BASH: Trying to extract a field from a variable, and place the result in a new variable

fribse · 03-01-2019, 08:05 AM

Hi All

I'm surely missing something here, so please help me out :-)
It's CentOS 7, and a bash script.

I have an array. In that array the lines have fields seperated by tabs.
I want to extract the sixth field and place it in a variable for use in a grep, so I thought of doing this:

This command extracts the field:
echo "$i" | cut -f 6

But how do I place the field in a variable?

If I do
extract="${echo "$i" | cut -f 6}"
if doesn't work because of the quote's inside the quotes.
Doing
extract=${echo "$i" | cut -f 6}
Gives me an error with 'bad substition'.
I also tried escaping the 'inner' quotes, but it's the same reply.

So... What to do :-)

berndbausch · 03-01-2019, 08:26 AM

Quote:

Originally Posted by fribse

If I do
extract="${echo "$i" | cut -f 6}"
if doesn't work because of the quote's inside the quotes.
Doing
extract=${echo "$i" | cut -f 6}
Gives me an error with 'bad substition'.

You don't need quotes around the $i.

But your problem is somewhere else. In order to assign the output of a command to a variable, you need to use the $(...) construct, e.g.

Code:

extract=$(echo $i | cut -f 6)

BW-userx · 03-01-2019, 08:36 AM

cli agrees,

Code:

userx@slackcurdvd.net:~
$ i="123456 78910"
 
$ ex=$(echo "$i" | cut -f6)
 
$ echo $ex
123456 78910
 
$ ex=$(echo "$i" | cut -f 6)
 
$ echo $ex
123456 78910

you might also want to look up cut
https://en.wikibooks.org/wiki/Cut

to chop off up to the 6th

Code:

${string:position}
$ echo ${i:6}
78910

ondoho · 03-01-2019, 03:03 PM

Quote:

Originally Posted by fribse

I have an array. In that array the lines have fields seperated by tabs.
I want to extract the sixth field and place it in a variable

your example doesn't use arrays, just strings.

however, in bash you can use arrays!
which would make the assignment somewhat easier:

Code:

extract="${array[5]}"

yes, that's 5 for the sixth element because the indeces start at 0 (zero).

grail · 03-02-2019, 02:16 AM

To OP, you have provided your attempt, but not an example of the input and your desired output.

This would best help to assist you in a solution. As has been pointed out, you mention arrays but use none in your example, so you may also need to show how you are assigning
the original variable, "i" in your example.

notKlaatu · 03-02-2019, 02:29 AM

You can obtain an item from a Bash array like this:

echo ${i[5]}

The braces insulate the array from being treated as a plain old variable. The number in the brackets is the item number, starting at 0.

fribse · 03-05-2019, 02:02 AM

Thankyou all for the invaluable help, doh, mistypings with the $(, thankyou Berndbusch! My bad eyes couldn't see that in putty.
Doing the '$(i:6)' won't that cut of at the 6'th character, and not the 6'th word?
You are right that this line is not an array, but I have several lines in an array, and are going through the array, to extract the same positition from each line (thus emulating a two dimensional array).

fribse · 03-05-2019, 02:26 AM

The script is now:

Code:

SEARCHP=""
for i in "${hits_array[@]}"
do
   echo "$i"
   SEARCHP+=$(echo "$i" | cut -f 7 )
   done <<< $i

And that works!
Now on to a grep with multiple pattern or :-) (where this string will be used).

Thankyou again for the help!

pan64 · 03-05-2019, 03:42 AM

Quote:

Originally Posted by fribse

If I do
extract="${echo "$i" | cut -f 6}"
if doesn't work because of the quote's inside the quotes.

This work, if you use ( ) instead of { }.

Code:

extract="$(echo "$i" | cut -f 6)"

Quote:

Originally Posted by fribse

The script is now:

Code:

SEARCHP=""
for i in "${hits_array[@]}"
do
   echo "$i"
   SEARCHP+=$(echo "$i" | cut -f 7 )
   done <<< $i

here i used outside the for loop and also as cycle variable. That will may cause confusion later.

Also try to use shellcheck to check/improve your script.

fribse · 03-05-2019, 04:24 AM

So the script changed a bit

Code:

SEARCHP=""
SEPARATOR="|"
for i in "${hits_array[@]}"
do
   echo "$i"
   SEARCHP+=$(echo "$i" | cut -f 7 | sed 's/.$//')$SEPARATOR
   done <<< $i

# Clean up the output to a OR list to be used in grep
SEARCHP=$(echo $SEARCHP | sed 's/.$//' )

So now I've gotten the grep created, I have to use both an 'OR' and an 'AND' grep.

grep -E "$SEARCHP" /var/log/maillog | grep "DKIM-Signature field added" | awk 'BEGIN{FS=OFS=" ";}{print $6,$11;}' | sed 's/.$//'

The first is the OR part, it has | in between the patterns from the script, but I can't seem to find a way to combine it to an AND, other than doing the grep again, on the results.
The last bits just takes the words I'm interested in, and removes the paranthesis that's shown at the end of the second field shown.

Weehee, it's getting there :-)

fribse · 03-05-2019, 04:29 AM

Quote:

Originally Posted by pan64

Also try to use shellcheck to check/improve your script.

Great tip, thankyou!