Bash Programming Cheat Sheet
January 1, 2010
Bash Programming Cheat
Sheet
Written By: ph34r
A quick cheat sheet for programmers who want to do shell
scripting. This is not intended to teach programming, etc. but
it is intended for a someone who knows one programming language
to begin learning about bash scripting.
Basics
All bash scripts must tell the o/s what to use as the
interpreter. The first line of any script should be:
#!/bin/bash
You must make bash scripts executable.
chmod +x filename
Variables
Create a variable – just assign value. Variables are
non-datatyped (a variable can hold strings, numbers, etc. with
out being defined as such).
varname=value
Access a variable by putting $ on the front of the name
echo $varname
Values passed in from the command line as arguments are
accessed as $# where #= the index of the variable in the array
of values being passed in. This array is base 1 not base 0.
command var1 var2 var3 …. varX
$1 contains whatever var1 was, $2 contains whatever var2 was,
etc.
Built in variables:
| $1-$N | Stores the arguments (variables) that were passed to the shell program from the command line. |
| $? | Stores the exit value of the last command that was executed. |
| $0 | Stores the first word of the entered command (the name of the shell program). |
| $* | Stores all the arguments that were entered on the command line ($1 $2 …). |
| “$@” | Stores all the arguments that were entered on the command line, individually quoted (“$1″ “$2″ …). |
Quote Marks
Regular double quotes (“like these”) make the shell ignore
whitespace and count it all as one argument being passed or
string to use. Special characters inside are still
noticed/obeyed.
Single quotes ‘like this’ make the interpreting shell ignore
all special characters in whatever string is being passed.
The back single quote marks (`command`) perform a different
function. They are used when you want to use the results of a
command in another command. For example, if you wanted to set
the value of the variable contents equal to the list of files
in the current directory, you would type the following command:
contents=`ls`, the results of the ls program are put in the
variable contents.
Logic and comparisons
A command called test is used to evaluate conditional
expressions, such as a if-then statement that checks the
entrance/exit criteria for a loop.
test expression
Or
[ expression ]
Numeric Comparisons
| int1 -eq int2 | Returns True if int1 is equal to int2. |
| int1 -ge int2 | Returns True if int1 is greater than or equal to int2. |
| int1 -gt int2 | Returns True if int1 is greater than int2. |
| int1 -le int2 | Returns True if int1 is less than or equal to int2 |
| int1 -lt int2 | Returns True if int1 is less than int2 |
| int1 -ne int2 | Returns True if int1 is not equal to int2 |
String Comparisons
| str1 = str2 | Returns True if str1 is identical to str2. |
| str1 != str2 | Returns True if str1 is not identical to str2. |
| str | Returns True if str is not null. |
| -n str | Returns True if the length of str is greater than zero. |
| -z str | Returns True if the length of str is equal to zero. (zero is different than null) |
File Comparisons
| -d filename | Returns True if file, filename is a directory. |
| -f filename | Returns True if file, filename is an ordinary file. |
| -r filename | Returns True if file, filename can be read by the process. |
| -s filename | Returns True if file, filename has a nonzero length. |
| -w filename | Returns True if file, filename can be written by the process. |
| -x filename | Returns True if file, filename is executable. |
Expression Comparisons
!expression
Returns true if expression is not true
expr1 -a expr2
Returns True if expr1 and expr2 are true. ( && , and
)
expr1 -o expr2
Returns True if expr1 or expr2 is true. ( ||, or )
Logic Con’t.
If…then
if [ expression ]
then
commands
fi
If..then…else
if [ expression ]
then
commands
else
commands
fi
If..then…else If…else
if [ expression ]
then
commands
elif [ expression2 ]
then
commands
else
commands
fi
Case select
case string1 in
str1)
commands;;
str2)
commands;;
*)
commands;;
esac
string1 is compared to str1 and str2. If one of these strings
matches string1, the commands up until the double semicolon (;
are executed. If neither str1 nor str2 matches string1, the
commands associated with the asterisk are executed. This is the
default case condition because the asterisk matches all
strings.
