Linux - Check If There Is an Empty Line at The End of a File

Linux - check if there is an empty line at the end of a file

Olivier Pirson's answer is neater than the one I posted here originally (it also handles empty files correctly). I edited my solution to match his.

In bash:

newline_at_eof()
{
    if [[ -s "$1" && -z "$(tail -c 1 "$1")" ]]
    then
        echo "Newline at end of file!"
    else
        echo "No newline at end of file!"
    fi
}

As a shell script that you can call (paste it into a file, chmod +x <filename> to make it executable):

#!/bin/bash
if [[ -s "$1" && -z "$(tail -c 1 "$1")" ]]
then
    echo "Newline at end of file!"
else
    echo "No newline at end of file!"
fi

How to detect file ends in newline?

@Konrad: tail does not return an empty line. I made a file that has some text that doesn't end in newline and a file that does. Here is the output from tail:

$ cat test_no_newline.txt
this file doesn't end in newline$ 

$ cat test_with_newline.txt
this file ends in newline
$

Though I found that tail has get last byte option. So I modified your script to:

#!/bin/sh
c=`tail -c 1 $1`
if [ "$c" != "" ]; then
    echo "no newline"
fi

Why is it recommended to have empty line in the end of a source file?

Many older tools misbehave if the last line of data in a text file is not terminated with a newline or carriage return / new line combination. They ignore that line as it is terminated with ^Z (eof) instead.

How can I test if line is empty in shell script?

Since read reads whitespace-delimited fields by default, a line containing only whitespace should result in the empty string being assigned to the variable, so you should be able to skip empty lines with just:

[ -z "$line" ] && continue

How do I get the last non-empty line of a file using tail in Bash?

You can use Awk:

awk '/./{line=$0} END{print line}' my_file.txt

This solution has the advantage of using just one tool.

How to add an empty line at the end of these commands?

So how can I add a blank line to the end of the file within the
scripts that convert fastq to fasta?

I would use GNU sed following replace

cat $1 >> final.fasta

using

sed '$a\\n' $1 >> final.fasta

Explanation: meaning of expression for sed is at last line ($) append newline (\n) - this action is undertaken before default one of printing. If you prefer GNU AWK then you might same behavior following way

awk '{print}END{print ""}' $1 >> final.fasta

Note: I was unable to test any of solution as you doesnot provide enough information to this. I assume above line is somewhere inside loop and $1 is always name of file existing in current working directory.

to check if a line before and after a string empty

Could you please try following if you are ok with awk.

awk -v string="Hello" '
FNR==NR{
  a[FNR]=$0
  next
}
($0==string) && a[FNR-1]=="" && a[FNR+1]==""{
  a[FNR-1]=a[FNR]=a[FNR-2]="del_flag"
}
END{
  for(i=1;i<=length(a);i++){
    if(a[i]!="del_flag"){
      print a[i]
    }
  }
}
'  Input_file  Input_file

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