Multiple shell parameter assignments prefixing a bash command

Question

In the bash manual section 3.7.4, it states

The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments, as described in Shell Parameters [section 3.4].

And a trivial example of this is

MYVAR=MYVALUE mycommand

I have read section 3.4 and I still can't figure out how to specify multiple parameter assignments. Note that the statement in 3.7.4 is definitely plural, implying that it is possible.

The following does not seem to work:

MYVAR1=abc MYVAR2=xyz mycommand

I am using bash version 4.1, 23 December 2009.

Works here. You're screwing something else up.

Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams

2011-10-22 18:30:49 +00:00
Commented Oct 22, 2011 at 18:30 — Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams, Commented Oct 22, 2011 at 18:30

Ned Deily · Accepted Answer · 2011-10-22 19:08:35Z

2

It should work. That is acceptable syntax. Here's an example:

$ cat a
#!/bin/sh
echo $MYVAR1
echo $MYVAR2
$ ./a


$ MYVAR1=abc MYVAR2=xyz ./a
abc
xyz
$

UPDATE: Your updated example given in your answer will work if you precede the simple command with the variables as required:

mycommand () { echo MYVAR1=[$MYVAR1]; echo MYVAR2=[$MYVAR2]; }
for f in ~/*.txt ; do MYVAR1=abc MYVAR2=xyz mycommand; done

edited Oct 22, 2011 at 19:08

answered Oct 22, 2011 at 18:37

Ned Deily

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Comments

system PAUSE · Accepted Answer · 2011-10-22 18:37:05Z

1

Oops, my example was over-simplified. This works fine:

mycommand () { echo MYVAR1=[$MYVAR1]; echo MYVAR2=[$MYVAR2]; }
MYVAR1=abc MYVAR2=xyz mycommand

but this does not:

mycommand () { echo MYVAR1=[$MYVAR1]; echo MYVAR2=[$MYVAR2]; }
MYVAR1=abc MYVAR2=xyz for f in ~/*.txt; do mycommand; done

and the key difference is this phrase:

for any simple command or function

A for command is a complex command, not "a simple command or function", according to section 3.2.

answered Oct 22, 2011 at 18:37

system PAUSE

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1 Comment

Ned Deily Over a year ago

But this does: for f in ~/*.txt ; do MYVAR1=abc MYVAR2=xyz mycommand; done

evan · Accepted Answer · 2011-10-22 18:51:25Z

0

I've never seen that method used.

You could always just pass in regular parameters to the script.

Updating for new example:

This works in both situations:

> mycommand () { echo MYVAR1=[$1]; echo MYVAR2=[$2]; }

> mycommand a b
MYVAR1=[a]
MYVAR2=[b]

> for f in ~/file*.txt; do mycommand a b; done
MYVAR1=[a]
MYVAR2=[b]
MYVAR1=[a]
MYVAR2=[b]

edited Oct 22, 2011 at 18:51

answered Oct 22, 2011 at 18:31

evan

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4 Comments

Ned Deily Over a year ago

It's perfectly legal and defined in the POSIX standard (it's not even a bashism): 'A "simple command" is a sequence of optional variable assignments and redirections, in any sequence, optionally followed by words and redirections, terminated by a control operator.' pubs.opengroup.org/onlinepubs/9699919799/utilities/…

evan Over a year ago

What should technically work and what is easy to make work are often two different things. Why beat yourself up over something that you can make work another way in two shakes of a lamb's tail?

Ned Deily Over a year ago

Huh? Passing a parameter and setting a variable are not the same thing. And this is basic shell stuff. There's nothing tough about it.

evan Over a year ago

If there wasn't anything tough about it, you wouldn't have the opportunity to answer a question on it.

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