Removing trailing / starting newlines with sed, awk, tr, and friends

Question

I would like to remove all of the empty lines from a file, but only when they are at the end/start of a file (that is, if there are no non-empty lines before them, at the start;

Answer 1

perl -0pe 's/^\n+|\n+(\n)$/\1/gs'

Answer 2

Here's an adapted sed version, which also considers "empty" those lines with just spaces and tabs on it.

sed -e :a -e '/[^[:blank:]]/,$!d; /^[[:space:]]*$/{ $d; N; ba' -e '}'

It's basically the accepted answer version (considering BryanH comment), but the dot . in the first command was changed to [^[:blank:]] (anything not blank) and the \n inside the second command address was changed to [[:space:]] to allow newlines, spaces an tabs.

An alternative version, without using the POSIX classes, but your sed must support inserting \t and \n inside […]. GNU sed does, BSD sed doesn't.

sed -e :a -e '/[^\t ]/,$!d; /^[\n\t ]*$/{ $d; N; ba' -e '}'

Testing:

prompt$ printf '\n \t \n\nfoo\n\nfoo\n\n \t \n\n' 



foo

foo



prompt$ printf '\n \t \n\nfoo\n\nfoo\n\n \t \n\n' | sed -n l
$
 \t $
$
foo$
$
foo$
$
 \t $
$
prompt$ printf '\n \t \n\nfoo\n\nfoo\n\n \t \n\n' | sed -e :a -e '/[^[:blank:]]/,$!d; /^[[:space:]]*$/{ $d; N; ba' -e '}'
foo

foo
prompt$

Answer 3

@dogbane has a nice simple answer for removing leading empty lines. Here's a simple awk command which removes just the trailing lines. Use this with @dogbane's sed command to remove both leading and trailing blanks.

awk '{ LINES=LINES $0 "\n"; } /./ { printf "%s", LINES; LINES=""; }'

This is pretty simple in operation.

• Add every line to a buffer as we read it.
• For every line which contains a character, print the contents of the buffer and then clear it.

So the only things that get buffered and never displayed are any trailing blanks.

I used printf instead of print to avoid the automatic addition of a newline, since I'm using newlines to separate the lines in the buffer already.

Answer 4

using awk:

awk '{a[NR]=$0;if($0 && !s)s=NR;}
    END{e=NR;
        for(i=NR;i>1;i--) 
            if(a[i]){ e=i; break; } 
        for(i=s;i<=e;i++)
            print a[i];}' yourFile

Answer 5

this can be solved easily with sed -z option

sed -rz 's/^\n+//; s/\n+$/\n/g' file
Hello

Welcome to
Unix and Linux

Answer 6

here's a one-pass solution in awk: it does not start printing until it sees a non-empty line and when it sees an empty line, it remembers it until the next non-empty line

awk '
    /[[:graph:]]/ {
        # a non-empty line
        # set the flag to begin printing lines
        p=1      
        # print the accumulated "interior" empty lines 
        for (i=1; i<=n; i++) print ""
        n=0
        # then print this line
        print
    }
    p && /^[[:space:]]*$/ {
        # a potentially "interior" empty line. remember it.
        n++
    }
' filename

Note, due to the mechanism I'm using to consider empty/non-empty lines (with [[:graph:]] and /^[[:space:]]*$/), interior lines with only whitespace will be truncated to become truly empty.