$cat a.txt 
hello
$cat b.txt 
killo
$ls
1  a.txt  b.txt  l.txt  trueangle  u.txt
$cat u.txt 
UPPERCASE
$cat l.txt 
uppercase
$cat a.txt >> b.txt >> l.txt >> u.txt 
$cat u.txt 
UPPERCASE
hello
$cat a.txt 
hello
$cat 
1          a.txt      b.txt      l.txt      trueangle/ u.txt      
$cat l.txt 
uppercase
$cat a.txt 
hello
$cat a.txt >> b.txt 
$cat b.txt 
killo
hello
$cat l.txt >> u.txt 
$cat u.txt 
UPPERCASE
hello
uppercase
$paste b.txt u.txt > final.txt
$cat final.txt 
killo	UPPERCASE
hello	hello
	uppercase
$sort -t: -k1 final.txt 
hello	hello
killo	UPPERCASE
	uppercase
$


$sort -t: -k3 --debug /etc/passwd
sort: using ‘en_US.UTF-8’ sorting rules
root:x:0:0:root:/root:/bin/bash
       ________________________
_______________________________
jeffrin:x:1000:1000:Jeffrin Jose T,,,:/home/jeffrin:/bin/bash
          ___________________________________________________
_____________________________________________________________
uuidd:x:100:101::/run/uuidd:/bin/false
        ______________________________
______________________________________
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
       ____________________________________________
___________________________________________________
Debian-exim:x:101:103::/var/spool/exim4:/bin/false
              ____________________________________
__________________________________________________
statd:x:102:65534::/var/lib/nfs:/bin/false
        __________________________________
__________________________________________
sshd:x:103:65534::/var/run/sshd:/usr/sbin/nologin
       __________________________________________
_________________________________________________
messagebus:x:104:106::/var/run/dbus:/bin/false
             _________________________________
______________________________________________
colord:x:105:109:colord colour management daemon,,,:/var/lib/colord:/bin/false
         _____________________________________________________________________
______________________________________________________________________________
usbmux:x:106:46:usbmux daemon,,,:/home/usbmux:/bin/false
         _______________________________________________
________________________________________________________
avahi:x:107:113:Avahi mDNS daemon,,,:/var/run/avahi-daemon:/bin/false
        _____________________________________________________________
_____________________________________________________________________
pulse:x:108:114:PulseAudio daemon,,,:/var/run/pulse:/bin/false
        ______________________________________________________
______________________________________________________________
speech-dispatcher:x:109:29:Speech Dispatcher,,,:/var/run/speech-dispatcher:/bin/sh
                    ______________________________________________________________
__________________________________________________________________________________
rtkit:x:110:116:RealtimeKit,,,:/proc:/bin/false
        _______________________________________
_______________________________________________
Debian-gdm:x:111:117:Gnome Display Manager:/var/lib/gdm3:/bin/false
             ______________________________________________________
___________________________________________________________________
saned:x:112:118::/var/lib/saned:/bin/false
        __________________________________
__________________________________________
ntp:x:113:120::/home/ntp:/bin/false
      _____________________________
___________________________________
geoclue:x:114:121::/var/lib/geoclue:/bin/false
          ____________________________________
______________________________________________
dnsmasq:x:115:65534:dnsmasq,,,:/var/lib/misc:/bin/false
          _____________________________________________
_______________________________________________________
fetchmail:x:116:65534::/var/lib/fetchmail:/bin/false
            ________________________________________
____________________________________________________
vde2-net:x:117:123::/var/run/vde2:/bin/false
           _________________________________
____________________________________________
timidity:x:118:125:TiMidity++ MIDI sequencer service:/etc/timidity:/bin/false
           __________________________________________________________________
_____________________________________________________________________________
epmd:x:119:128::/var/run/epmd:/bin/false
       _________________________________
________________________________________
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
         ______________________________________
_______________________________________________
systemd-timesync:x:120:129:systemd Time Synchronization,,,:/run/systemd:/bin/false
                   _______________________________________________________________
__________________________________________________________________________________
systemd-network:x:121:130:systemd Network Management,,,:/run/systemd/netif:/bin/false
                  ___________________________________________________________________
_____________________________________________________________________________________
systemd-resolve:x:122:131:systemd Resolver,,,:/run/systemd/resolve:/bin/false
                  ___________________________________________________________
_____________________________________________________________________________
systemd-bus-proxy:x:123:132:systemd Bus Proxy,,,:/run/systemd:/bin/false
                    ____________________________________________________
________________________________________________________________________
proftpd:x:124:65534::/run/proftpd:/bin/false
          __________________________________
____________________________________________
ftp:x:125:65534::/srv/ftp:/bin/false
      ______________________________
____________________________________
_apt:x:126:65534::/nonexistent:/bin/false
       __________________________________
_________________________________________
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
        __________________________________
__________________________________________
bin:x:2:2:bin:/bin:/usr/sbin/nologin
      ______________________________
____________________________________
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
           _________________________________________
____________________________________________________
sys:x:3:3:sys:/dev:/usr/sbin/nologin
      ______________________________
____________________________________
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
         ___________________________________________
____________________________________________________
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
       ______________________________________________________
_____________________________________________________________
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
      __________________________________________
________________________________________________
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
        _________________________________________________________________________
_________________________________________________________________________________
sync:x:4:65534:sync:/bin:/bin/sync
       ___________________________
__________________________________
games:x:5:60:games:/usr/games:/usr/sbin/nologin
        _______________________________________
_______________________________________________
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
      _________________________________________
_______________________________________________
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
         _________________________________________________
__________________________________________________________
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
     _______________________________________
____________________________________________
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
