Re: Probably very stupid script/bash question
Am 2008-03-05 14:13:33, schrieb Brian:
teststring=one two three four five six
{ read A B C D E F; } ( echo $teststring )
echo Data received = $E Bytes
END OF REPLIED MESSAGE
This look a little bit weird. Why not use:
8
read A B C D E F EOF
one two three four five six
EOF
8
and
8
teststring=one two three four five six
read A B C D E F EOF
$teststring
EOF
echo Data received = $E Bytes
8
no need for a stupid resources eating subschell expecialy on a ARM.
:-)
Thanks, Greetings and nice Day
Michelle Konzack
Systemadministrator
24V Electronic Engineer
Tamay Dogan Network
Debian GNU/Linux Consultant
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Re: Probably very stupid script/bash question
Am 2008-03-05 13:10:37, schrieb Bob McGowan: Mark Clarkson wrote: On Wed, 05 Mar 2008 12:16:02 -0800 Bob McGowan [EMAIL PROTECTED] wrote: Brian wrote: So can you explain exactly what the first ( echo $teststring ) does exactly please? man bash 8-- echo EOD Here Documents This type of redirection instructs the shell to read input from the current source until a line containing only word (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input for a command. The format of here-documents is: [-]word here-document delimiter No parameter expansion, command substitution, arithmetic expansion, or pathname expansion is performed on word. If any characters in word are quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. If word is unquoted, all lines of the here-document are subjected to parameter expansion, com- mand substitution, and arithmetic expansion. In the latter case, the character sequence \newline is ignored, and \ must be used to quote the characters \, $, and `. If the redirection operator is -, then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion. EOD 8-- Thanks, Greetings and nice Day Michelle Konzack Systemadministrator 24V Electronic Engineer Tamay Dogan Network Debian GNU/Linux Consultant -- Linux-User #280138 with the Linux Counter, http://counter.li.org/ # Debian GNU/Linux Consultant # Michelle Konzack Apt. 917 ICQ #328449886 +49/177/935194750, rue de Soultz MSN LinuxMichi +33/6/61925193 67100 Strasbourg/France IRC #Debian (irc.icq.com) signature.pgp Description: Digital signature
Re: Probably very stupid script/bash question
Michelle Konzack wrote:
Am 2008-03-05 14:13:33, schrieb Brian:
teststring=one two three four five six
{ read A B C D E F; } ( echo $teststring )
echo Data received = $E Bytes
END OF REPLIED MESSAGE
This look a little bit weird. Why not use:
8
read A B C D E F EOF
one two three four five six
EOF
8
and
8
teststring=one two three four five six
read A B C D E F EOF
$teststring
EOF
echo Data received = $E Bytes
8
no need for a stupid resources eating subschell expecialy on a ARM.
:-)
Actually, the example code is a trivial example used to illustrate how
to use process substitution. A real case might have a series of
commands running, or a single large application, etc., from which you
want to read the output in some asynchronous way.
In some cases, using a here doc would be an option, but in the cases
mentioned above, you don't know what the text will be until you fetch
it. It's a run time issue.
--
Bob McGowan
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Re: Probably very stupid script/bash question
Michelle Konzack wrote: Am 2008-03-05 13:10:37, schrieb Bob McGowan: Mark Clarkson wrote: On Wed, 05 Mar 2008 12:16:02 -0800 Bob McGowan [EMAIL PROTECTED] wrote: Brian wrote: So can you explain exactly what the first ( echo $teststring ) does exactly please? man bash 8-- ### deleted 'here' doc usage ### The syntax ... (command ...) is *not* a here document. You should refer back to the original discussion for details. A brief synopsis is: '(command ...)' will substitute the command output into a named piped, which is then read. Using the second '', separated by white space, will then redirect input from the named pipe to a builtin command, such as read. If you take the example command and remove the space between the two less than symbols, you get a syntax error. -- Bob McGowan smime.p7s Description: S/MIME Cryptographic Signature
Re: Probably very stupid script/bash question
Mark Clarkson wrote:
On Tue, 04 Mar 2008 18:46:05 +0100
Brian [EMAIL PROTECTED] wrote:
I could not get this to work, the shell complains:
./dirvish-mail.sh: 98: Syntax error: redirection unexpected
Interesting to note that this does not work under busybox. I think
this is rather an esoteric but often useful bash feature. (It also
needs /proc so is not available in some special cases.)
