Parsing multi-line variables using grep

Question

I am trying to figure out why this does not work and of course how to address it, I have a long list of dates in a variable and would like to count the number of occurrences using grep, it seems like splitting a variable over new lines does not work as expected? Example,

$ list="2015-a 2015-b 2016-a" ; count=`echo $list | tr " " \\n | grep 2015 | wc -l` ; echo $count
1


$ list="2015-a,2015-b,2016-a" ; count=`echo $list | tr , \\n | grep 2015 | wc -l` ; echo $count
1


$ list="2015-a,2015-b,2016-a" ; count=`echo $list | sed s/,/\\n/g | grep 2015 | wc -l` ; echo $count
1

Any ideas?

Answer 1

The problem is with the way backticks interpret \\\\ :

Backslashes () inside backticks are handled in a non-obvious manner:

  $ echo "`echo \\\\a`" "$(echo \\\\a)" a \\a $ echo "`echo \\\\\\\\a`" "$(echo \\\\\\\\a)" \\a \\\\a # Note that this is true for *single quotes* too! $ foo=`echo '\\\\'`; bar=$(echo '\\\\'); echo "foo is $foo, bar is $bar" foo is \\, bar is \\\\ 

So instead of saying:

$ echo "`echo $list | tr " " \\n`"
2015-an2015-bn2016-a

You have to say:

$ echo "`echo $list | tr " " \\\\n`"
2015-a
2015-b
2016-a

Even though it is best to use $() because backticks are deprecated:

$ echo "$(echo $list | tr " " '\n')"
2015-a
2015-b
2016-a

If you still want to use backticks, the cleanest solution is to use " " as a wrapper instead of escaping with such a \\\\\\\\ :

$  echo "`echo $list | tr " " "\n"`"
2015-a
2015-b
2016-a

All of this can be read in Why is $(...) preferred over ... (backticks)? .

---

All in all, if you just want to count how many words contain 2015 you may consider using grep -o as suggested in the comments or maybe something more robust like this awk :

awk '{for (i=1;i<=NF;i++) if ($i~2015) count++; print count}'

See some examples:

$ awk '{for (i=1;i<=NF;i++) if ($i~2015) s++; print s}' <<< "2015-a 2015-b 2016-a"               2
$ awk '{for (i=1;i<=NF;i++) if ($i~2015) s++; print s}' <<< "2015-a 2015-b 2016-a 20152015-c"
3