I use below command to filter the file for # and empty lines.
But, how can we grep uncommented and blank lines in linux with single grep.

[root@localhost ~]# cat test | grep -v ^# | grep -v ^$

For simple cases, my go-to pattern for this is:

$ egrep '^[^#]'

This pattern matches lines which begin with some character OTHER than a pound sign.

If you extend the definition of 'blank' line to include a line entirely of whitespace, then the pattern fails, as it will match such a line. The pattern also fails if you allow arbitrary whitespace before the pound sign in a comment line (as Apache, bash, and others do).

If those cases are important to you, this pattern is better:

$ egrep '^[[:blank:]]*[^[:blank:]#]'

For example:

$ cat test

# comment

  # spaces then comment

config # then comment

before empty line



after empty line

space only on next line





tab only on next line



$ egrep '^[[:blank:]]*[^[:blank:]#]' test

config # then comment

before empty line

after empty line

space only on next line

tab only on next line

$

IIUC, you want to show non-blank and non-commented lines. You can do that with -e using a single grep command:

grep -v -e "^#" -e "^$" test

For example, if your test looks like this:

#a

uncommented line



#comment below blank line

output will be:

$ grep -v -e "^#" -e "^$" test

uncommented line

consider the file:

valid config line 1

# Comment

valid config line 2 # Comment

Blank line between



These two

One space in the line between



These two

If you consider the cases:

• Lines starting with #


• Blank lines

You can use cat file | grep -v '^$|^#' or cat file | grep -v '^($|#)'

And you will get something like:

valid config line 1

valid config line 2 # Comment

Blank line between

These two

One space in the line between



These two

However, I would consider also lines that start with a config line and then have an inline comment (supported in several config files), lines that are not blank but have only spaces, for this in a single command, I would use sed:

cat file | sed '/^(#|[[:space:]]*$)/d;s/#.*//g'

Obtaining:

valid config line 1

valid config line 2 

Blank line between

These two

One space in the line between

These two

Explanation

• 
  [[:space:]]*$ matches 0 or more spaces before the end of the line


• 
  ^(a|b) matches lines starting with a or b, using # as a and [[:space:]]*$ as b will match all lines starting with #, blank lines, and lines that only have spaces.


• 
  /match/d deletes all the matching lines


• 
  ; separates sed commands


• 
  s/a/b/g replaces a with b globally. Using #.* as a and an empty b will remove all the matching comments after a line.

Hope it helps. Regards

Got it!

[root@localhost ~]# cat file | grep -v ^'$|#'