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

So I'm going to borrow part of @dogbane's answer for this, since that sed line for removing the leading blank lines is so short...

tac is part of coreutils, and reverses a file. So do it twice:

tac file | sed -e '/./,$!d' | tac | sed -e '/./,$!d'

It's certainly not the most efficient, but unless you need efficiency, I find it more readable than everything else so far.

Answer 2

I'd like to introduce another variant for gawk v4.1+

result=($(gawk '
    BEGIN {
        lines_count         = 0;
        empty_lines_in_head = 0;
        empty_lines_in_tail = 0;
    }
    /[^[:space:]]/ {
        found_not_empty_line = 1;
        empty_lines_in_tail  = 0;
    }
    /^[[:space:]]*?$/ {
        if ( found_not_empty_line ) {
            empty_lines_in_tail ++;
        } else {
            empty_lines_in_head ++;
        }
    }
    {
        lines_count ++;
    }
    END {
        print (empty_lines_in_head " " empty_lines_in_tail " " lines_count);
    }
' "$file"))

empty_lines_in_head=${result[0]}
empty_lines_in_tail=${result[1]}
lines_count=${result[2]}

if [ $empty_lines_in_head -gt 0 ] || [ $empty_lines_in_tail -gt 0 ]; then
    echo "Removing whitespace from \"$file\""
    eval "gawk -i inplace '
        {
            if ( NR > $empty_lines_in_head && NR <= $(($lines_count - $empty_lines_in_tail)) ) {
                print
            }
        }
    ' \"$file\""
fi

Answer 3

In bash, using cat, wc, grep, sed, tail and head:

# number of first line that contains non-empty character
i=`grep -n "^[^\B*]" <your_file> | sed -e 's/:.*//' | head -1`
# number of hte last one
j=`grep -n "^[^\B*]" <your_file> | sed -e 's/:.*//' | tail -1`
# overall number of lines:
k=`cat <your_file> | wc -l`
# how much empty lines at the end of file we have?
m=$(($k-$j))
# let strip last m lines!
cat <your_file> | head -n-$m
# now we have to strip first i lines and we are done 8-)
cat <your_file> | tail -n+$i

Man, it's definitely worth to learn "real" programming language to avoid that ugliness!

Answer 4

This AWK script will do the trick:

BEGIN {
    ne=0;
}

/^[[:space:]]*$/ {
    ne++;
}

/[^[:space:]]+/ {
    for(i=0; i < ne; i++)
        print "";
    ne=0;
    print
}

The idea is simple: empty lines do not get echoed immediately. Instead, we wait till we get a non-empty line, and only then we first echo out as much empty lines as seen before it, and only then echo out the new non-empty line.