df Command

Often you need a subset of all filesystem, for example just Oracle file systems.  Common convention is that Oracle filesystems begin with /u0. 

# df -k | grep /u0

/dev/vx/dsk/oradg/u02vol 12582912 8717924 3744252    70%    /u02
/dev/vx/dsk/oradg/u01vol  8796160 5563610 3131556    64%    /u01
/dev/vx/dsk/oradg/u04vol 10035200 1247888 8519534    13%    /u04
/dev/vx/dsk/oradg/u03vol 12582912 2524060 9744542    21%    /u03

we can use cut, Perl of awk to extract the fourth column, which is the available space in the filesystem.

$ df -k | grep /u0|awk '{ print $4 }'

3744252
3132546
8519534
9744542

Create the script to check space in all file systems

Now that we see the command, we can place this command inside a loop to evaluate the free space for each filesystem.  Note that our command is placed inside the Korn shell for loop, and the command is enclosed in back-ticks (the key immediately above the tab key).

check_filesystem_size.ksh
#!/bin/ksh

for i in `df -k | grep /u0 `
do
   # If any filesystem has less than 100k, issue an alert

   freespace=echo $i | awk '{ print $4 }'
   if [ $freespace -lt 100 ]
   then
      mail -s "On server $HOSTNAME filesystem $i has less then 100K of free space ($freespace)" joe.user@company.com < df -k | grep "/u0" 
   fi
done

We can execute this script each 10 minutes from crontab (Linux cron capability to specify increment is used):

#****************************************************************
# Filesystem free space below 100K alert
#****************************************************************
1/10 * * * * /home/oracle/check_filesystem_size.ksh > dev/null >&1

Old News

Checking Disk Performance and Disk Usage

Linux comes with the /sbin/hdparm program that can be used to control IDE or ATAPI hard drives that are common on most PCs. One feature of the hdparm program is to use the -t option to determine the rate at which data is read from the disk into a buffer in memory. For example, here's the result of typing /sbin/hdparm -t /dev/hda on one system:
/dev/hda:

/dev/hda:
 Timing buffered disk reads: 178 MB in  3.03 seconds = 58.81 MB/sec

The command requires the IDE drive's device name (/dev/hda for the first hard drive and /dev/hdb for the second hard drive) as an argument. If you have an IDE hard drive, you can try this command to see how fast data is read from your system's disk drive.
To display the space available in the currently mounted file systems, use the df command. If you want a more readable output from df, type the following command:

df -h

Here's a typical output from this command:

Filesystem            Size  Used Avail Use% Mounted on
/dev/hda5             7.1G  3.9G  2.9G  59% /
/dev/hda3              99M   18M   77M  19% /boot
none                  125M     0  125M   0% /dev/shm
/dev/scd0             2.6G  2.6G     0 100% /media/cdrecorder

As As this example shows, the -h option causes the df command to show the sizes in gigabytes (G) and megabytes (M).

To check the disk space being used by a specific directory, use the du command and specify the -h option to view the output in kilobytes (K) and megabytes (M), as shown in the following example:

du -h /var/log
Here's a typical output of that command:

152K    /var/log/cups
4.0K    /var/log/vbox
4.0K    /var/log/httpd
508K    /var/log/gdm
4.0K    /var/log/samba
8.0K    /var/log/mail
4.0K    /var/log/news/OLD
8.0K    /var/log/news
4.0K    /var/log/squid
2.2M    /var/log

The du command displays the disk space used by each directory, and the last line shows the total disk space used by that directory. If you want to see only the total space used by a directory, use the -s option. For example, type du -sh /home to see the space used by the /home directory. The command produces an output that looks like this:

89M