How to UNCOMMENT a line that contains a specific string using Sed?

Question

Yes, to comment line containing specific string with sed, simply do:

sed -i '/<pattern>/s/^/#/g' file

And to uncomment it:

sed -i '/<pattern>/s/^#//g' file

In your case:

sed -i '/2001/s/^/#/g' file    (to comment out)
sed -i '/2001/s/^#//g' file    (to uncomment)

Option "g" at the end means global change. If you want to change only a single instance of pattern, just skip this.

Try this sed command,

sed -i '/^#.* 2001 /s/^#//' file

To complement @Avinash Raj's helpful answer with a more generic, POSIX-compliant solution.

• Toggles commenting of lines that match a specifiable string that must occur as a separate word anywhere on the line.
• The comment character (string) is also specifiable.

Note that the solution is awk-based, because a robust portable solution with sed is virtually impossible due to the limitations of POSIX' basic regular expressions.

awk -v commentId='#' -v word='2001' '
  $0 ~ "(^|[[:punct:][:space:]])" word "($|[[:punct:][:space:]])" { 
    if (match($0, "^[[:space:]]*" commentId))
      $0 = substr($0, RSTART + RLENGTH)
    else
      $0 = commentId $0
  } 
  { print }
  ' file > tmpfile.$$ && mv tmpfile.$$ file

• (^|[[:punct:][:space:]]) and ($|[[:punct:][:space:]]) are the POSIX extended regex equivalents of the \< and \> word-boundary assertions known from other regex dialects.
• Whitespace after the comment char is preserved, but not before it.
• When prepending the comment char to a line, it is directly prepended, without whitespace.
• Thus, if you only toggle comments with this solution, all whitespace is preserved.
• POSIX awk doesn't offer in-place updating (neither does POSIX sed, incidentally), hence the output is first captured in a temporary file and that file then replaces the original on success.

Quick example of how to comment and uncomment a line in a file.

Sample file :

umask 027
TMOUT=600

Lets now backup the file (just for laughs) and comment out and un comment:

# backup file (because we should always do this)
cp /etc/bash.bashrc /etc/bash.bashrc.$(date '+%Y-%m-%d,%H:%M:%S')

# original: TMOUT=600   , result :# TMOUT=600
sed -i '/[^#]/ s/\(^TMOUT=600.*$\)/#\ \1/' /etc/bash.bashrc

# original # TMOUT=600   ,result :TMOUT=600
sed -i '/^#.*TMOUT=600.*$/s/^#\ //' /etc/bash.bashrc