setting a positional parameter in a function in bash

Question

I'm making a function to easly convert my strings to arrays as I need to.

I am somewhat running into a weird issue. I am still new to bash and this is really bugging me. Would anybody be able to shed some light onto this?

convert.sh

#!/bin/bash
convert2array () {
read -a $1_arr <<< $1
}

mx=$(dig +short google.com mx | cut -d' ' -f 2 | sed 's/\.$//')

convert2array "$mx"

echo ${mx_arr[@]}

Output:

bash -x convert2array.sh 
++ sed 's/\.$//'
++ cut '-d ' -f 2
++ dig +short google.com mx
+ mx='alt2.aspmx.l.google.com
alt3.aspmx.l.google.com
alt1.aspmx.l.google.com
aspmx.l.google.com
alt4.aspmx.l.google.com'
+ convert2array mx
+ read -a mx_arr
+ echo 585911
585911

mklement0 · Accepted Answer · 2016-12-19 04:57:40Z

Try the following:

convert2array () {
  # Bash v4+ alternative: `readarray -t` instead of `IFS=$'\n' read -d '' -ra`
  IFS=$'\n' read -d '' -ra "$1" <<<"$2"
}

mx=$(dig +short google.com mx | cut -d' ' -f 2 | sed 's/\.$//')

convert2array mx_arr "$mx"

printf '%s\n' "${mx_arr[@]}"

As for what you tried:

$1 inside convert2array is not the name of your input variable, $mx, but its value.
You need to pass in the name of the variable you want to declare (possibly after modifying the input name) explicitly, as a separate argument.
read by default only reads the 1st line of the input, whereas you're passing multiple lines.
-d '' makes read read all lines, and IFS=$'\n' makes read read each line as a whole.
In Bash v4+, using builtin readarray, IFS=$'\n' read -d '' -ra can be replaced with readarray -t.

A streamlined version of your command would be to read the dig ... output lines directly into an array, using the Bash v4+ readarray builtin:

readarray -t mx_arr < <(dig +short google.com mx | cut -d' ' -f 2 | sed 's/\.$//')

Bash v3.x alternative:

IFS=$'\n' read -d ''  -ra mx_arr < <(dig +short google.com mx | cut -d' ' -f 2 | sed 's/\.$//')

Community · Accepted Answer · 2017-05-23 12:17:05Z

2

You can directly store the dig results in the array

 declare -a results=( $( dig +short google.com mx | cut -d' ' -f 2 | sed 's/\.$//' ) )
 echo "${results[@]}"

Also, you need not use cut here, sed alone would suffice.

declare -a results=( $( dig +short google.com mx  | sed -E 's/^[[:digit:]]*[[:blank:]]*(.*)\.$/\1/') )
echo "${results[@]}"
aspmx.l.google.com
alt1.aspmx.l.google.com
alt2.aspmx.l.google.com
alt4.aspmx.l.google.com
alt3.aspmx.l.google.com

See [ bash arrays ],[ command subsctitution ] and [ positional parameters ].

^{You're warned : The output can be but in one format. Though ( $( .. ) ) is an anti-pattern as pointed out in comment#1 , for this case, it would suffice.}

edited May 23, 2017 at 12:17

CommunityBot

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answered Dec 19, 2016 at 4:19

sjsam

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5 Comments

mklement0 Over a year ago

By double-quoting the command substitution you're invariably creating an array with a single element containing all lines, which is not the intent. Without double-quoting, the output lines are invariably subject to word-splitting and globbing, and even though that happens to work in this case, ( $(...) ) is not a generally robust way to read command output into an array (unless you set $IFS and turn off globbing, which is not worth the effort).

sjsam Over a year ago

@mklement0 : Thanks for the pointer. I thought about it but then forgot to take the double quotes off in a rush to post an answer :) Changed now

sjsam Over a year ago

@mklement0 : Also, I agree to the second part of your comment

mklement0 Over a year ago

I'm glad to hear that, but I really wish you'd preface your solution with a prominent warning to that effect. I believe that ( $(...) ) is an anti-pattern that should not be promoted. It's seductively simple, but it will come back to bite you - see mywiki.wooledge.org/…

Мона_Сах Over a year ago

@mklement0 : Well said, reminds me of there are dark corners in the Bourne shell, and people use all of them- Chet Ramey

muru · Accepted Answer · 2016-12-19 04:46:47Z

1

Just a note on what went wrong in your function:

convert2array () {
read -a $1_arr <<< $1
}

Here, you're reading in the contents of $1, which is the name of the variable, when you want to read the contents of the variable itself. You can use indirection here:

convert2array () {
  read -a $1_arr <<< ${!1}
}

As others have noted, there are easier ways to get your output in arrays.

answered Dec 19, 2016 at 4:46

muru

4,9811 gold badge38 silver badges84 bronze badges

Comments

agc · Accepted Answer · 2016-12-19 05:49:54Z

Provided the array is only a one-off, for a read-only output loop, POSIX shell can do it:

set -- `dig +short google.com mx` ; \
while [ "$2" ] ; do echo "${2%.*}" ; shift 2 ; done

Output:

alt2.aspmx.l.google.com
alt3.aspmx.l.google.com
alt4.aspmx.l.google.com
alt1.aspmx.l.google.com
aspmx.l.google.com

If the output needs further fussing, pipe the echo to whatever else it is that needs doing.

Note: The above code contains a simple kludge to avoid sed and cut -- instead just output only the even number array members, and use parameter expansion "Remove Smallest Suffix Pattern" to remove that '.' at the end.

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