read command

nymus7 · July 22, 2005, 4:10am

'Morning

vmstat 1 1|sed 1,2d|awk '{printf("%s\n",$1)}'|read var
echo $var

This syntax run on AIX (ksh) but not on linux (bash).
I think that problem is the read command, because the following syntax is ok :

vmstat 1 1|sed 1,2d|awk '{printf("%s\n",$1)}'

Could someone help me!

regards
nymus

blowtorch · July 22, 2005, 4:22am

Strange, you syntax seems to be fine! I am checking the man page for bash on the net, you should not be getting an error. Just to check, can you skip the "\n" in the awk?

nymus7 · July 22, 2005, 4:34am

Yeah strange, I'm getting any errors! the echo command send me only a blank line...
I've just skipping the "\n" in the awk but it's the same result!
Thanks for your help
nymus

nymus7 · July 22, 2005, 4:43am

I've made new test:
This syntax doesn't run:

vmstat 1 1|sed 1,2d|awk '{printf("%s\n",$1)}'|read var
echo $var

But this syntax is running:

vmstat 1 1 |sed 1,2d |awk '{printf("%s\n",$1) }'|read var;echo $var

Why...? where's the problem
Thx

r2007 · July 22, 2005, 4:52am

FYI. From BASH Frequently-Asked Questions

E4) If I pipe the output of a command into `read variable', why doesn't
the output show up in $variable when the read command finishes?

This has to do with the parent-child relationship between Unix
processes. It affects all commands run in pipelines, not just
simple calls to `read'. For example, piping a command's output
into a `while' loop that repeatedly calls `read' will result in
the same behavior.

Each element of a pipeline runs in a separate process, a child of
the shell running the pipeline. A subprocess cannot affect its
parent's environment. When the `read' command sets the variable
to the input, that variable is set only in the subshell, not the
parent shell. When the subshell exits, the value of the variable
is lost.

Many pipelines that end with `read variable' can be converted
into command substitutions, which will capture the output of
a specified command. The output can then be assigned to a
variable:
grep ^gnu /usr/lib/news/active | wc -l | read ngroup
can be converted into
ngroup=$$grep ^gnu /usr/lib/news/active | wc -l$
This does not, unfortunately, work to split the text among
multiple variables, as read does when given multiple variable
arguments. If you need to do this, you can either use the
command substitution above to read the output into a variable
and chop up the variable using the bash pattern removal
expansion operators or use some variant of the following
approach.

Say /usr/local/bin/ipaddr is the following shell script:

#! /bin/sh
host `hostname` | awk '/address/ {print $NF}'

Instead of using
/usr/local/bin/ipaddr | read A B C D
to break the local machine's IP address into separate octets, use
OIFS="$IFS"
IFS=.
set -- $$/usr/local/bin/ipaddr$
IFS="$OIFS"
A="$1" B="$2" C="$3" D="$4"
Beware, however, that this will change the shell's positional
parameters. If you need them, you should save them before doing
this.

This is the general approach -- in most cases you will not need to
set $IFS to a different value.

Some other user-supplied alternatives include:

read A B C D << HERE
$(IFS=.; echo $(/usr/local/bin/ipaddr))
HERE

and, where process substitution is available,

read A B C D < <(IFS=.; echo $(/usr/local/bin/ipaddr))

nymus7 · July 22, 2005, 5:26am

Thanks for your reply!
But my script doesn't run with new syntax : read var << echo $(vmstat 1 1 |...)
I've read on the bash manuel that the read command is a "Bash Builtin Commands" and should running like in other shell.
What's that means? Is bash a restrictive shell...?
I'll must to change all my AIX scripts for my linux box really a bad things
Regards,ny

blowtorch · July 22, 2005, 5:54am

Hey r2007, thanks for that FAQ. But this works just fine from ksh. Maybe it has something to do with the manner in which forks are called by the shell..

nymus7 · July 22, 2005, 6:08am

Hi guys

blowtorch has perhaps right! or are my environment variables not right??
It's very strange because the only manner that the 'read command' run on my box is like so:

echo "enter a number"
read number
echo $number

Anyway.. thanks for your help
ny

Perderabo · July 22, 2005, 8:22am

nymus7:

Hi guys

blowtorch has perhaps right! or are my environment variables not right??
It's very strange because the only manner that the 'read command' run on my box is like so:
echo "enter a number"
read number
echo $number
Anyway.. thanks for your help
ny

Not strange at all. That is the expected result and it's explained by that FAQ article that r2007 quoted. A pipeline like:
somecommand | read variable
doesn't work because a subshell is spawned to execute that read command. That subshell sets "variable" and then it exits. The original shell now resumes and it doesn't have a copy of the value of "variable".

That syntax works in ksh because the Korn shell arranges that when the last command in a pipeline is a shell built-in command, that command is run in the context of the cuurent shell. This means that despite appearances ksh will not treat "somecommand | read variable" as true pipeline. This feature works in ksh alone. Not even pdksh supports this syntax and this is the most frequently reported pdksh "bug".

nymus7 · July 22, 2005, 11:10am

Ok, great! That's means that many commands in my ksh script will don't run in bash!
How to know the difference between the ksh and the bash shell and wich commands will products a subshell??
What's the best way to learn the bash shell because I've seen on internet some bash tutorial but they don't speak about subshell and many commands are described like ksh?
What's means "pdksh"??
:-)thanks

Perderabo · July 22, 2005, 11:18am

There's a lot of stuff that is different between bash and ksh. pdksh is a shell that was written to be as close as possible to ksh. Unlike ksh, pdksh is open source. Get a good ksh book and a good bash book and read them both. Then write a few hundred shell scripts in both languages. At that point the differences will seem clear.

nymus7 · July 22, 2005, 11:24am

Well, I've understood...
Thanks Perderabo and have a nice weekend!
nymus

r2007 · July 23, 2005, 1:15am

be careful, there should have a blank character between the two "<"s
read var < < echo $(vmstat 1 1 |...)

nymus7 · July 25, 2005, 10:19am

Hi r2007

I'm receiving the following error with a blank character between the two "<"s
-bash: syntax error near unexpected token `<'

r2007 · July 25, 2005, 10:31am

sorry, i am not look into your script last time. it shold like this[untested]:

read var < <(vmstat 1 1 |...)

there has a blank between tow "<"s, and no blank between "<" and "("

Unbeliever · July 25, 2005, 10:38am

Out of interest ... is there any reason you cant use back quotes? For example:

var=`vmstat 1 1|sed 1,2d|awk '{printf("%s\n",$1)}'`

or slightly simpler:

var=`vmstat 1 1|sed 1,2d|awk '{print($1)}'`

nymus7 · July 28, 2005, 3:39am

Hi r2007
Great, it's running!
Thanks