SED uncomment line - How to uncomment a line or all lines with SED

Sed is a very powerful linux command line tool. It is able to replace either static characters or based on regular expressions.

I will give here a few examples on how to uncomment all lines or specific line based on regular expression.

Sed uncomment all lines in a file:

Code:

user@linux:~# cat file.conf
line1
#line2
line3A
#setnence abc
sentence def
#sentence bcd
user@linux:~# sed 's/^#\(.*\)/\1/g' file.conf    
line1
line2
line3A
setnence abc
sentence def
sentence bcd

Sed uncomment specific line:

Code:

user@linux:~# cat file.conf
line1
#line2
line3A
#setnence abc
sentence def
#sentence bcd
user@linux:~# sed 's/^#line2/line2/g' file.conf             
line1
line2
line3A
#setnence abc
sentence def
#sentence bcd

Sed uncomment specific lines based on regular expression - uncomment lines containing bc combination:

Code:

user@linux:~# cat file.conf                     
line1
#line2
line3A
#setnence abc
sentence def
#sentence bcd
user@linux:~# sed 's/^#\(.*\)bc\(.*\)/\1bc\2/g' file.conf               
line1
#line2
line3A
setnence abc
sentence def
sentence bcd

All the above examples operate on a memory copy of the file, not on the file on the disk. Use "-i" switch of sed to uncomment lines in the file.

Quote:

-i[SUFFIX], --in-place[=SUFFIX]

edit files in place (makes backup if extension supplied)

Code:

user@linux:~# cat file.conf                     
line1
#line2
line3A
#setnence abc
sentence def
#sentence bcd
user@linux:~# sed -i 's/^#\(.*\)bc\(.*\)/\1bc\2/g' file.conf
user@linux:~# cat file.conf
line1
#line2
line3A
setnence abc
sentence def
sentence bcd

More on sed replacement based on regular expressions:
- \(.*\) represents a regular expression matching group for any characters zero or multiple times.
- \1 and \2 represent the first and second regex matching groups in the match criteria. They are used in the replacement part of sed command.