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Accepted
carillonator
Asked: 2012-03-30 15:53:37 +0800 CST

bash/sed/awk/etc remove every other newline

  • 772

a bash commands outputs this:

Runtime Name: vmhba2:C0:T3:L14
Group State: active
Runtime Name: vmhba3:C0:T0:L14
Group State: active unoptimized
Runtime Name: vmhba2:C0:T1:L14
Group State: active unoptimized
Runtime Name: vmhba3:C0:T3:L14
Group State: active
Runtime Name: vmhba2:C0:T2:L14
Group State: active

I'd like to pipe it to something to make it look like this:

Runtime Name: vmhba2:C0:T1:L14 Group State: active 
Runtime Name: vmhba3:C0:T3:L14 Group State: active unoptimized
Runtime Name: vmhba2:C0:T2:L14 Group State: active
[...]

i.e. remove every other newline

I tried ... |tr "\nGroup" " " but it removed all newlines and ate up some other letters as well. thanks

bash awk sed

8 Answers

  1. Best Answer

    can't test right now, but

    ... | paste - -

    should do it

    • 99

  2. One possibility is:

    awk 'ORS=NR%2?" ":"\n"'

    If the line number is evenly divisible by 2, end with a new line, otherwise, end with a space.

    (Tested on: CentOS 6, GNU Awk 3.1.7)

    Using sed (see explanation):

    sed ':a;N;$!ba;s/\nGroup/ Group/g'

    Further reading:

    • 12

  3. If you want to use sed, there's no reason to read the whole file into memory. You can merge every other line like this:

    sed 'N;s/\n/ /' inputfile

    Use any character you'd like instead of the space.

    Here's another way using awk:

    awk '{printf "%s", $0; if (getline) print " " $0; else printf "\n"}' inputfile

    The if/else handles the case where there is an odd number of lines in the file. Without it, the odd last line gets printed twice. Otherwise, for comparison, you could do:

    awk '{printf "%s", $0; getline; print " " $0}'
    • 9

  4. The most idiomatic way to do it in awk is as follows:

    awk 'ORS=NR%2?FS:RS' file

    It outputs:

    Runtime Name: vmhba2:C0:T3:L14 Group State: active
Runtime Name: vmhba3:C0:T0:L14 Group State: active unoptimized
Runtime Name: vmhba2:C0:T1:L14 Group State: active unoptimized
Runtime Name: vmhba3:C0:T3:L14 Group State: active
Runtime Name: vmhba2:C0:T2:L14 Group State: active

    To explain it we need to define each one of the built-in variables:

    • RS record separator. Defaults to \n (new line).
    • ORS output record separator. Defaults to \n (new line).
    • FS field separator. Defaults to (space).
    • NR number of record.

    As the default record separator is the new line, a record is, as default, a line.

    NR%2 is the modulus of NR/2, so that it will be either 0 or 1. 0 for even lines and 1 for odd lines.

    var=condition?condition_if_true:condition_if_false is the ternary operator.

    All together, saying ORS=NR%2?FS:RS we are defining the output record separator:

    • if the number of record is on the form 2k + 1, that is, on even lines, then the output record separators is set to FS, that is, a space.
    • if the number of record is on the form 2k, that is, on odd lines, then the output record separators is set to RS, that is, a new line.

    This way, odd lines end with a space, that is then joined with the next line. After that line, a new line is printed.

    More info in Idiomatic awk.

    • 4

  5. This works for me on Linux:

    ... | tr "\\n" " "

    This replaces an empty space for a newline character; you must escape the newline character for things to work correctly.

    • 3

  6. In bash:

    ... | while read l1; do read l2; echo "$l1 $l2"; done
    • 2

  7. If Perl is an option:

    perl -pe 's/\n/ / if $. % 2 == 1' file

    s/\n/ / replaces newline with space
    $. is the line number

    • 1

  8. How about using grep ?

    .. | grep -v '^Group State'
    • -2