The other solution from Alex:
echo $teststring | ( read A B C D E F; )
Of course, both solutions work perfectly under bash.
This should not have worked under bash either since it is still running
in a sub-shell, so the variables are lost when read finishes.
Mark,
of course you are correct. I had written the following test script:
=
#!/bin/bash
teststring=one two three four five six
{ read A B C D E F; } ( echo $teststring )
echo Data received = $E Bytes
echo $teststring | ( read A B C D E F; )
echo Data received = $E Bytes
=
$E was still set from the first read call. Duuh.
So can you explain exactly what the first ( echo $teststring )
does exactly please?
Cheers Brian
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Re: Probably very stupid script/bash question
Brian wrote:
Mark Clarkson wrote:
On Tue, 04 Mar 2008 18:46:05 +0100
Brian [EMAIL PROTECTED] wrote:
I could not get this to work, the shell complains:
./dirvish-mail.sh: 98: Syntax error: redirection unexpected
Interesting to note that this does not work under busybox. I think
this is rather an esoteric but often useful bash feature. (It also
needs /proc so is not available in some special cases.)
The other solution from Alex:
echo $teststring | ( read A B C D E F; )
Of course, both solutions work perfectly under bash.
This should not have worked under bash either since it is still running
in a sub-shell, so the variables are lost when read finishes.
Mark,
of course you are correct. I had written the following test script:
=
#!/bin/bash
teststring=one two three four five six
{ read A B C D E F; } ( echo $teststring )
echo Data received = $E Bytes
echo $teststring | ( read A B C D E F; )
echo Data received = $E Bytes
=
$E was still set from the first read call. Duuh.
So can you explain exactly what the first ( echo $teststring )
does exactly please?
Cheers Brian
It looks like the construct ' (...)' is an undocumented (at least it's
not in the man page for bash) feature that redirects the output of a
command in such a way as to look like it's from a file. I base this on
the following observations:
1. Reading the man page for bash, it states that if a builtin command
(such as 'read') is used in a pipe, the command is run in a sub shell.
This is why echo 1 2 3 | read A B C followed by echo $A $B $C prints
only a blank line. The assignment is in a sub shell and lost when
execution comes back to the parent.
2. If I have a file with more than one line in it and the first line
has no white space, and I do this:
read A B C file
The variable A will have the first line of the file in it, with all
subsequent variables empty. If the first line has white space, then as
many variables as there are fields in the line will contain values.
3. if I do this, using the ' (...)' construct with an echo that
produces two lines:
{ read A B C D E F; } ( echo -e one two three\nfour five )
The first three variables will have the first three values, but the last
three variables will be empty.
4. See 5, below.
Since this operator appears to follow the same rules as file
redirection, it would seem logical to assume it mimics file type I/O
redirection. But this is a WAG ('wild ass guess' for those that don't
know) and may be way off base.
In any case, I'd be interested in knowing where you found this construct.
Further, for those interested in actually using it, these are the rules
I've figured out, so far:
1. The format is exact, you must have one or more white spaces between
the two 'less than' symbols and NO space between the 'less than' and the
'left parenthesis'. White space before or after the four characters
appears to be immaterial, it works with or without it.
2. Changing the operator to ' {' doesn't work, only works with
parenthesis (not unexpected, just checking:).
3. Space after the left parenthesis or before the right parenthesis is
not needed. Works with or without it.
4. The command to the left of the operator does not need to be
isolated. Both:
{ read A B C D E F; } ( echo )
and:
read A B C D E F ( echo )
work the same.
