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I want to get an alert when my disk is full (parsing df output).

Sadly, parsing the output of df really is the most reliable way to determine how full a disk is, on most operating systems. However, please note that this is a "least bad" answer, not a "best" answer. Parsing any command-line reporting tool's output in a program is never pretty. The purpose of this FAQ is to try to describe all the problems this approach is known to encounter, and work around them.

The first, biggest problem with df is that it doesn't work the same way on all operating systems. Unix is divided largely into two families -- System V and BSD. On BSD-like systems (including Linux, in this case), df gives a human-readable report:

However, on System-V-like systems, the output is completely different:

So, your first obstacle will be recognizing that you may need to use a different command depending on which OS you're on (e.g. bdf on HP-UX); and that there may be some OSes where it's simply not possible to do this with a shell script at all.

For the rest of this article, we'll assume that you've got a system with a BSD-like df command.

The next problem is that the output format of df is not consistent across platforms. Some plaforms use 6 columns of output. Some use 7. Some platforms (like Linux) use 1-kilobyte blocks by default when reporting the actual space used or available; others, like OpenBSD or IRIX, use 512-byte blocks by default, and need a -k switch to use kilobytes.

Worse, often a line of output will be split into multiple lines on the screen. For example (Linux):

If the device name is sufficiently long (very common with network-mounted file systems), df may split the output onto two lines in an attempt to preserve the columns for human readability. Or it may not... see, for example, OpenBSD 4.3:

Most versions of df give you a -P switch which is intended to standardize the output... sort of. Older versions of OpenBSD still split lines of output even when -P is supplied, but Linux will generally force the output for each file system onto a single line.

Therefore, if you want to write something robust, you can't assume the output for a given file system will be on a single line. We'll get back to that later.

You can't assume the columns line up vertically, either:

So, what can you actually do?

Removing the % sign, comparing the number to a specified threshold, scheduling an automatic way to run the script, etc. are left as exercises for you.

2012-07-01 04:05