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Malte Forkel 06-06-2008 10:59 AM

bash syntax question
 
I recently came across the following example for switching time zones:

$ date
Fr 6. Jun 12:49:30 CEST 2008
$ TZ=UTC date
Fr 6. Jun 10:49:35 UTC 2008

Could someone please explain to me the general shell syntax involved here? I did not know that I could put an assignment and a command on one line. My dull approach would have been

$ echo $TZ

$ TZ=UTC
$ export TZ
$ date
Fr 6. Jun 10:54:29 UTC 2008
$ unset TZ
$ date
Fr 6. Jun 12:54:39 CEST 2008

Thanks in advance,
Malte


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Alfredo Finelli 06-06-2008 12:35 PM

bash syntax question
 
On Friday 06 June 2008 12:59, Malte Forkel wrote:
> I recently came across the following example for switching time
> zones:
>
> $ date
> Fr 6. Jun 12:49:30 CEST 2008
> $ TZ=UTC date
> Fr 6. Jun 10:49:35 UTC 2008
>
> Could someone please explain to me the general shell syntax involved
> here? I did not know that I could put an assignment and a command on
> one line. My dull approach would have been [...]

The 'date' command runs in a child process for which the bash shell has
to determine the environment. Parameter assignments are inherited in
that environment (i.e. the 'date' command sees them) in two cases:
either when the variable is not only declared but also exported or when
the assignment is provided on the command line right before the command
itself. From the man page:

The environment for any simple command or function may
be augmented temporarily by prefixing it with parameter
assignments, as described above in PARAMETERS. These assignment
statements affect only the environment seen by that command.

For more information you may want to read the "Command execution
environment" section in the Bash man page.

Best regards.

Alfredo


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Malte Forkel 06-06-2008 02:40 PM

bash syntax question
 
Alfredo Finelli schrieb:
The 'date' command runs in a child process for which the bash shell has
to determine the environment. Parameter assignments are inherited in
that environment (i.e. the 'date' command sees them) in two cases:
either when the variable is not only declared but also exported or when
the assignment is provided on the command line right before the command
itself. From the man page:


The environment for any simple command or function may
be augmented temporarily by prefixing it with parameter
assignments, as described above in PARAMETERS. These assignment
statements affect only the environment seen by that command.

For more information you may want to read the "Command execution
environment" section in the Bash man page.


Alfrede, thanks for your help! No I understand. And its good to know that I'm using an official feature :-)



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