5. Just like '' from a file, the position of ' (...)' can be
anywhere on the line. This line works just like the examples above,
though the ones used above are certainly more understandable to human eyes:
(echo one two three four five six) read A B C D E F
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Re: Probably very stupid script/bash question
On Wed, 05 Mar 2008 12:16:02 -0800 Bob McGowan [EMAIL PROTECTED] wrote: Brian wrote: So can you explain exactly what the first ( echo $teststring ) does exactly please? In any case, I'd be interested in knowing where you found this construct. The bash man page seems to be one of the few that can be read a thousand times and /still/ find something new in it each time! The following extract is from 'man bash' ;-) ,although what it doesn't make clear is that (list) is preceeded by ' ', which I guess is why it's often missed. blockquote Process Substitution Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files. It takes the form of (list) or (list). The process list is run with its input or out- put connected to a FIFO or some file in /dev/fd. The name of this file is passed as an argument to the current command as the result of the expansion. If the (list) form is used, writing to the file will pro- vide input for list. If the (list) form is used, the file passed as an argument should be read to obtain the output of list. When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion. /blockquote Cheers Mark. -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
Re: Probably very stupid script/bash question
Mark Clarkson wrote:
On Wed, 05 Mar 2008 12:16:02 -0800
Bob McGowan [EMAIL PROTECTED] wrote:
Brian wrote:
So can you explain exactly what the first ( echo $teststring )
does exactly please?
In any case, I'd be interested in knowing where you found this
construct.
The bash man page seems to be one of the few that can be read a
thousand times and /still/ find something new in it each time!
The following extract is from 'man bash' ;-) ,although what it doesn't
make clear is that (list) is preceeded by ' ', which I guess is why
it's often missed.
blockquote
Process Substitution
Process substitution is supported on systems that support
named pipes (FIFOs) or the /dev/fd method of naming open files. It
takes the form of (list) or (list). The process list is run with
its input or out- put connected to a FIFO or some file in /dev/fd. The
name of this file is passed as an argument to the current command as
the result of the expansion. If the (list) form is used, writing to
the file will pro- vide input for list. If the (list) form is
used, the file passed as an argument should be read to obtain the
output of list.
When available, process substitution is performed
simultaneously with parameter and variable expansion, command
substitution, and arithmetic expansion.
/blockquote
Cheers
Mark.
Interesting. But...
If I do the process substitution using a stand alone programs, it
works as described:
$ wc (echo this is a test)
1 4 15 /dev/fd/63
And, note the '/dev/fd' device, just as the man page describes.
If I put the above, verbatim, in a script file, it also works exactly as
described.
It breaks when the command used is a builtin command:
$ read list (echo this is a test)
bash: read: `/dev/fd/63': not a valid identifier
In this case, I typed a CR after the first line and it hung, in fact
it was waiting for input to the 'read' builtin. So I typed a '^D', with
the error message then following.
The string ' (' does not appear anywhere in the bash man page, hence
my failure to make the association with 'process substitution'.
But, my question is still not completely answered. What prompted Brian
(or the original poster, if different), to add a second '' to the '('?
I suppose the next question should be what type of bug is this? Is it a
documentation issue, where the ' (...)' syntax simply needs explaining
or is it a bash bug, in that a 'read list (...)' should work?
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Re: Probably very stupid script/bash question
On Wed, 05 Mar 2008 13:10:37 -0800
Bob McGowan [EMAIL PROTECTED] wrote:
Mark Clarkson wrote:
On Wed, 05 Mar 2008 12:16:02 -0800
Bob McGowan [EMAIL PROTECTED] wrote:
Brian wrote:
So can you explain exactly what the first ( echo
$teststring ) does exactly please?
In any case, I'd be interested in knowing where you found this
construct.
The bash man page seems to be one of the few that can be read a
thousand times and /still/ find something new in it each time!
The following extract is from 'man bash' ;-) ,although what it
doesn't make clear is that (list) is preceeded by ' ', which I
guess is why it's often missed.
blockquote
Process Substitution
Process substitution is supported on systems that support
named pipes (FIFOs) or the /dev/fd method of naming open files. It
takes the form of (list) or (list). The process list is run
with its input or out- put connected to a FIFO or some file
in /dev/fd. The name of this file is passed as an argument to
the current command as the result of the expansion. If the (list)
form is used, writing to the file will pro- vide input for
list. If the (list) form is used, the file passed as an argument
should be read to obtain the output of list.
When available, process substitution is performed
simultaneously with parameter and variable expansion, command
substitution, and arithmetic expansion.
/blockquote
Cheers
Mark.
Interesting. But...
