$tsort 
1
tsort: -: input contains an odd number of tokens
$tsort 
1
2
1
2
$tsort 
0
1
2
3
4
5
0
2
4
1
3
5
$tsort 
0
1
2
3
4
5
0
2
4
1
3
5
$

http://www.geeksforgeeks.org/topological-sorting/
https://www.quora.com/What-are-some-real-world-applications-of-topological-sort

$sudo strace -p "`pidof ping`"
strace: Process 2447 attached
poll([{fd=3, events=POLLIN|POLLERR}], 1, 738) = 1 ([{fd=3, revents=POLLIN}])
recvmsg(3, {msg_name(16)={sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("208.118.235.148")}, msg_iov(1)=[{"E(\0T<\336\0\0.\1\322\213\320v\353\224\300\250\0d\0\0\224\367\t\217\0lt\252\253V"..., 192}], msg_controllen=32, [{cmsg_len=32, cmsg_level=SOL_SOCKET, cmsg_type=0x1d /* SCM_??? */}], msg_flags=0}, MSG_DONTWAIT) = 84
write(1, "64 bytes from wildebeest.gnu.org"..., 84) = 84
poll([{fd=3, events=POLLIN|POLLERR}], 1, 666) = 0 (Timeout)
sendmsg(3, {msg_name(16)={sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("208.118.235.148")}, msg_iov(1)=[{"\10\0\241\363\t\217\0mu\252\253V\0\0\0\0^<\16\0\0\0\0\0\20\21\22\23\24\25\26\27"..., 64}], msg_controllen=0, msg_flags=0}, MSG_CONFIRM) = 64
recvmsg(3, {msg_name(16)={sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("208.118.235.148")}, msg_iov(1)=[{"E(\0T<\337\0\0.\1\322\212\320v\353\224\300\250\0d\0\0\251\363\t\217\0mu\252\253V"..., 192}], msg_controllen=32, [{cmsg_len=32, cmsg_level=SOL_SOCKET, cmsg_type=0x1d /* SCM_??? */}], msg_flags=0}, 0) = 84
write(1, "64 bytes from wildebeest.gnu.org"..., 84) = 84
poll([{fd=3, events=POLLIN|POLLERR}], 1, 667) = 0 (Timeout)
sendmsg(3, {msg_name(16)={sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("208.118.235.148")}, msg_iov(1)=[{"\10\0~\361\t\217\0nv\252\253V\0\0\0\0\200=\16\0\0\0\0\0\20\21\22\23\24\25\26\27"..., 64}], msg_controllen=0, msg_flags=0}, MSG_CONFIRM) = 64
recvmsg(3, {msg_name(16)={sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("208.118.235.148")}, msg_iov(1)=[{"E(\0T<\340\0\0.\1\322\211\320v\353\224\300\250\0d\0\0\206\361\t\217\0nv\252\253V"..., 192}], msg_controllen=32, [{cmsg_len=32, cmsg_level=SOL_SOCKET, cmsg_type=0x1d /* SCM_??? */}], msg_flags=0}, 0) = 84
write(1, "64 bytes from wildebeest.gnu.org"..., 84) = 84
poll([{fd=3, events=POLLIN|POLLERR}], 1, 678) = 0 (Timeout)
sendmsg(3, {msg_name(16)={sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("208.118.235.148")}, msg_iov(1)=[{"\10\0\1\357\t\217\0ow\252\253V\0\0\0\0\374>\16\0\0\0\0\0\20\21\22\23\24\25\26\27"..., 64}], msg_controllen=0, msg_flags=0}, MSG_CONFIRM) = 64
recvmsg(3, {msg_name(16)={sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("208.118.235.148")}, msg_iov(1)=[{"E(\0T<\341\0\0.\1\322\210\320v\353\224\300\250\0d\0\0\t\357\t\217\0ow\252\253V"..., 192}], msg_controllen=32, [{cmsg_len=32, cmsg_level=SOL_SOCKET, cmsg_type=0x1d /* SCM_??? */}], msg_flags=0}, 0) = 84
write(1, "64 bytes from wildebeest.gnu.org"..., 84) = 84
poll([{fd=3, events=POLLIN|POLLERR}], 1, 663^Cstrace: Process 2447 detached
 
$sudo strace -p "`pidof ping`"
strace: must have PROG [ARGS] or -p PID
Try 'strace -h' for more information.
$


