newline character in POSIX shell

Question

I'm parsing output of avahi-browse tool and my script should be POSIX compatible.

I'm doing it next way:

      local _dnssd=`avahi-browse -apt`
      if [ -z "$_dnssd" ]; then
        echo "No info"
      else
        IFS='
    ' # it's new line character in IFS
        for _row in $_dnssd
         do  
          local _tmpIFP="$IFS"
          IFS=";"
            case "$_row" in
            ...
            esac
            IFS="$_tmpIFS"
        done
      fi

I really don't like line with newline assignment to IFS. Is it possible to replace it in better way?

I tried some suggestions from stackoverflow, but it doesn't work:

IFS=$(echo -e '\n')

avahi-browse output:

+;br0;IPv4;switch4B66E4;_http._tcp;local
+;br0;IPv4;switch4B66E4;_csco-sb._tcp;local

fedorqui, I can't use any bashism. It would be executed on POSIX shell. — Jurasic, Feb 19 '14 at 13:28

score 6 · Accepted Answer · answered Feb 19 '14 at 14:10

6

Add a space after \n in the IFS variable, then remove that space again:

IFS="$(printf '\n ')" && IFS="${IFS% }"
#IFS="$(printf '\n ')" && IFS="${IFS%?}"
printf '%s' "$IFS" | od -A n -c

answered Feb 19 '14 at 14:10

larz

76
1

chepner · Answer 2 · 2014-02-19T15:47:17.980

It's better to use a while loop than trying to iterate over a string that contains the entire output.

avahi-browse -apt | while IFS=";" read field1 field2 ...; do
    case ... in
    ...
    esac
done

Note you should need one name per field for the read command. The ... is just a placeholder, not valid shell syntax for a variable number of fields.

This simply does nothing if the program produces no output. If you really need to detect that case, try

avahi-browse -apt | {
    read line || { echo "No info"; exit; }
    while : ; do
        IFS=";" read field1 field2 ... <<EOF
        $line
EOF
        case ... in
        ...
        esac
        read line || break
    done
}

In both cases, any variables set in the right-hand side of the pipe are local to that shell. If you need to set variables for later use, you'll need to make some further adjustments.

what if field could have spaces like =;eth0;IPv4;\039s\032remote\032desktop\032on\032O;VNC Remote Access;local;O.local;192.168.26.19;5900. After IFS=";" it would be impossible to detect whole field. — Jurasic, Feb 19 '14 at 15:42
By setting `IFS` to a semicolon, the spaces are treated as part of a field, rather than as a field separator. — chepner, Feb 19 '14 at 15:46

linuxball · Answer 3 · 2019-04-02T08:41:26.620

If one can rely* that IFS has its default value (space, tab, new line) then one can simply strip the first two characters (space and tab) and the new line character will remain:

IFS=${IFS#??}

*You can rely on it if IFS has not been modified by the script before and if it is a POSIX shell (as the topic implies):

The shell shall set IFS to <space><tab><newline> when it is invoked.

See http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_05_03

newline character in POSIX shell

3 Answers3