Category: Console

The three load-average values in the first line of top output are:

The three load-average values in the first line of top output are the 1-minute, 5-minute and 15-minute average. (These values also are displayed by other commands, such as uptime, not only top.) That means, reading from left to right, one can examine the aging trend and/or duration of the particular system state. The state in question is CPU load—not to be confused with CPU percentage. In fact, it is precisely the CPU load that is measured, because load averages do not include any processes or threads waiting on I/O, networking, databases or anything else not demanding the CPU. It narrowly focuses on what is actively demanding CPU time. This differs greatly from the CPU percentage. The CPU percentage is the amount of a time interval (that is, the sampling interval) that the system’s processes were found to be active on the CPU. If top reports that your program is taking 45% CPU, 45% of the samples taken by top found your process active on the CPU. The rest of the time your application was in a wait. (It is important to remember that a CPU is a discrete state machine. It really can be at only 100%, executing an instruction, or at 0%, waiting for something to do. There is no such thing as using 45% of a CPU. The CPU percentage is a function of time.) However, it is likely that your application’s rest periods include waiting to be dispatched on a CPU and not on external devices. That part of the wait percentage is then very relevant to understanding your overall CPU usage pattern.

Testing whether a string is null.

Testing whether a string is null.

#!/bin/bash
# str-test.sh: Testing null strings and unquoted strings,
#+ but not strings and sealing wax, not to mention cabbages and kings . . .

# Using if [ … ]

# If a string has not been initialized, it has no defined value.
# This state is called “null” (not the same as zero!).

if [ -n $string1 ] # string1 has not been declared or initialized.
then
echo “String “string1″ is not null.”
else
echo “String “string1″ is null.”
fi # Wrong result.
# Shows $string1 as not null, although it was not initialized.

echo
Continue reading “Testing whether a string is null.”

Strace – trace system calls and signals.

Strace is a useful diagnostic, instructional, and debugging tool. System administrators, diagnosticians and trouble-shooters will find it invaluable for solving problems with programs for which the source is not readily available since they do not need to be recompiled in order to trace them. Students, hackers and the overly-curious will find that a great deal can be learned about a system and its system calls by tracing even ordinary programs. And programmers will find that since system calls and signals are events that happen at the user/kernel interface, a close examination of this boundary is very useful for bug isolation, sanity checking and attempting to capture race conditions.
Debugging why some process isn’t connecting to a remote server can be exceedingly frustrating. DNS can fail, connect can hang, the server might send something unexpected back etc. You can use tcpdump to analyze a lot of that, and that too is a very nice tool, but a lot of the time strace will give you less chatter, simply because it will only ever return data related to the syscalls generated by “your” process. If you’re trying to figure out what one of hundreds of running processes connecting to the same database server does for example (where picking out the right connection with tcpdump is a nightmare), strace makes life a lot easier.
Continue reading “Strace – trace system calls and signals.”

Write ISO to USB stick.

After inserting your USB stick, you can find out what device it is using the following command:

grep -Ff <(hwinfo --disk --short) <(hwinfo --usb --short)

Finally, once you've found your block device, write the image to it. Point 'dd' to the full path such as '/home/user/Downloads/Linux-x86_64.iso'

umount /dev/sdX
dd if=/path/to/downloaded.iso of=/dev/sdX

Resizing images on Linux – ImageMagick convert command

With the resolution of today’s digital cameras it is often necessary to resize the files to make them smaller. You may want to reduce the file size before publishing them on the web, or sending in an email. There is however the convert command which is part of the ImageMagick suite. If you are running Linux then you most likely have that installed already, just man convert for more information. The most basic way to use convert is to give a file at a time on the command line:

convert -resize 50% original.jpg newsize.jpg

Alternatively you could replace the percentage with the actual size such as 640×480

convert -resize 640x480 original.jpg newsize.jpg

Where this really comes into it’s own is when you have a directory full of images that you want to convert. This can be used by using the xargs and find commands in conjunction with convert.

find . -iname "*.jpg" | xargs -l -i convert -resize 640x480 {} ../small/{}

How to use bind mounts in Linux.

Bind mounting is quite simple. Instead of mounting a block device into a particular path you are mounting one path into another path.

mount -o bind /home/andzia /ftp/andzia

You will now see this reflected when running the mount command:

mount | grep var
/home/andzia on /ftp/andzia type none (rw,bind)

If you want this to persist across reboots, you’ll just need to update your /etc/fstab with the bind mount as well.

/home/andzia /ftp/andzia none bind 0 0

Get a hardware information using a CLI.

Dmidecode is a tool for dumping a computer’s DMI (some say SMBIOS) table contents in a human-readable format. This table contains a description of the system’s hardware components, as well as other useful pieces of information such as serial numbers and BIOS revision. Thanks to this table, you can retrieve this information without having to probe for the actual hardware. While this is a good point in terms of report speed and safe‐ness, this also makes the presented information possibly unreliable.


dmidecode |grep -i product