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You can evaluate a command and store the answer in a variable.
For example if you want to store the current date in a variable you can use:
current_date=`date`
or the newer method:
current_date=$(date)
echo $current_date
Evaluate a variable
If you have a variable which contains a command you can evaluate (execute) this command with 'eval':
Arithmetic
Arithmetic calculations
Arithmetic calculation can be done within double round brackets. Examples:
year=2013
next_year=$(( year + 1 ))
or
(( next_year = year + 1 ))
echo $next_year
2014
% : modulo / remainder
echo $(( 10 % 3 ))
1
Just be careful and never try arithmetic operations on numbers starting with a zero!!!
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'\' prevents the following character from being "interpreted". There are several "special characters" (see above) in shell which have a certain "meaning" and functions as little "commands".
Unfortunately '\' has different meanings depending on where/how it gets used.
Examples:
$ person=Alex
$ echo $person
Alex
$ echo \$person
$person => No evaluation done !!!
$ echo "\$person"
$person => No evaluation done !!!
$ echo '\$person'
Alex => Evaluation done !!!
When used as the very last character in a line it will surpress the "new line".
$ cat > text_file << EOF
> Hello
> How are you?
> EOF
$ cat text_file
Hello
How are you?
$ cat > text_file << EOF
> Hello, \
> how are you?
> EOF
$ cat text_file
Hello, how are you?
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