How to remove empty lines from begin and end of file?

Question

I would like to remove empty lines from begin and the end of file, but not remove empty lines between lines. I think sed or awk would be the solution.

Source:

1:
2:
3:line1
4:
5:line2
6:
7:
8:

Output:

1:line1
2:
3:line2

Answer 1

Try this,

To remove blank lines from the begin of a file:

sed -i '/./,$!d' filename

To remove blank lines from the end of a file:

sed -i -e :a -e '/^\n*$/{$d;N;ba' -e '}' file

To remove blank lines from begin and end of a file:

sed -i -e '/./,$!d' -e :a -e '/^\n*$/{$d;N;ba' -e '}' file

From man sed,

-e script, --expression=script -> add the script to the commands to be executed

b label -> Branch to label; if label is omitted, branch to end of script.

a -> Append text after a line (alternative syntax).

$ -> Match the last line.

n N -> Add a newline to the pattern space, then append the next line of input to the pattern space. If there is no more input then sed exits without processing any more commands.

Answer 2

This little awk program will remove empty lines at the start of a file:

awk 'NF {p=1} p'

So we can combine that with tac that reverses lines and get:

awk 'NF {p=1} p' file | tac | awk 'NF {p=1} p' | tac

line1

line2

---

Stealing @guillermo chamorro's command substitution trick:

awk 'NF {p=1} p' <<< "$(< file)"

Answer 3

If the file is small enough to fit memory requirements:

$ perl -0777 -pe 's/^\n+|\n\K\n+$//g' ip.txt
line1

line2

• -0777 to slurp entire input file
• ^\n+ one or more newlines from start of string
• \n\K to prevent deleting newline character of last non-empty line
• \n+$ one or more newlines at end of string

Answer 4

I proppose this:

printf '%s\n' "$(cat file)" | sed '/[a-z]/,$!d'

It will print the whole text except start-end blank lines. So, if we extend the example:

(blank)
(blank)
line1

line2
line1

line2
line1

line2
line1



line2
(blank)
(blank)

It will output:

line1

line2
line1

line2
line1

line2
line1




line2

Answer 5

A simple 2 pass approach just for completeness:

$ awk 'NR==FNR{if (NF) { if (!beg) beg=NR; end=NR } next} FNR>=beg && FNR<=end' file file
line1

line2

The above treats lines of only blank chars as empty. If instead you only want lines with no chars at all to be considered empty then just change NF to /./.

Answer 6

Not written the code, but there has to be an efficient algorithm for any size file along these lines.

(a) Read and ignore empty lines until the first non-empty.

(b) Read and print non-empty lines until the next empty.

(c) Count (n) the empty lines until the next non-empty.

(d) If you hit non-empty, print n newlines and you are back in state (b).

(e) If you hit EOF, you are done -- discarded n empty lines before EOF.

Answer 7

command

sed -n '/[a-zA-Z]/,/[a-zA-Z]/p' file| awk 'OFS=":"{$2=$1;$1=NR;print }'

output

1 line1
2 
3 line2