Replace or append a line with string within a file

Question

I am looking for advice on how to search a part of a string within a file and replace the complete line OR append that string to that file if not found. I "played" with sed for a while now, but couldn't get it to work as expected.

I need to add:

/swapfile   none    swap    sw    0   0

to /etc/fstab (on Ubuntu 14.04 - Trusty Tahr).

Conditions:

• If any line starting with /swapfile is present in /etc/fstab, remove that line and replace with the string provided above
• If more than one line starting with /swapfile is found, remove them all and append the string above to the end of the file
• If no /swapfile is present in /etc/fstab, append the string to /etc/fstab
• The command must not show console output and must be a "one-liner" (due to automation purposes with puppet)

I am confident that's possible, but I simply didn't find a related tutorial about using sed in the way I need it.

I used sudo sed -i '$a/swapfile none swap sw 0 0' /etc/fstab but this only appends the string :(

Answer 1

You can do this with sed — it's Turing-complete. But it isn't the best tool for the job. Sed doesn't have a convenient way of remembering that it's already made a replacement.

What you can relatively easily do with sed is to blank all the lines starting /swapfile, and add a new one at the end:

sed -i '$! s/^\/swapfile[\t ]//; $s/\(^\/swapfile.*\)\?$/\n\/swapfile none swap sw/' /etc/fstab

but beyond that we're quickly getting into territory where I wouldn't leave such sed code for another sysadmin to maintain, especially when a simple, readable combination of shell commands would do a better job:

{ </etc/fstab grep -v '/swapfile[\t ]'; echo '/swapfile none swap sw'; } >/etc/fstab.new && mv /etc/fstab.new /etc/fstab

If you want to preserve the existing position of the /swapfile line if it's there and only modify the file if it needs modifying, a combination of shell logic and awk is a better tool. I've used multiple lines here for clarity but you can put all the code on the same line if you like. As a bonus, if the file already contained the intended line (with exact spacing), it won't be modified.

awk '
  /\/swapfile[\t ]/ {if (replaced) {next}
                     else {replaced=1; print "/swapfile none swap sw"}}
  1 {print}
  END {if (!replaced) print "/swapfile none swap sw"}
' /etc/fstab >/etc/fstab.new &&
if cmp -s /etc/fstab.new /etc/fstab.new; then
  rm /etc/fstab.new;
else
  mv /etc/fstab.new /etc/fstab;
fi

Answer 2

Since you are automating this with Puppet, it's better to let Puppet handle the fstab for you. Use the mount resource type.

Something like:

mount{'swapfile':
    name    => 'none',
    fstype  => 'swap',
    ensure  => mounted,
    atboot  => true,
    device  => '/swapfile',
    options => 'sw',
}

should work.

If this absolutely has to be done with a one-liner (which, for the record, I'm still against), the following perl should help:

perl -pi -e 'BEGIN{$string = "/swapfile   none    swap    sw    0   0"} s!^/swapfile.*!$string! && $x++;END{print "$string\n" unless $x}' /etc/fstab

Broken down into several lines for clarity:

perl -pi -e '
             BEGIN{$string = "/swapfile   none    swap    sw    0   0"}
             s!^/swapfile.*!$string! && $x++; # Modify line if it exists
             END{print "$string\n" unless $x} # Append it if it wasn't found
            ' /etc/fstab