       ____________________________________
___________________________________________
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
       __________________________________________
_________________________________________________
$sort -t: -k3 --debug /etc/passwd
$sort -t: -k3 --debug /etc/passwd
sort: using ‘en_US.UTF-8’ sorting rules
root:x:0:0:root:/root:/bin/bash
       ________________________
_______________________________
jeffrin:x:1000:1000:Jeffrin Jose T,,,:/home/jeffrin:/bin/bash
          ___________________________________________________
_____________________________________________________________
uuidd:x:100:101::/run/uuidd:/bin/false
        ______________________________
______________________________________
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
       ____________________________________________
___________________________________________________
Debian-exim:x:101:103::/var/spool/exim4:/bin/false
              ____________________________________
__________________________________________________
statd:x:102:65534::/var/lib/nfs:/bin/false
        __________________________________
__________________________________________
sshd:x:103:65534::/var/run/sshd:/usr/sbin/nologin
       __________________________________________
_________________________________________________
messagebus:x:104:106::/var/run/dbus:/bin/false
             _________________________________
______________________________________________
colord:x:105:109:colord colour management daemon,,,:/var/lib/colord:/bin/false
         _____________________________________________________________________
______________________________________________________________________________
usbmux:x:106:46:usbmux daemon,,,:/home/usbmux:/bin/false
         _______________________________________________
________________________________________________________
avahi:x:107:113:Avahi mDNS daemon,,,:/var/run/avahi-daemon:/bin/false
        _____________________________________________________________
_____________________________________________________________________
pulse:x:108:114:PulseAudio daemon,,,:/var/run/pulse:/bin/false
        ______________________________________________________
______________________________________________________________
speech-dispatcher:x:109:29:Speech Dispatcher,,,:/var/run/speech-dispatcher:/bin/sh
                    ______________________________________________________________
__________________________________________________________________________________
rtkit:x:110:116:RealtimeKit,,,:/proc:/bin/false
        _______________________________________
_______________________________________________
Debian-gdm:x:111:117:Gnome Display Manager:/var/lib/gdm3:/bin/false
             ______________________________________________________
___________________________________________________________________
saned:x:112:118::/var/lib/saned:/bin/false
        __________________________________
__________________________________________
ntp:x:113:120::/home/ntp:/bin/false
      _____________________________
___________________________________
geoclue:x:114:121::/var/lib/geoclue:/bin/false
          ____________________________________
______________________________________________
dnsmasq:x:115:65534:dnsmasq,,,:/var/lib/misc:/bin/false
          _____________________________________________
_______________________________________________________
fetchmail:x:116:65534::/var/lib/fetchmail:/bin/false
            ________________________________________
____________________________________________________
vde2-net:x:117:123::/var/run/vde2:/bin/false
           _________________________________
____________________________________________
timidity:x:118:125:TiMidity++ MIDI sequencer service:/etc/timidity:/bin/false
           __________________________________________________________________
_____________________________________________________________________________
epmd:x:119:128::/var/run/epmd:/bin/false
       _________________________________
________________________________________
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
         ______________________________________
_______________________________________________
systemd-timesync:x:120:129:systemd Time Synchronization,,,:/run/systemd:/bin/false
                   _______________________________________________________________
__________________________________________________________________________________
systemd-network:x:121:130:systemd Network Management,,,:/run/systemd/netif:/bin/false
                  ___________________________________________________________________
_____________________________________________________________________________________
systemd-resolve:x:122:131:systemd Resolver,,,:/run/systemd/resolve:/bin/false
                  ___________________________________________________________
_____________________________________________________________________________
systemd-bus-proxy:x:123:132:systemd Bus Proxy,,,:/run/systemd:/bin/false
                    ____________________________________________________
________________________________________________________________________
proftpd:x:124:65534::/run/proftpd:/bin/false
          __________________________________
____________________________________________
ftp:x:125:65534::/srv/ftp:/bin/false
      ______________________________
____________________________________
_apt:x:126:65534::/nonexistent:/bin/false
       __________________________________
_________________________________________
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
        __________________________________
__________________________________________
bin:x:2:2:bin:/bin:/usr/sbin/nologin
      ______________________________
____________________________________
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
           _________________________________________
____________________________________________________
sys:x:3:3:sys:/dev:/usr/sbin/nologin
      ______________________________
____________________________________
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
         ___________________________________________
____________________________________________________
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
       ______________________________________________________
_____________________________________________________________
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
      __________________________________________
________________________________________________
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
        _________________________________________________________________________
_________________________________________________________________________________
sync:x:4:65534:sync:/bin:/bin/sync
       ___________________________
__________________________________
games:x:5:60:games:/usr/games:/usr/sbin/nologin
        _______________________________________
_______________________________________________
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
      _________________________________________
_______________________________________________
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
         _________________________________________________
__________________________________________________________
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
     _______________________________________
____________________________________________
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
       ____________________________________
___________________________________________
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
       __________________________________________
_________________________________________________
$