If I do the process substitution using a stand alone programs, it
works as described:
$ wc (echo this is a test)
1 4 15 /dev/fd/63
And, note the '/dev/fd' device, just as the man page describes.
If I put the above, verbatim, in a script file, it also works exactly
as described.
It breaks when the command used is a builtin command:
$ read list (echo this is a test)
bash: read: `/dev/fd/63': not a valid identifier
In this case, I typed a CR after the first line and it hung, in
fact it was waiting for input to the 'read' builtin. So I typed a
'^D', with the error message then following.
The string ' (' does not appear anywhere in the bash man page,
hence my failure to make the association with 'process substitution'.
But, my question is still not completely answered. What prompted
Brian (or the original poster, if different), to add a second '' to
the '('?
I suppose the next question should be what type of bug is this? Is
it a documentation issue, where the ' (...)' syntax simply needs
explaining or is it a bash bug, in that a 'read list (...)' should
work?
This should explain it:
$ exec 44
$ echo this is a test 4
$ wc /dev/fd/4
1 4 15 /dev/fd/4
which of course is equivalent to
$ wc (echo this is a test)
and is not a redirection!
The man page tells me that (list) substitutes the list and it's up to
the user to choose what to do with it. I chose to redirect it, hence
the additional ' '.
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Re: Probably very stupid script/bash question
Mark Clarkson wrote: On Wed, 05 Mar 2008 13:10:37 -0800 Bob McGowan [EMAIL PROTECTED] wrote: Mark Clarkson wrote: On Wed, 05 Mar 2008 12:16:02 -0800 Bob McGowan [EMAIL PROTECTED] wrote: Brian wrote: So can you explain exactly what the first ( echo $teststring ) does exactly please? Deleted details available elsewhere This should explain it: $ exec 44 $ echo this is a test 4 $ wc /dev/fd/4 1 4 15 /dev/fd/4 which of course is equivalent to $ wc (echo this is a test) and is not a redirection! The man page tells me that (list) substitutes the list and it's up to the user to choose what to do with it. I chose to redirect it, hence the additional ' '. I think I get it. Following through with this, regarding the error seen with 'read' and the substitution only form: 'bash: read: `/dev/fd/4': not a valid identifier', I did this, based on your setup above: $ read a b c d /dev/fd/4 1 2 3 4 bash: read: `/dev/fd/4': not a valid identifier $ read a b c d /dev/fd/4 $ echo $c $d a test Which is just another way of illustrating that you need to redirect the substitution. Thanks. -- Bob McGowan smime.p7s Description: S/MIME Cryptographic Signature
Re: Probably very stupid script/bash question
Mark Clarkson wrote: On Wed, 05 Mar 2008 13:10:37 -0800 Bob McGowan [EMAIL PROTECTED] wrote: If I do the process substitution using a stand alone programs, it works as described: $ wc (echo this is a test) 1 4 15 /dev/fd/63 I couldn't find the correct place to interject this question in the thread, so I fairly randomly selected this location... As far as I can tell, the following 4 commands should all behave the same, but the last one hangs. Can anyone see why? $ cat (echo foo) foo $ bash -c 'cat (echo foo)' foo $ echo foo | bash -c 'cat' foo $ bash -c 'cat' (echo foo) -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
Re: Probably very stupid script/bash question
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 William Pursell wrote: I couldn't find the correct place to interject this question in the thread, so I fairly randomly selected this location... As far as I can tell, the following 4 commands should all behave the same, but the last one hangs. Can anyone see why? $ cat (echo foo) foo $ bash -c 'cat (echo foo)' foo $ echo foo | bash -c 'cat' foo $ bash -c 'cat' (echo foo) bash is not passing it's stdin to cat's, cat is also in a subshell. -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFHzy8mLeTfO4yBSAcRAnU1AJwPDDxW8w6lg2uluPdPqnmHdpA2OgCgjtpM yzemPNl95uLFTZY+XwmJxOU= =kLad -END PGP SIGNATURE- -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
Re: Probably very stupid script/bash question
On Wed March 5 2008 15:36:50 William Pursell wrote: As far as I can tell, the following 4 commands should all behave the same, but the last one hangs. Can anyone see why? $ cat (echo foo) foo $ bash -c 'cat (echo foo)' foo $ echo foo | bash -c 'cat' foo $ bash -c 'cat' (echo foo) The last command is of the form bash -c string file. bash file reads commands from a file, but bash -c string reads commands from the string instead, and apparently ignores the file argument. The file argument is normally omitted when -c string is used. --Mike Bird -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
Re: Probably very stupid script/bash question
Georg Neis wrote:
Brian wrote:
The following does not (the value is empty):
echo $teststring | { read A B C D E F; }
echo Data received = $E Bytes --- $E is empty
I assume it has something to do with the read command being executed in
a subshell.
