remount read-only / to as read/write

mount -o remount,rw /

FFS or BSD partition mounting in Linux

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Null Mount or Bind Mount (Mount a folder in linux)

mount --bind /usr/u01 /u01
mount --bind /usr/u02 /u02

in /etc/fstab

/usr/u01        /u01    auto            bind                    0       0
/usr/u02        /u02    auto            bind                    0       0

Note that when mount command is used usually df won't show the mounted null partitions. But when fstab is modified and mount -a is used, df will show the newly mounted partitions.

df -h
/usr/u01               25G  274M   25G   2% /u01
/usr/u02               25G  274M   25G   2% /u02