the trouble with |read

kc8tbe · 07-28-2004, 02:59 PM

I sort of expected this to work:

Code:

unset STRING #make sure we start with a clean slate
echo "wassup" | read STRING # note that this line produces no output
echo $STRING # though it ought to, this line produces no output

I've also tried some variations of read, including those that ignore the newline character. For some reason, "wassup" isn't assigned to $STRING. Since line 2 produces no output, it's safe to assume that "wassup" is being piped to read. And since running line 2 with "read -t 3" returns instantaneously (instead of waiting three seconds), it's safe to assume that read is seeing some data. Anyone know what I'm doing wrong?

jlliagre · 07-28-2004, 03:12 PM

This is a bash "feature", read is run by a subshell so your variable is set but lost immediatly.
Your example works under ksh though.

kc8tbe · 07-28-2004, 06:01 PM

The variable is still "lost" if the entire procedure occurs within a shell script instead of being run line by line by the user.

Any idea how to make bash not "lose" my variable? Using ksh exclusively for this purpose seems like a poor solution. (Although I might just switch to ksh altogether...)

osvaldomarques · 07-28-2004, 06:51 PM

Hi kc8tbe,
The common way to get this content would be

Code:

STRING=`echo "wassup"`

.
If you need to pipe the result of an operation, you can do it by

Code:

echo "wassup" |
while read STRING
do
  echo $STRING
done

jlliagre · 07-28-2004, 07:12 PM

You can also workaround this issue like this:

Code:

echo "wassup" |
(
read STRING
echo $STRING
...
)

Beware that after the block is done, the STRING variable is lost too.

Another annoying thing is that inside the block, you need to explicitly set the input if you want interaction with the user.
eg.

Code:

read reply < /dev/tty

Both problems applies to osvaldo's while loop too.

Cedrik · 07-29-2004, 01:46 AM

Maybe kc8tbe just want to echo "wassup" just after read the input

Code:

read STRING && echo "wassup"
echo $STRING

[edit]
After read more carefully, I feel my suggestion slighty out of lead

osvaldomarques seems to be right with STRING=`echo "wassup"`

Hko · 07-29-2004, 06:45 AM

Quote:

Originally posted by kc8tbe
The variable is still "lost" if the entire procedure occurs within a shell script instead of being run line by line by the user.

I think you didn't understand it correctly when jlliagre said:

Quote:

This is a bash "feature", read is run by a subshell so your variable is set but lost immediatly.

In case of bash, "read" is not a program, but a builtin command. Then realize that a pipe ('|') connects the output of one process to the input of another. So, to have "read" read from a pipe there is another process needed. Since "read" is built inside bash, another bash-process is started to run "read". The reading of the variable occurs in a different process, which terminates inmediatly after running "read". When process isn't there anymore, all of its variables are lost.

This is no different from the command line or in a single script!

osvaldomarques' soltution is the "standard" way to read the output of a command (either builtin, or a seperate program or process):

Code:

# The classic syntax
STRING=`echo "wassup"`

# newer, bash's syntax (less comaptible, but has some advantages)
STRING=$(echo "wassup")

In this simple case of assigning a string to a variable, it's a "detour" to do this. To assign a string to a variable, you wou'd normally do:

Code:

STRING="wassup"

.

If one these doesn't do what you need for your specific purpose, please explain what it is you are trying to get done by the script.

kc8tbe · 07-29-2004, 07:48 AM

Thanks everyone! I had completely forgotten about the backtick (`). STRING = `echo "wassup"` does what I want it to.

For the curious, I'm writing a script to update /etc/hosts. Here's an excerpt:

Code:

#!/bin/sh
# Get an ip address for the domain name specified on the commandline.
IPADDR=`host $1 | egrep -o "(([0-9]{3}|[0-9]{2}|[0-9]{1})(\.?)){4}"`
# Do something useful with the $IPADDR (i.e. write it to a file).
# ...
# echo $IPADDR for the world to see,
echo $IPADDR