$sudo cat /proc/crypto 
name         : crct10dif
driver       : crct10dif-pclmul
module       : crct10dif_pclmul
priority     : 200
refcnt       : 1
selftest     : passed
internal     : no
type         : shash
blocksize    : 1
digestsize   : 2

name         : crc32
driver       : crc32-pclmul
module       : crc32_pclmul
priority     : 200
refcnt       : 1
selftest     : passed
internal     : no
type         : shash
blocksize    : 1
digestsize   : 4

name         : crc32c
driver       : crc32c-intel
module       : crc32c_intel
priority     : 200
refcnt       : 1
selftest     : passed
internal     : no
type         : shash
blocksize    : 1
digestsize   : 4

name         : lzo
driver       : lzo-generic
module       : kernel
priority     : 0
refcnt       : 5
selftest     : passed
internal     : no
type         : compression

name         : crct10dif
driver       : crct10dif-generic
module       : kernel
priority     : 100
refcnt       : 2
selftest     : passed
internal     : no
type         : shash
blocksize    : 1
digestsize   : 2

name         : aes
driver       : aes-generic
module       : kernel
priority     : 100
refcnt       : 1
selftest     : passed
internal     : no
type         : cipher
blocksize    : 16
min keysize  : 16
max keysize  : 32

name         : sha1
driver       : sha1-generic
module       : kernel
priority     : 0
refcnt       : 1
selftest     : passed
internal     : no
type         : shash
blocksize    : 64
digestsize   : 20

name         : md5
driver       : md5-generic
module       : kernel
priority     : 0
refcnt       : 1
selftest     : passed
internal     : no
type         : shash
blocksize    : 64
digestsize   : 16

name         : digest_null
driver       : digest_null-generic
module       : kernel
priority     : 0
refcnt       : 1
selftest     : passed
internal     : no
type         : shash
blocksize    : 1
digestsize   : 0

name         : compress_null
driver       : compress_null-generic
module       : kernel
priority     : 0
refcnt       : 1
selftest     : passed
internal     : no
type         : compression

name         : ecb(cipher_null)
driver       : ecb-cipher_null
module       : kernel
priority     : 100
refcnt       : 1
selftest     : passed
internal     : no
type         : blkcipher
blocksize    : 1
min keysize  : 0
max keysize  : 0
ivsize       : 0
geniv        : 

name         : cipher_null
driver       : cipher_null-generic
module       : kernel
priority     : 0
refcnt       : 1
selftest     : passed
internal     : no
type         : cipher
blocksize    : 1
min keysize  : 0
max keysize  : 0

$


$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