Difference between $LOGNAME and logname

Question

When the echo $LOGNAME and logname commands run normally, I get identical results from them:

pandya@pandya-desktop:~$ echo $LOGNAME
pandya
pandya@pandya-desktop:~$ logname
pandya

Is there any difference between them?

Answer 1

From Environment variables:

$LOGNAME is same as $USER which gives

The name of the currently logged-in user. This variable is set by the system. You probably shouldn't change its value manually.

From man logname:

logname - print user´s login name

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Explained differently used by following example:

pandya@pandya-desktop:~$ sudo su
root@pandya-desktop:/home/pandya# echo $LOGNAME
root
root@pandya-desktop:/home/pandya# logname
pandya
root@pandya-desktop:/home/pandya# exit
exit
pandya@pandya-desktop:~$

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Here you can see the difference after logging as root in a terminal,

• $LOGNAME gives the name of the currently logged-in user in the terminal (i.e root)
• Whereas logname prints the user's login name who logged in to the session (i.e. pandya)

Answer 2

The utility logname is broken in 16.04, apparently on purpose because it's possible to hack the value it returns, which could be a security flaw. https://bugzilla.gnome.org/show_bug.cgi?id=747046 I had been using logname in some scripts, and found it useful because it always returned the same value (my login name) whether I was calling it from from a my own level or from a sudo invocation, whereas the environment variables $USER and $LOGNAME do not. I wasn't worried about someone hacking into my computer and modifying the value it returns. Alas.... I have found a workaround. I created a file in my home directory as such

echo $USER > ~/.logname

When I want to access my logname (e.g., assigning to a Bash variable g_logname), I do so thusly:

declare g_logname="$(<~/.logname)";

This works for me, whether as myself or at the root level through sudo, which maintains the assignment of "~" as my home directory. I can create a .logname file for the home directory of each user on the system. Yes, someone could hack into my computer and change these files, but I'm not really worried about that. I just want my scripts to work.