Tests [ 0 -eq $UID ] don't work as in bash

Tests [ 0 -eq $UID ] don't work as in bash

[Andrey Voropaev]

Hello!

I'm sorry if this is FAQ question, but googling didn't report it, so
I'm asking here.

In our scripts we often use constructs like

##############
if [ 0 -eq $UID ]
then
        echo "This script must not be run by root user!"
        exit 1
fi
#################

This works fine in bash, but dash fails with warning " [: -ne:
argument expected". Is this intended feature? How to work around it in
dash then?

Best regards
Andrei Voropaev

Re: Tests [ 0 -eq $UID ] don't work as in bash

[Paul Smith]

On Wed, 2016-09-07 at 16:44 +0200, Andrey Voropaev wrote:
> if [ 0 -eq $UID ]

The variable UID is not defined to be automatically set by the shell in
POSIX; having it set is a bash extension.  dash doesn't set it
automatically for you.

You'll have to set it yourself:

  UID=$(id -u)

Also you should quote variable references, in general:

  if [ 0 -eq "$UID" ]

Be sure that if your scripts rely on bash-isms you start them with
#!/bin/bash and if you don't want them to rely on bash-isms, you start
them with #!/bin/sh.

Re: Tests [ 0 -eq $UID ] don't work as in bash

[Stephane Chazelas]

2016-09-07 11:55:00 -0400, Paul Smith:
> On Wed, 2016-09-07 at 16:44 +0200, Andrey Voropaev wrote:
> > if [ 0 -eq $UID ]
> 
> The variable UID is not defined to be automatically set by the shell in
> POSIX; having it set is a bash extension.  dash doesn't set it
> automatically for you.
> 
> You'll have to set it yourself:
> 
>   UID=$(id -u)
> 
> Also you should quote variable references, in general:
> 
>   if [ 0 -eq "$UID" ]
> 
> Be sure that if your scripts rely on bash-isms you start them with
> #!/bin/bash and if you don't want them to rely on bash-isms, you start
> them with #!/bin/sh.
[...]

Note that both zsh and bash have it, both from the start, both
copied from tcsh ($uid in tcsh).

In zsh, you can also set UID (and USERNAME and EUID, and GID...)
to change your uid (assuming you have the permission to do so).

$ sudo zsh -c 'id; UID=1000; id; UID=0'
uid=0(root) gid=0(root) groups=0(root)
uid=1000(chazelas) gid=0(root) groups=0(root)
zsh:1: failed to change user ID: operation not permitted

In bash, UID is read-only.

-- 
Stephane