WHAT I AM USING

• zsh


• MacOS Mojave 14.10

WHAT I WANT TO DO

$ echo $USERNAME

myusername

$ export USERNAME=newvalue

$ echo $USERNAME

newvalue

WHAT IS HAPPENING

$ echo $USERNAME

myusername

$ export USERNAME=newvalue

$ echo $USERNAME

myusername

WHAT I TRIED

Tried to set USERNAME in .bashrc, .zshrc, .profile and nothing changes...

Tried to unset and reset and nothing changes...

The USERNAME shell variable is special in the Z shell.

It is always the account name of the user ID of the shell process.

You won't observe the behaviour that you observed in the Almquist, Watanabe, Korn, or Bourne Again shells. This variable is just an ordinary shell variable, that starts out unset, as far as they are concerned. Here's the 93 Korn shell, for example:

$ echo $USERNAME



$ USERNAME=wibble

$ echo $USERNAME

wibble

$

In the Z shell it starts out as the account name of the UID of the shell process. An attempt to set it will attempt to change that UID. This of course fails if you are not the superuser and leaves the variable back as it was before.

If you had run the Z shell as the superuser, however, you would have seen both the USERNAME shell variable and your shell process's user ID change.

root # echo $USERNAME

root

root # USERNAME=JdeBP

JdeBP %

Further reading

• Paul Falstad et al. (2015-12-02). "Parameters Set By The Shell". Z Shell Manual. 5.2.