# gcc getuid.c -o getuid
# strace ./getuid
execve("./getuid", ["./getuid"], 0x7ffce778f8a0 /* 12 vars */) = 0
brk(NULL)                               = 0x563fe210a000
......
write(1, "uid is 0\n", 9uid is 0
)               = 9
exit_group(0)                           = ?
+++ exited with 0 +++

It shows uid is 0. Run the program as another user:

# strace -u nan ./getuid
......
write(1, "uid is 1000\n", 12uid is 1000
)           = 12
exit_group(0)                           = ?
+++ exited with 0 +++

This time, uid is 1000.