Iteration (Loops)
for var1 in list
do
commands
done
This executes once for each item in the list. This list can be
a variable that contains several words separated by spaces
(such as output from ls or cat), or it can be a list of values
that is typed directly into the statement. Each time through
the loop, the variable var1 is assigned the current item in the
list, until the last one is reached.
while [ expression ]
do
commands
done
until [ expression ]
do
commands
done
Functions
Create a function:
fname(){
commands
}
Call it by using the following syntax: fname
Or, create a function that accepts arguments:
fname2 (arg1,arg2…argN){
commands
}
And call it with: fname2 arg1 arg2 … argN
Orignal Location :http://linux-sxs.org/programming/bashcheat.html
VNC tools
August 18, 2009
VNC packages +ves
UltraVNC easy movement of files, looks better than tightVNC but not as good as RealVNC
TightVNC easy movement of files + works with mac natively
RealVNC faster and better looking , more polished vivek’s Vote
Of Course all of these do most of the major stuff
http://www.realvnc.com/cgi-bin/download.cgi
Norms, Crest factor and multitones
August 18, 2009
Norms and Crest Factor
An n – norm 
is defined as
The most common norm is 
& 
= rms value
and
= max( |u(t)| )
The creast factor of a non-zero signal is defined as the ratio of peak to rms value
CF(u) = 
1
A CF of 1 indicates a signal which only stays at 
Therefore a signal with a low crest factor spends most of the time near the peek values.
A creast factor of 1 implies a square wave of amplitude
In terms of amplitude distribution –
, where 
is the total length of set E
the proportion of the time a where |u(t)| > a
= min{a|
=0}
and
Which is a weighted integral of the amplitude distribution or if we plot a graph of vs a, 
is the 
todo: update the crest factor definition in Wiki http://en.wikipedia.org/wiki/Normed_vector_space
Crest factor of a multi-tone Signal
For a following multitone signal with N tones
We have mean, = 
, regardless of phase 
The goal is to minimize the crest factor
Pitch Detection Algorithms
August 8, 2008
- Auto-correlation Algorithm
http://cnx.org/content/m11714/latest/
Speech Detection – in presense of Background Noise
July 22, 2008
measure energy and zero crossing.
- Voiced Signals : high energy – low frequency (low zero crossing)
- Unvoiced Signals : low energy – high frequency (big zero crossing)
- Noise : Low energy + low frequency (overall low zero crossing)
Vowels :
Consonants :
formants : Peak of frequency spectrum
LPC
Another method, which is used to obtain a frequency spectrum is that of Linear Predictive Coding(LPC). This is the most successful method in widespread use today. The idea behind LPC is that the values of the signal can be expressed as a linear combination of the preceding values. That is, if s(i) is the amplitude at time i,
s(i) = a1*s(i-1) + a2*s(i-2) + … + ap*s(i-p)
When the input data is filled in, this becomes a system of linear equations which can be solved to determine the values of a1 through ap. These values then produce a very noise free signal, which clearly identifies the formants. Typical values for p are 10-12.
Logarithms on floats
July 22, 2008
Logarithms are usually computed by using a an iterative algorithm like taylor series or another iterative algorithm.
Easy Solution: Floating point representation of a number separates the number into an exponent, a sign and a mantissa. The number represented is then N=M*2^E.
The log of that number would be LOG(N)=LOG(M * 2^E)=LOG(M) + E*LOG(2). If we assume M is normalized, 1<=M<2, then 0 <=LOG(M) <=LOG(2). The LOG of the exponent gives us the coarse estimate of the LOG (to 6dB), which can be found by just multiplying the exponent by a constant (LOG(2) in the appropriate base). (A constant offset may need to be added to the exponent e.g. for IEEE representation we need to compensate for offset of 128 in offset)
For Finer precision obtain an estimate of the log of mantissa and add that to the logarithm of the exponent. The logarithm of the mantissa can be found using a small look-up table addressed by the most significant bits of M. By only using 32 entry long table (5 bits) an accuracy of around 0.25 dB can be obtained.
Other Log for floating point notes from the web
