bash script command output execution doesn't assign full output when using backticks

Question

I used many times [``] to capture output of command to a variable. but with following code i am not getting right output.

#!/bin/bash
export XLINE='($ZWP_SCRIP_NAME),$ZWP_LT_RSI_TRIGGER)R),$ZWP_RTIMER'
echo 'Original XLINE'
echo $XLINE 
echo '------------------'
echo 'Extract all word with $ZWP'
#works fine
echo $XLINE  | sed -e 's/\$/\n/g' | sed -e 's/.*\(ZWP[_A-Z]*\).*/\1/g'  | grep ZWP
echo '------------------'
echo 'Assign all word with $ZWP to XVAR'
#XVAR doesn't get all the values
export XVAR=`echo $XLINE  | sed -e 's/\$/\n/g' | sed -e 's/.*\(ZWP[_A-Z]*\).*/\1/g' | grep ZWP` #fails
echo "$XVAR"

and i get:

Original XLINE
($ZWP_SCRIP_NAME),$ZWP_LT_RSI_TRIGGER)R),$ZWP_RTIMER
------------------
Extract all word with $ZWP
ZWP_SCRIP_NAME
ZWP_LT_RSI_TRIGGER
ZWP_RTIMER
------------------
Assign all word with $ZWP to XVAR
ZWP_RTIMER

why XVAR doesn't get all the values?

however if i use $() to capture the out instead of ``, it works fine. but why `` is not working?

...and you don't need export either, unless you want your subprocesses to have access to your variables. And using all-caps names for your own variables contravenes POSIX convention (see fourth paragraph of pubs.opengroup.org/onlinepubs/009695399/basedefs/…) — Charles Duffy
– Charles Duffy, Commented Dec 18, 2015 at 18:05
BTW, sed -e 's/\$/\n/g' doesn't do what you think it does. Frankly, tr '$' '\n' would be a better alternative. — Charles Duffy
– Charles Duffy, Commented Dec 18, 2015 at 18:08
Several of the mistakes here are covered in the bash tag wiki. I suggest you review it. — tripleee
– tripleee, Commented Dec 18, 2015 at 18:10
The backticks should work the same as $(), but in strange ways they don't. Here is a simpler example showing a case where backticks don't work as expected: ubuntuforums.org/showthread.php?t=1293395 — Markku K.
– Markku K., Commented Dec 18, 2015 at 19:37

mklement0 · Accepted Answer · 2015-12-18 19:55:26Z

4

Having GNU grep you can use this command:

XVAR=$(grep -oP '\$\KZWP[A-Z_]+' <<< "$XLINE")

If you pass -P grep is using Perl compatible regular expressions. The key here is the \K escape sequence. Basically the regex matches $ZWP followed by one or more uppercase characters or underscores. The \K after the $ removes the $ itself from the match, while its presence is still required to match the whole pattern. Call it poor man's lookbehind if you want, I like it! :)

Btw, grep -o outputs every match on a single line instead of just printing the lines which match the pattern.

If you don't have GNU grep or you care about portability you can use awk, like this:

XVAR=$(awk -F'$' '{sub(/[^A-Z_].*/, "", $2); print $2}' RS=',' <<< "$XLINE")

edited Dec 18, 2015 at 19:55

mklement0

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answered Dec 18, 2015 at 18:11

hek2mgl

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7 Comments

Charles Duffy Over a year ago

What are you doing here that depends on -P? Strikes me as completely needless reliance on a newer version of GNU grep than otherwise would be necessary. (OTOH, if you used a negative lookbehind to exclude the $, that would make sense).

Charles Duffy Over a year ago

You're still using -o and -P, both of those GNUisms, in the latter form. Might at least change the latter to -E to support older GNU grep.

hek2mgl Over a year ago

@CharlesDuffy Oh, sorry. I just missed that.

Charles Duffy Over a year ago

Being able to backslash-escape ERE syntax (ie. \+) to use them in BREs is actually a nonportable extension too; better to use -E.

hek2mgl Over a year ago

Oh, never knew. But \{1,\} should be portable, isn't it?

|

Charles Duffy · Accepted Answer · 2015-12-18 18:18:31Z

1

First, the smallest change that makes your code "work":

echo "$XLINE" | tr '$' '\n' | sed -e 's/.*\(ZWP[_A-Z]*\).*/\1/g' | grep ZWP_

The use of tr replaces a sed expression that didn't actually do what you thought it did -- try looking at its output to see.

One sane alternative would be to rely on GNU grep's -o option. If you can't do that...

zwpvars=( )                                     # create a shell array
zwp_assignment_re='[$](ZWP_[[:alnum:]_]+)(.*)'  # ...and a regex
content="$XLINE"
while [[ $content =~ $zwp_assignment_re ]]; do
  zwpvars+=( "${BASH_REMATCH[1]}" )             # found a reference
  content=${BASH_REMATCH[2]}                    # stuff the remaining content aside
done

printf 'Found variable: %s\n' "${zwpvars[@]}"

edited Dec 18, 2015 at 18:18

answered Dec 18, 2015 at 18:09

Charles Duffy

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3 Comments

Markku K. Over a year ago

echo "$XLINE" | sed -e 's/\$/\n/g' seems to work OK for me ...replaces the '$' characters with newlines.

Charles Duffy Over a year ago

@MarkkuK., doesn't work for me -- maybe we were testing against different implementations of sed.

Charles Duffy Over a year ago

Excellent call about the backticks impacting backslash interpretation, btw.

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