$cat u.txt 
UPPERCASE
$cat l.txt
cat: l.txt: No such file or directory
$tr A-Z a-z < u.txt > l.txt
$cat u.txt 
UPPERCASE
$cat l.txt 
uppercase
$


$a=`time`

real	0m0.000s
user	0m0.000s
sys	0m0.000s
$a
bash: a: command not found
$a=`ls`
$a
bash: a: command not found
$echo a
a
$echo $a
a.txt b.txt trueangle
$echo `a`
bash: a: command not found

$a=`ls`
$b=time
$echo $b
time
$b=`time`

real	0m0.000s
user	0m0.000s
sys	0m0.000s
$echo $b

$b=`time`

real	0m0.000s
user	0m0.000s
sys	0m0.000s
$echo $b

$b=date
$echo $b
date
$b=`date`
$echo $b
Fri Apr 22 22:50:27 IST 2016
$


http://mywiki.wooledge.org/BashFAQ/032

>>> import struct
>>> dir()   # show the names in the module namespace
['__builtins__', '__doc__', '__name__', 'struct']
>>> dir(struct)   # show the names in the struct module
['Struct', '__builtins__', '__doc__', '__file__', '__name__',
 '__package__', '_clearcache', 'calcsize', 'error', 'pack', 'pack_into',
 'unpack', 'unpack_from']
>>> class Shape(object):
        def __dir__(self):
            return ['area', 'perimeter', 'location']
>>> s = Shape()
>>> dir(s)
['area', 'perimeter', 'location']

https://docs.python.org/2/library/functions.html#dir

$com=world
$echo com
com
$echo $com
world
$x=hello
$echo ${x}{com}
hello{com}
$echo ${x}${com}
helloworld
$echo $x$com
helloworld
$echo ${x}world
helloworld
$echo ${x} world
hello world
$echo $x world
hello world
$echo $xcom

$echo $x com
hello com
$echo $x $com
hello world
$echo $x$com
helloworld
$echo $xcom

$echo ${x}com
hellocom
$


$your_id=${USER}-on-${HOSTNAME}
$echo "$your_id"
jeffrin-on-debian
$your_id=$USER-on-$HOSTNAME
$echo "$your_id"
jeffrin-on-debian
$echo ${USER}5
jeffrin5
$echo $USER5

$echo $USER 5
jeffrin 5
$echo $USER 5
jeffrin 5
$your_id=hello
$echo $your_id5

$echo $your_id 5
hello 5
$echo ${your_id}5
hello5
$

http://www.tldp.org/LDP/abs/html/parameter-substitution.html
http://guide.bash.academy/03.variables.html

In computer science, a nondeterministic algorithm is an
algorithm that, even for the same input, can exhibit
different behaviors on different runs, as opposed to a
deterministic algorithm.


https://en.wikipedia.org/wiki/Nondeterministic_algorithm

$ls
trueangle
$echo hello > a.txt
$echo killo > b.txt
$cat {a,b}.txt
hello
killo
$pwd
/home/jeffrin/temp
$cd  ~
$pwd
/home/jeffrin
$cd tr
trueangle/     trueangle.old/ 
$pwd
/home/jeffrin
$cd -
/home/jeffrin/temp
$pwd
/home/jeffrin/temp
$ls
a.txt  b.txt  trueangle
$