General Purpose utilities commands in Linux

1. Calender
cal: Command to see calender for any specific month or a complete year
Usage: 
$ cal [ [month] year]
Example:
$ cal april 2009
Output:
April 2009
Su Mo Tu We Th Fr Sa
 1  2  3  4
5   6  7  8  9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30

2. Date
date: displays the current date
Usage:
$ date
Output:
Tue Apr 21 21:33:49 IST 2009
user$ date +"%D %H:%M:%S"
04/21/09 21:35:02
Options:
d - The da of the month (1-31)
y - The last two digits of the year
H,M,S - Hour Minute and second respectively
D - the date in mm/dd/yy

3. echo
echo: Print message on the terminal
Usage:
echo ""
Example:
$ echo "Welcome to the workshop"
Output:
Welcome to the workshop

4. printf
printf: Print the formatted message on the terminal
Usage:
printf ""
Example:
$ printf "the amount is %d\n" 100
Output:
the amount is 100

5. Calculator
bc: A text based calculator
Usage:
$ bc
2*10+20-9+4/2 [Input]
33 [Output]
[ctrl+d] [Quit]

xcalc is graphical based calculator

6. script
script command records your session and stores it in a file
Usage:
$ script
Script started, file is typescript
$ echo "this is a sample script"
this is a sample script
$ [ctrl+d]
Script done, file is typescript

By default if you dont specify any file name the contents will be stored in file name typescipt
Usage:
$ cat typescript
Script started on Tuesday 21 April 2009 10:07:00
$ echo "this is a sample script"
this is a sample script
$
Script done on Tuesday 21 April 2009 10:07:34 PM

7. passwd
passwd command allows you to change your password
Usage:
$ passwd
Changing password for user123.
(current) UNIX password:
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

8. WHO: Who are the users?
who command tells you the users currently logged on to the system
Usage:
$ who
root     :0           Sep  4 22:47
root     pts/1        Sep  4 22:48 (:0.0)

9. man - The reference Manual
man displays the documentation for a command
Usage:
man
Example:
$ man ls
ls - list directory contents
SYNOPSIS
ls [OPTION]... [FILE]...
DESCRIPTION
List information about the FILEs (the
none of -cftuvSUX nor --sort.