Command aliases in Linux and UNIX

Aliases can be used to perform complex commands but are set in the shell like environmental variables.

If you find yourself frequently using a fairly complicated command like the one below and want to be able to run it without typing the whole thing you can use the alias command to set up a shortcut.

$ alias alert=’tail -200 /u01/installed/software/mypgm/alert.log|more’

Now you can execute this whole command simply by typing alert. Many of the features of shell scripting are available in aliases including the pipe (|) used to send the output of one command to the input of another.

If you would like to have an alias automatically set up every time you enter a shell you can add the alias definition to your .profile file (or whatever file your default shell executes at login.)

A small set of well written aliases can save you a lot of keystrokes, just be careful not to make aliases with the same name as commands you use unless you want the alias to replace that command when you type it.

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linux, unix, system administration, sysadmin

Astroglide leak

Here’s a slippery situation I’m glad I’m not in… Don Burleson points out an article on Homeland Stupidity about a security breach from the Astroglide folks.

The major personal lubricant company accidentally exposed thousands of customers personal information on the company’s website, some of whom only signed up for a free sample. To compound the problem the data was cached on Google even after it was taken down from the Astroglide website.

While it would be easy to become desensitized to all these data breaches, but they’re a good reminder to take a look at security at all levels.

information technology, identity theft

Viewing strings of text in binary files on Linux and UNIX

If a user encounters a binary file and does not know what it is used for or where it came from, they may gain some insight into its origins and use by searching for character strings within the file. If the cat command is used to view a binary file, the user will get a screen full of garbage that will more often than not change the display characteristics. Instead, the strings command should be used, as demonstrated in the following examples:

Find All Strings in the Binary File

$ strings echo
/lib/ld-linux.so.2
libc.so.6
stdout
getopt_long
__fpending
getenv
...
Copyright (C) 2002 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
%s (%s) %s
Written by %s.
%s %s
memory exhausted

Again the above output has been abbreviated to save space, but you can see that there is some useful information here. Just knowing that “This is free software” and that it is copyrighted by the Free Software Foundation can give you some great insight on where this came from and why it might be there.

Finding Occurrences of a String in a Binary File

Here we show how the output of the strings command can be piped into the grep command to look for specific words within a binary file.

$ strings echo|grep GLIBC
GLIBC_2.3
GLIBC_2.1.3
GLIBC_2.1
GLIBC_2.0
GLIBC_2.2

This shows how grep can be used to limit the output of a command to only lines that contain certain text.

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unix, linux, text, search, find

More Linux and UNIX find command options

Like many Linux and UNIX commands, the find command supports a long list of options. I covered several of the options in a previous article but there are even more useful options. Here are some of them:

Doing things with what we find

The –exec option gives find the powerful ability to execute commands on the files found. The syntax is a little tricky but an example is usually all it takes to get it right.

Before using the -exec option, especially with a powerful command like rm I recommend performing the same find without the –exec. By doing this you will see exactly which files you will be affecting when you run the final command.

The following is a practical example that finds files less than three days old with the .txt extension and deletes them.

Finding .txt Files < 3 Days Old and Delete Them

$ find . -name '*.txt' -mtime -3 -exec rm {} \;
$ ls –lt
total 8
-rw-r--r-- 1 tclark authors 2229 Jan 13 21:35 declaration.txt
-rw-rw-r-- 1 tclark presidents 1310 Jan 13 17:48 gettysburg.txt

The –exec option allows you to put any command after it. Here we have used rm but it is often useful to use this option with cp or chmod. Within the command to be run there must be two curly brackets {}. find will execute the command for each file it finds substituting the file name (and path) where the curly brackets are. Finally the end of the –exec option is signaled by an escaped semicolon (\;). The –exec option should always be the last option given in a find command.

The find command is great for finding files and directories but next we’ll look at some options for finding other things on the system.

Dealing with “Permission denied” in find

If you use find a lot (and you probably will) you will sometimes run into the problem where you get just pages and pages of output like this:

$ find / -name '*.txt'
find: /var/lib/dav: Permission denied
find: /var/lib/nfs/statd: Permission denied
find: /var/lib/dhcpv6: Permission denied
find: /var/lib/slocate: Permission denied
find: /var/lib/xdm/authdir: Permission denied
find: /var/lib/php/session: Permission denied
find: /var/log/samba: Permission denied
find: /var/log/ppp: Permission denied
find: /var/log/audit: Permission denied
find: /var/log/squid: Permission denied
...

This is find telling you there are certain directories you don’t have permissions to search. This can make it very difficult to find the useful output of the find as it can be mixed in with the permissions errors.

To ignore these (and any other) errors and just get the results of what you can find we can use a special redirect at the end of the command. Redirecting output will be covered in more detail in the chapter on shell scripting, but suffice it to say that in this command 2>/dev/null is redirecting the error output to nowhere.

$ find / -name '*.txt' 2>/dev/null
/var/www/icons/small/README.txt
/usr/X11R6/lib/X11/rgb.txt
/usr/X11R6/lib/X11/doc/Xprint_FAQ.txt
/usr/lib/4Suite/tests/Xml/Core/count.txt
...

While it would not be a good idea to redirect the error output all the time (usually you want to know when something has gone wrong) in this case of the find command it can be very useful.

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find, search, unix, linux, system administration, sysadmin

Solaris prtdiag output

Vikrant posted a comment on my previous post on getting hardware information in Solaris asking if I could explain the output of the prtdiag command in Solaris. Unfortunately the output varies quite a bit depending on what hardware you have, but here’s the output from my Ultra10.

$ /usr/platform/`uname -i`/sbin/prtdiag
System Configuration: Sun Microsystems sun4u Sun Ultra 5/10 UPA/PCI (UltraSPARC-IIi 440MHz)
System clock frequency: 110 MHz
Memory size: 1024 Megabytes

========================= CPUs =========================

Run Ecache CPU CPU
Brd CPU Module MHz MB Impl. Mask
--- --- ------- ----- ------ ------ ----
0 0 0 440 2.0 12 9.1

========================= IO Cards =========================

Bus# Freq
Brd Type MHz Slot Name Model
--- ---- ---- ---- -------------------------------- ----------------------
0 PCI-1 33 1 ebus
0 PCI-1 33 1 network-SUNW,hme
0 PCI-1 33 2 SUNW,m64B ATY,GT-C
0 PCI-1 33 3 ide-pci1095,646
0 PCI-2 33 2 pci108e,1000-pci108e,1000
0 PCI-2 33 2 SUNW,hme-pci108e,1001 SUNW,qsi-cheerio

No failures found in System
===========================

The “System Configuration” line shows vendor and model information as well as the processor version and speed. “System clock frequency” is the bus speed on the motherboard of the system. The processor speed is typically a multiple of the clock frequency.

The “Memory size” shows the total memory in the system. On most server-class systems there is additional output to show what size memory modules are in each slot in the system. This can be very useful for determining if memory can be added or if it will need to go in place of existing chips.

The “CPU” section has detailed information on each processor in the system. Again, this is far more interesting in a larger, multi-processor system. All the processors in a machine should have identical information. I don’t believe Sun systems allow mixing different processors.

The “I/O Cards” section will have information on cards added to the system but may also list I/O devices (drive controllers etc.) built into the motherboard.

So that’s the highlights. If anyone wants to send me the prtdiag output from a larger system I’ll gladly add that here with some details.

sun, solaris, system administration, sysadmin, unix