UNIX for Dummies Questions & Answers

Hey, guys!

Trying to research this is such a pain since the read command itself is a common word. Try searching "unix OR linux read command examples" or using the command substitution keyword.

So, I wanted to use a command statement similar to the following.

This is kinda taken from another question I posted recently about string manipulation and extracting portions of a string (in order to store to variable, via command substitution).

$ TEMPFILE=`ls -1 *31AUG2010.txt`
yields $TEMPFILE=filetransfers_31AUG2010.txt
$ read DD MM YY<enter>
> `echo ${TEMPFILE:14:2}` `echo ${TEMPFILE:16:3}` `echo ${TEMPFILE:19:4}`<enter>
which yields:
$DD=`echo
$MM=${TEMPFILE:14:2}`
$YY=`echo ${TEMPFILE:16:3}` `echo ${TEMPFILE:19:4}`

As you can see, I had intended for each variable to store the output from each command substitution set, but instead stored the literal values between each space encountered on the following input lines. BOOOOO!!

I don't even know if what I'm attempting to do is possible, I wanted to do this in a for loop, storing a new corresponding value for each variable per command set iteration.

Anybody have any experience with this, or how to get command substitution output to work with the read command?

I know that I could just do the command substitution independently for each variable in the loop, but now I'm just curious to know if it's possible to accomplish in one line using the read command.

This is just for my curiosity, but I'd appreciate if somebody could educate me on this matter.

Your read command is backwards -

Code:

echo "${TEMPFILE:14:2}  ${TEMPFILE:16:3} ${TEMPFILE:19:4}" | read a b c

IFS sets the record separator for a read, by default it is space(s). This means the first text block gets put in a, second in b, third in the c variable.

You can also create arrays like that:

Code:

#!/bin/ksh
set -A arr ${TEMPFILE:14:2}  ${TEMPFILE:16:3} ${TEMPFILE:19:4}
echo ${arr[0]}
echo ${arr[1]}
echo ${arr[2]}

bash uses declare -a arrayname value1 value2 ... valuen
plus you can assign values directly to each element of an array.

I was reading that because each segment of a pipeline is handled inna child shell of the parent, that setting multiple variables using output from preceding segments of a pipeline into a read command statement only stores the variables for the read arguments in the subshell, so the values assigned to the variables are not returned to the parent process.

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?

I was however able to successfully get read to take the space-delimted output from a command substitution statement, using a Here-Document.

See below.

This was done using bourne shell.

$ read YY MM DD << EOF
> `echo "${TEMPFILE:14:2} ${TEMPFILE:16:3} ${TEMPFILE:19:4}"`
> EOF
$ echo $YY $MM $DD
10 09 06


I hope this helps anybody else trying to accomplish the same thing. It worked as intended...eventually, after several attempts to play with damm I/O redirection with file descriptors and such. WHY CAN'TCHU JUST DO WHAT I WANT, UNIX?! YOU STUPID ROBOT!

~another rant brought to you by your friendly neighborhod programmar