sample session related to “command expansion” in Bash shell

$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

using the dir() Built-in function in Python programming

>>> 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

sample session using “variable expansion” with Bash shell

$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

getting choices listed directly with the first TAB

$p
Display all 428 possibilities? (y or n)
$p
bash: p: command not found
$cat .inputrc 
set show-all-if-ambiguous on
$

related text:
Specifies the duration Readline will wait for a character when reading an ambiguous key sequence (one that can form a complete key sequence using the input read so far, or can take additional input to complete a longer key sequence).

enclose one or more commands inside a pair of parentheses

$ls
1
$( ls date )
ls: cannot access date: No such file or directory
$( `ls` date )
bash: 1: command not found
$cat 1
B$
$( ls `date` )
ls: cannot access Mon: No such file or directory
ls: cannot access Feb: No such file or directory
ls: cannot access 22: No such file or directory
ls: cannot access 21:36:34: No such file or directory
ls: cannot access IST: No such file or directory
ls: cannot access 2016: No such file or directory
$( `ls` `date` )
bash: 1: command not found
$( ls time )
ls: cannot access time: No such file or directory
$( ls `time` )

real	0m0.000s
user	0m0.000s
sys	0m0.000s
1
$( ls pwd )
ls: cannot access pwd: No such file or directory
$ls pwd
ls: cannot access pwd: No such file or directory
$( ls ) ( pwd )
bash: syntax error near unexpected token `('
$ls date
ls: cannot access date: No such file or directory
$`ls` date
bash: 1: command not found
$ls `date`
ls: cannot access Mon: No such file or directory
ls: cannot access Feb: No such file or directory
ls: cannot access 22: No such file or directory
ls: cannot access 21:41:17: No such file or directory
ls: cannot access IST: No such file or directory
ls: cannot access 2016: No such file or directory
$ (ls  pwd )
ls: cannot access pwd: No such file or directory
$x =1
bash: x: command not found
$x = 1
bash: x: command not found
$$x = 1
bash: =: command not found
$ (x = 1)
bash: x: command not found
$x=1
$echo $x
1
$(echo $x)
1
$echo $x
1
$(x=2;echo $x)
2
$echo $x
1
$(x=2 echo $x)
1
$echo $x
1
$(x=2;echo $x)
2
$


http://www.tldp.org/LDP/abs/html/subshells.html
http://unix.stackexchange.com/questions/138463/do-parentheses-really-put-the-command-in-a-subshell