Diff between '{..}' and {'..'}
Hi,
Could anyone tell me the difference between
......| awk '{.......}'
AND
......| awk {'......'}
And also when(in what situation) these are used?
Thanks in advance,
Anil.
anilby
comp.lang.awk
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2005-01-12, 22:37(-08), Anil:
> Hi,
>
> Could anyone tell me the difference between
>
> .....| awk '{.......}'
>
> AND
>
> .....| awk {'......'}
>
>
> And also when(in what situation) these are used?
> Thanks in advance,
[...]
The second one should never be used.
The difference is at the shell level, not at the awk level.
'...' are strong shell quotes, which means that the text inside
single quotes is passed literally as one single argument to awk.
If you put "{" and "}" outside the quotes, they may be treated
specially depending on the shell.
For instance the bash, zsh and some versions of ksh shells (and
csh-like shells, but those have a different syntax), use {...}
to perform what is called brace expansion.
cmd foo{bar,baz}
is expanded into 3 arguments: "cmd", "foobar", "foobaz"
So in
awk {'print "'\'foo,bar\''"'}
awk would take three arguments: <awk>, <print "'foo> and <bar'">
which means run the <print "'foo> code on the file named
<bar'">, while in
awk '{print "'\'foo,bar\''"}'
awk would take two arguments: <awk> and <{print "'foo,bar'"}>
which means run the <{print "'foo,bar'"}> code on stdin.
awk {'print "foo"'}
would be the same as
awk '{print "foo"}'
(except for csh-like shells) as there's no unquoted commas
inbetween the braces (so, it's no candidate for brace
expansion).
--
Stephane
|
0 |
|
Stephane
1/13/2005 9:00:32 AM
Thanks for the reply Stephane.
Similarly in what situations shud I use
awk '{........}' < filename
AND
awk '{.........}' filename
(The "<" sign before filename)
|
|
0 |
|
Anil
1/13/2005 10:03:40 AM
Anil wrote:
> Thanks for the reply Stephane.
>
> Similarly in what situations shud I use
>
> awk '{........}' < filename
Here the shell reads the file and presents
it to standard input of the AWK interpreter.
The file name is unknown to AWK.
> AND
>
> awk '{.........}' filename
Here it is the AWK interpreter which
opens the file and reads the data.
The FILENAME variable can be read in
this case.
Last week someone pointed out that the
second variant should be preferred over
the first one but I cant remember the
reason.
|
|
0 |
|
ISO
1/13/2005 10:10:53 AM
2005-01-13, 02:03(-08), Anil:
> Thanks for the reply Stephane.
>
> Similarly in what situations shud I use
>
> awk '{........}' < filename
> AND
>
> awk '{.........}' filename
>
> (The "<" sign before filename)
In the first case, the file is opened by the shell, in the
second, it is opened by awk.
You may prefer the first one as it makes a consistent way (with
consistent error messages) to open a file all along your script,
or to avoid problems with filenames containing equal signs (awk
take them for variable assignements if the = is not preceded by
a /). It also means that awk is not run if the shell can't open
the file (which saves some execution time).
You will prefer the second if there's more than one file to
open, or if you want awk to have access to the name of the file
(through FILENAME or ARGV variables).
If you need to open a file called file=name, use
awk '...' ./file=name
Or:
awk '...' < file=name
That means that:
awk '...' "$file"
is not correct unless you've made sure $file can't be looking
like a variable assignment.
--
Stephane
|
|
0 |
|
Stephane
1/13/2005 10:28:02 AM
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