Re: [PLUG] Bash question

[JP Vossen <jp@jpsdomain.org>]

On 05/29/2016 09:22 PM, Christopher Barry wrote:
> On Sun, 29 May 2016 17:26:58 +0000 (UTC)
> Gregory Deal <1deal@comcast.net> wrote:
>
> > Even after checking the "learning bash" book and searching, I'm having
> > a little trouble explaining some bash functioning. I'm trying to
> > process some command output, making an array of its output lines. I
> > was trying to check for the "no output" case (which goes right back to
> > a prompt) by interrogating the final array length. But even with no
> > command output, I get a length of 1. Could someone explain this?
> > Thanks.
> >
> > Script:
> >
> > #!/bin/bash
> >
> > # DTD specifies 0 or 1 <rules>, so check if any before processing.
> > echo "Rules content:"
> > xmlpathval.py ~/program1/sample2.xml "/sync/rules[1]/text()" |
> > while IFS= read -r line; do
> > echo "-->'$line'"
> > done
> > echo "IFS='$IFS'"
> >
> > rulesInfo=$(xmlpathval.py ~/program1/sample2.xml
> > "/sync/rules[1]/text()") <<<<<<< raw output shown below if [[ -z
> > $rulesInfo ]]; then echo "No rules found."
> > fi
> > echo "rulesInfo='$rulesInfo'"
> >
> > IFS=
> > unset rulesArray
> > readarray -t rulesArray <<< "$rulesInfo"
> > echo "length=${#rulesArray[@]}"
> > if [[ ${#rulesArray[@]} -eq 0 ]]; then
> > echo "empty"
> > else
> > echo "not empty"
> > fi
> > echo "rulesArray(single echo)=${rulesArray[@]}"
> > echo rulesArray from loop=
> > for element in "${rulesArray[@]}"
> > do
> > echo "'$element'"
> > done
> >
> > Input:
> >
> > gkd@sisko:~/program1$ xmlpathval.py sample2.xml
> > "/sync/rules[1]/text()" gkd@sisko:~/program1$ echo $?
> > 0
> >
> > Results :
> >
> > gkd@sisko:~/scripts$ ./test.sh
> > Rules content:
> > IFS='
> > '
> > No rules found.
> > rulesInfo=''
> > length=1
> > not empty
> > rulesArray(single echo)=
> > rulesArray from loop=
> > ''
> > gkd@sisko:~/scripts$
>
> t=( '' )
> echo ${#t[@]}
> 1
>
> yet,
>
> [[ ${t[@]} ]] && echo yes
> <no output>
>
> therefore,
>
> test for an actual value in ${rulesArray[@]} as well
>
> (( ${#rulesArray[0]} )) && [[ "${rulesArray[@]}" ]] && do stuff


May be useful:
* http://www.gnu.org/software/bash/manual/html_node/Arrays.html
* http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_10_02.html
* http://www.thegeekstuff.com/2010/06/bash-array-tutorial/

As shown in your `echo "length=${#rulesArray[@]}"` you are getting 
*something* into that array.  I'd try `echo "length=${#rulesArray[@]}" | 
hexdump -C` to see what it is, though I suspect a trailing "\n" is 
creating an empty element.

That said, you could probably use something like this to test:
	if [ "${#rulesArray[@]}" -eq 0 -a -n "${rulesArray[@]}" ]; then
Or even simpler
	if [ -n "${rulesArray[@]}" ]; then

If there is a first element that is not empty...well then you are "not 
empty".  See `help test` for the meanings of "-a", "-n", etc.  (`help` 
is "man" for bash internal commands. :)

I have to admit I find bash arrays very confusing because of all the 
"random" punctuation.  I want to add a bunch more on that stuff if/when 
we ever get around to the 2nd edition of the _bash Cookbook_.

You might also look at `help read` and the "-a array" option.

Later,
JP
--  -------------------------------------------------------------------
JP Vossen, CISSP | http://www.jpsdomain.org/ | http://bashcookbook.com/
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