cat command examples

Concatenate Files and Number the Lines

cat -n file1 file2
When working with poems and source code, it's really nice to have numbered lines so that references are clear. If you want to generate line numbers when you use cat, add the -n option (or --number).

$ cat -n housman_-_rue.txt quarles_-_the_world.txt
1 WITH rue my heart is laden
2 For golden friends I had,
3 For many a rose-lipt maiden
4 And many a lightfoot lad.
5 By brooks too broad for leaping
6 The lightfoot boys are laid;
7 The rose-lipt girls are sleeping
8 In fields where roses fade.
9 The world's an Inn; and I her guest.
10 I eat; I drink; I take my rest.
11 My hostess, nature, does deny me
12 Nothing, wherewith she can supply me;
13 Where, having stayed a while, I pay
14 Her lavish bills, and go my way.

Line numbers can be incredibly useful, and cat provides a quick and dirty way to add them to a file.

Note

For a vastly better cat, check out dog (more information is available at http://opensource.weblogsinc.com/2005/02/17/why-dogs-are-betters-than-cats). Instead of local files, you can use dog to view the HTML source of web pages on stdout, or just a list of images or links on the specified web pages. The dog command converts all characters to lowercase or vice versa; converts line endings to Mac OS, DOS, or Unix; and even allows you to specify a range of characters to output (lines 525, for instance). Not to mention, the man page for dog is one of the funniest ever. This is one dog that knows a lot of new tricks!

source scott granemann

host command examples

To quickly find the IP address associated with a domain name, use the host command:

$ host www.granneman.com
www.granneman.com is an alias for granneman.com.
granneman.com has address 216.23.180.5
www.granneman.com is an alias for granneman.com.
www.granneman.com is an alias for granneman.com.
granneman.com mail is handled by 30 bhoth.pair.com.


$ host 65.214.39.152
152.39.214.65.in-addr.arpa domain name pointer web.bloglines.com.

tail command examples

View the Constantly Updated Last Lines of a File or Files

tail -f
tail -f --pid=PID# terminates after PID dies.
The great thing about log files is that they constantly change as things happen on your system. The tail command shows you a snapshot of a file, and then deposits you back on the command line. Want to see the log file again? Then run tail again...and again...and again. Blech!
With the -f (or --follow) option, tail doesn't close. Instead, it shows you the last 10 lines of the file (or a different number if you add -n to the mix) as the file changes, giving you a way to watch all the changes to a log file as they happen. This is wonderfully useful if you're trying to figure out just what is happening to a system or program.
For instance, a web server's logs might look like this:
Note
In order to save space, I've removed the IP address, date, and time of the access.

$ tail -f /var/log/httpd/d20srd_org_log_20051201
"GET /srd/skills/bluff.htm HTTP/1.1"...
"GET /srd/skills/senseMotive.htm HTTP/1.1"...
"GET /srd/skills/concentration.htm HTTP/1.1"...
"GET /srd/classes/monk.htm HTTP/1.1"...
"GET /srd/skills/escapeArtist.htm HTTP/1.1"...

spoofing MAC addresses

You can even change (or "spoof") the hardware MAC address for your network device. This is usually only necessary to get around some ISPs' attempts to link Internet service to a specific machine. Be careful with spoofing your MAC address because a mistake can conflict with other network devices, causing problems. If you do decide to spoof your MAC, make sure you use ifconfig by itself to first acquire the default MAC address so you can roll back to that later (by the way, the MAC address shown in this command is completely bogus, so don't try to use it).

# ifconfig eth0 hw ether 00:14:CC:00:1A:00

lsof examples

List a User's Open Files

lsof -u
If you want to look at the files a particular user has open (and remember that those include network connections and devices, among many others), add the -u option to lsof, followed by the username (remember that lsof must be run as root).

Note
In order to save space, some of the data you'd normally see when you run lsof has been removed in this and further examples.

# lsof -u scott

List Users for a Particular File

lsof [file]
In the previous section, you saw what files a particular user had open. Let's reverse that, and see who's using a particular file. To do so, simply follow lsof with the path to a file on your system. For instance, let's take a look at who's using the SSH daemon, used to connect remotely to this computer (remember that lsof must be run as root).


List Processes for a Particular Program

To find out the full universe of other files associated with a particular running program, follow lsof with the -c option, and then the name of a running (and therefore "open") program

lsof -c [program]

Ex:

# lsof -c sshd
COMMAND PID USER NAME
sshd 10542 root /lib/ld-2.3.5.so
sshd 10542 root /dev/null
sshd 10542 root 192.168.0.170:ssh->192.168.0.100:4577 (ESTABLISHED)
sshd 10548 scott /usr/sbin/sshdp
sshd 10548 scott 192.168.0.170:ssh->192.168.0.100:4577 (ESTABLISHED)


source scott granemann

Quickly Find Out What a Command Does Based on Its Name

man -f
If you know a command's name but don't know what it does, there's a quick and dirty way to find out without requiring you to actually open the man page for that command. Use the -f option (or --whatis), and the command's synopsis appears.

$ man -f ls
ls (1) - list directory contents


source scott grannemann

Rebuild man's Database of Commands

man -u
Occasionally you'll try to use man to find out information about a command and man reports that there is no page for that command. Before giving up, try again with the -u option (or --update), which forces man to rebuild the database of commands and man pages it uses. It's often a good first step if you think things aren't quite as they should be.

$ man ls
No manual entry for ls
$ man -u ls
LS(1) User Commands LS(1)
NAME
ls - list directory contents
SYNOPSIS
ls [OPTION]... [FILE]...
[Listing condensed due to length]