Saturday, 9 February 2008

Bash Programming Cheat Sheet


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

Powered by ScribeFire.

No comments:

My photo
London, United Kingdom
twitter.com/zhengxin

Facebook & Twitter