Conditionally remove lines with less than 4 characters

Question

I want to trim lines less than 4 characters, except if the line begins with # or !.

Sample input:

aa
bbb
 
   
dasasdsad
! f
#
!
# s
a
&B@*!

Output:

dasasdsad
! f
#
!
# s
&B@*!

Answer 1

With grep:

< file.in grep -E '^[#!]|.{4}' > file.out

That is, select lines that either start with # or ! or contain a sequence of 4 characters.

Or with awk:

< file.in awk '/^[#!]/ || length >= 4' > file.out

Or with sed:

< file.in sed -e '/^[#!]/b' -e '/.\{4\}/!d' > file.out

Answer 2

You can try something like this, code not tested:

while read line;
do
var=$(echo $line | awk '{ print length }')
if [ $var -le 4 ]; then
echo $line >> tmp.file
fi
done < your.file

This are 2 steps, while read, it writes the lines of less than 4 characters to tmp.file. With comm -23 your.file tmp.file you compare both files, suppress common lines and > you.new.file is the final saved version