sed: How to delete lines matching a pattern that c

Question

Suppose a file /etc/fstab contains the following:

/dev/xvda1 / ext4 defaults 1 1
/dev/md0    /mnt/ibsraid    xfs defaults,noatime    0   2
/mnt/ibsraid/varlog /var/log    none    bind    0   0
/dev/xvdb   None    auto    defaults,nobootwait 0   2

I want to delete the line starting with /dev/xvdb. So I tried:

$ sed '/^/dev/xvdb/d' /etc/fstab
sed: -e expression #1, char 5: extra characters after command
$ sed '?^/dev/xvdb?d' /etc/fstab
sed: -e expression #1, char 1: unknown command: `?'
$ sed '|^/dev/xvdb|d' /etc/fstab
sed: -e expression #1, char 1: unknown command: `|'

None of these worked. I tried changing the delimiters to ? and | because doing this works for the sed substitution command when a pattern contains /.

I am using GNU Sed 4.2.1 on Debian.

Answer 1

You were very close. When you use a nonstandard character for a pattern delimiter, such as |pattern|, the first use of that character must be escaped:

$ sed '\|^/dev/xvdb|d' /etc/fstab
/dev/xvda1 / ext4 defaults 1 1
/dev/md0    /mnt/ibsraid    xfs defaults,noatime    0   2
/mnt/ibsraid/varlog /var/log    none    bind    0   0

Similarly, one can use:

sed '\?^/dev/xvdb?d' /etc/fstab

Lastly, it is possible to use slashes inside of /pattern/ if they are escaped in the way that you showed in your answer.

Answer 2

Why the obsession with using forward slash as your delimiter? Just use something else, like a comma:

sed ',^/dev/xvdb,d' /etc/fstab

or a colon:

sed ':^/dev/xvdb:d' /etc/fstab

Or whatever makes it easiest to read. The delimiter can be any character. The convention is to use a forward slash, but when it becomes awkward, switch it to something else.

Note, if you want to change the file itself, rather than output the result, you need the "in place" flag -i:

sed -i ':^/dev/xvdb:d' /etc/fstab

Answer 3

After some digging, I found that it is possible to escape the / in the pattern string using \. So this works:

$ sed '/^\/dev\/xvdb/d' /etc/fstab