Question

I am trying to find the virtual file that contains the current users id. I was told that I could find it in the proc directory, but not quite sure which file.

Answer 1

You actually want /proc/self/status, which will give you information about the currently executed process.

Here is an example:

$ cat /proc/self/status Name:   cat State:  R (running) Tgid:   17618 Pid:    17618 PPid:   3083 TracerPid:      0 Uid:    500 500 500 500 Gid:    500 500 500 500 FDSize: 32 Groups: 10 488 500  VmPeak:     4792 kB VmSize:     4792 kB VmLck:         0 kB VmHWM:       432 kB VmRSS:       432 kB VmData:      156 kB VmStk:        84 kB VmExe:        32 kB VmLib:      1532 kB VmPTE:        24 kB Threads:    1 SigQ:   0/32268 SigPnd: 0000000000000000 ShdPnd: 0000000000000000 SigBlk: 0000000000000000 SigIgn: 0000000000000000 SigCgt: 0000000000000000 CapInh: 0000000000000000 CapPrm: 0000000000000000 CapEff: 0000000000000000 Cpus_allowed:   00000003 Mems_allowed:   1 voluntary_ctxt_switches:    0 nonvoluntary_ctxt_switches: 3 

You probably want to look at the first numbers on the Uid and Gid lines. You can look up which uid numbers map to what username by looking at /etc/passwd, or calling the relevant functions for mapping uid to username in whatever language you’re using.

Ideally, you would just call the system call getuid() to look up this information, doing it by looking at /proc/ is counterproductive.