Sed Text Processing – Sed Deletes All Lines Instead of Selected Line

sedtext processing

I want to remove one Line from /etc/fstab and use the sed command for that, but it removes everything within my file.

My fstab:

/dev/mapper/vg00-lv_root /                       xfs     defaults        0 0
/dev/mapper/vg00-lv_backup /backup                 xfs     defaults        0 0
UUID=30d58f5e-a432-4739-98f9-3f2ceb66fc03 /boot                   ext4    defaults        1 2
/dev/mapper/vg00-lv_wsbackup /wsbackup               xfs     defaults        0 0
UUID=a59bad9c-8c89-4b20-aadd-e83c4ab22a6c swap                    swap    defaults        0 0
10.10.10.10 /mnt/wsbackup cifs user=user,password=password,domain=domain,vers=2.1 0 0

My script:

MNTDIR="/mnt/wsbackup"
if grep -q "$MNTDIR" "/etc/fstab"
then
 sed -i -n "\:$MNTDIR:d" /etc/fstab
else
 ...
fi

If I use the command sed "\:$MNTDIR:d" /etc/fstab within my script the output I get looks like I want it to:

[root@labor-ssd ~]# sh check.sh
/dev/mapper/vg00-lv_root /                       xfs     defaults        0 0
/dev/mapper/vg00-lv_backup /backup                 xfs     defaults        0 0
#/dev/sdb /backup       ext4 defaults   0 0
UUID=30d58f5e-a432-4739-98f9-3f2ceb66fc03 /boot                   ext4    defaults        1 2
/dev/mapper/vg00-lv_wsbackup /wsbackup               xfs     defaults        0 0
UUID=a59bad9c-8c89-4b20-aadd-e83c4ab22a6c swap                    swap    defaults        0 0
INFO: user mount configuration removed

But the changes are not written to the file so I use the -i option to write the changes to the file. The file is completely empty afterwards and I don't know why.

Best Answer

The /etc/fstab file becomes empty because you're not only using -i but also -n.

The -n option turns off the "default p (print) command" which is usually triggered at the end of each cycle.

With -n, your script truly outputs nothing, not even if you remove the -i option, because there is no p command in the script, not even the default p at the end of the cycle, which would have printed all lines that weren't deleted. Since the sed script outputs nothing, telling sed to do the changes in-place with -i empties the file.

So, to resolve your issue, remove -n from your command.

Personally, I would have written the code as

mntdir=/mnt/wsbackup

cp /etc/fstab /etc/fstab.orig
sed "\\:$mntdir:d" /etc/fstab.orig >/etc/fstab

This allows us to keep a copy of the original fstab file.

Alternatively,

mntdir=/mnt/wsbackup

sed -i.orig "\\:$mntdir:d" /etc/fstab

which would do pretty much the same thing, depending on what sed implementation you use.

Wrap that in a test on grep -q -F -e "$mntdir" /etc/fstab (similarly to what you did already) if you need to avoid doing anything to the file if the $mntdir string is not found in it, i.e.

mntdir=/mnt/wsbackup

grep -q -F -e "$mntdir" /etc/fstab &&
sed -i.orig "\\:$mntdir:d" /etc/fstab
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