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Re: built-in '[' and '/usr/bin/[' yield different results

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  Re: built-in '[' and '/usr/bin/[' yield different results Eric Blake <eblake@redhat.com> - 2018-11-13 11:54 -0600

#14814 — Re: built-in '[' and '/usr/bin/[' yield different results

On 11/13/18 10:29 AM, Service wrote:
>      # ensure that file1 exists and that file2 does not exist

There's your problem. It is inherently ambiguous what timestamp to use 
when a file is missing (infinitely new or infinitely old, or always an 
error for not existing); bash's -nt picked one way, while other shells 
have picked the other.  POSIX is silent on the matter (-nt is an 
extension outside of POSIX), so there is nothing portable you can rely on.

>      /bin/touch file1
>      /bin/rm -f file2
>      # built-in
>      if          [ file1 -nt file2 ]; then echo nt; else echo not_nt; fi
>      # external
>      if /usr/bin/[ file1 -nt file2 ]; then echo nt; else echo not_nt; fi
> 
>      # Output is as expected:
>      nt
>      nt

That is, bash's builtin '[' and coreutil's external '[' happened to pick 
the same thing: a missing file is treated as infinitely old.

> 
>      2. This does not work:
> 
>      # Put the above commands into a script, say check.sh
>      # Run with: /bin/sh < check.sh
>      # Or      : /bin/sh ./check.sh
>      # Or      : /usr/bin/env ./check.sh
> 
>      # Output is always not ok:
>      not_nt
>      nt

Most likely, this is because your /bin/sh is not bash, but probably 
dash, and dash has picked a missing file as being treated as always an 
error.  That does not make it a bug in bash, though, but a difference in 
behavior of your /bin/sh.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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