Yes.
So how can I extract the parts I want into variables to use them later
on in the script?
$ teststring='field1 field2 field3'
$ set -- $teststring
$ echo $2
field2
Regards,
Georg
Georg,
this works perfectly thanks.
Cheers Brian
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Re: Probably very stupid script/bash question
Mark Clarkson wrote:
On Mon, 2008-03-03 at 19:48 +0100, Brian wrote:
echo $teststring | { read A B C D E F; }
echo Data received = $E Bytes --- $E is empty
{ read A B C D E F; } ( echo $teststring )
echo Data received = $E Bytes --- $E is empty
Robomod,
I could not get this to work, the shell complains:
./dirvish-mail.sh: 98: Syntax error: redirection unexpected
The other solution from Alex:
echo $teststring | ( read A B C D E F; )
did not work either.
Then I got to wondering about why a few methods that I had tried before
posting did not work either, although they were documented many times. I
was doing the testing on an NSLU2 which uses Busybox, and its default
shell sh and not bash (I also have an NSLU2 with Debian on it, there
the shell is bash). So I assume this has something to do with busybox's
shell.
Of course, both solutions work perfectly under bash.
Cheers Brian
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Re: Probably very stupid script/bash question
On Tue, 04 Mar 2008 18:46:05 +0100 Brian [EMAIL PROTECTED] wrote: I could not get this to work, the shell complains: ./dirvish-mail.sh: 98: Syntax error: redirection unexpected Interesting to note that this does not work under busybox. I think this is rather an esoteric but often useful bash feature. (It also needs /proc so is not available in some special cases.) The other solution from Alex: echo $teststring | ( read A B C D E F; ) Of course, both solutions work perfectly under bash. This should not have worked under bash either since it is still running in a sub-shell, so the variables are lost when read finishes. -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
Probably very stupid script/bash question
Hi,
I tried using bash to split a string. This works OK:
echo $teststring | { read A B C D E F; echo Data received = $E Bytes; }
The following does not (the value is empty):
echo $teststring | { read A B C D E F; }
echo Data received = $E Bytes --- $E is empty
I assume it has something to do with the read command being executed in
a subshell.
So how can I extract the parts I want into variables to use them later
on in the script?
Thanks in advance
Cheers Brian
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Re: Probably very stupid script/bash question
On Mon, Mar 03, 2008 at 07:48:49PM +0100, Brian wrote:
Hi,
I tried using bash to split a string. This works OK:
echo $teststring | { read A B C D E F; echo Data received = $E Bytes; }
The following does not (the value is empty):
echo $teststring | { read A B C D E F; }
echo Data received = $E Bytes --- $E is empty
maybe
echo $teststring | ( read A B C D E F; )
why do you need the {} or ()
I assume it has something to do with the read command being executed in
a subshell.
So how can I extract the parts I want into variables to use them later
on in the script?
Thanks in advance
Cheers Brian
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Re: Probably very stupid script/bash question
On Mon, 2008-03-03 at 19:48 +0100, Brian wrote:
echo $teststring | { read A B C D E F; }
echo Data received = $E Bytes --- $E is empty
{ read A B C D E F; } ( echo $teststring )
echo Data received = $E Bytes --- $E is empty
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Re: Probably very stupid script/bash question
Brian wrote:
The following does not (the value is empty):
echo $teststring | { read A B C D E F; }
echo Data received = $E Bytes --- $E is empty
I assume it has something to do with the read command being executed in
a subshell.
Yes.
So how can I extract the parts I want into variables to use them later
on in the script?
$ teststring='field1 field2 field3'
$ set -- $teststring
$ echo $2
field2
Regards,
Georg
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