$ls -l
total 0
-rw-r--r-- 1 jeffrin jeffrin 0 Jan 27 00:13 hello
-rw-r--r-- 1 jeffrin jeffrin 0 Jan 27 00:14 world
$pwd
/home/jeffrin/sample
$env -i HOME=/home/jeffrin `cd`
HOME=/home/jeffrin
$pwd
/home/jeffrin/sample
$ls
hello  world
$env -i bash
jeffrin@debian:/home/jeffrin/sample$ echo $TERM
dumb
jeffrin@debian:/home/jeffrin/sample$ echo $PS1
${debian_chroot:+($debian_chroot)}\u@\h:\w\$
jeffrin@debian:/home/jeffrin/sample$ export PS1=>
bash: syntax error near unexpected token `newline'
jeffrin@debian:/home/jeffrin/sample$ export PS1=">"
>unset $PS1
bash: unset: `>': not a valid identifier
>unset PS1 
export PS1=$
$


$mkdir data
$cd data/
$ls
$pwd
/home/jeffrin/data
$echo hello > file1.txt
$echo world > file2.txt
$ls -l
total 8
-rw-r--r-- 1 jeffrin jeffrin 6 Jan 22 22:44 file1.txt
-rw-r--r-- 1 jeffrin jeffrin 6 Jan 22 22:44 file2.txt
$getfacl -R . > permissions.txt
$cat permissions.txt 
# file: .
# owner: jeffrin
# group: jeffrin
user::rwx
group::r-x
other::r-x

# file: permissions.txt
# owner: jeffrin
# group: jeffrin
user::rw-
group::r--
other::r--

# file: file1.txt
# owner: jeffrin
# group: jeffrin
user::rw-
group::r--
other::r--

# file: file2.txt
# owner: jeffrin
# group: jeffrin
user::rw-
group::r--
other::r--

$chmod 777 file2.txt 
$ls -l file2.txt 
-rwxrwxrwx 1 jeffrin jeffrin 6 Jan 22 22:44 file2.txt
$setfacl --restore=permissions.txt 
$ls -l file2.txt 
-rw-r--r-- 1 jeffrin jeffrin 6 Jan 22 22:44 file2.txt
$


$cat name.txt 
1 ram
2 shyam
3 raju
4 biju
$cat company.txt
1 google
2 facebook
3 twitter
4 ibm
$join company.txt name.txt 
1 google ram
2 facebook shyam
3 twitter raju
4 ibm biju
$join name.txt company.txt
1 ram google
2 shyam facebook
3 raju twitter
4 biju ibm
$


$ss -t
State      Recv-Q Send-Q                            Local Address:Port                                             Peer Address:Port                
ESTAB      0      0                                 192.168.0.100:57982                                           216.58.197.34:https                
ESTAB      0      0                                 192.168.0.100:56572                                           216.58.196.98:https                
ESTAB      0      0                                 192.168.0.100:51440                                           216.58.220.34:https                
ESTAB      0      0                                 192.168.0.100:56850                                          216.58.196.110:https                
ESTAB      0      0                                 192.168.0.100:42946                                           216.58.220.46:https                
ESTAB      0      0                                 192.168.0.100:42758                                           216.58.197.46:https                
ESTAB      0      0                                 192.168.0.100:32912                                           216.58.220.33:https                
ESTAB      0      0                                 192.168.0.100:40452                                           216.58.220.38:https                
ESTAB      0      0                                 192.168.0.100:51128                                           182.79.251.80:https                
ESTAB      0      0                                 192.168.0.100:38490                                           216.58.220.35:https                
ESTAB      0      0                                 192.168.0.100:37378                                           216.58.196.97:https                
ESTAB      0      0                                 192.168.0.100:41954                                          216.58.196.100:https                
CLOSE-WAIT 1      0                                 192.168.0.100:43954                                          74.125.100.153:https                
ESTAB      0      0                                 192.168.0.100:42908                                           216.58.197.74:https                
ESTAB      0      0                                 192.168.0.100:37384                                           216.58.196.97:https                
ESTAB      0      0                                 192.168.0.100:56818                                          216.58.196.110:https                
ESTAB      1430   0                                 192.168.0.100:51130                                           182.79.251.80:https                
$ss -s
Total: 875 (kernel 0)
TCP:   22 (estab 16, closed 1, orphaned 0, synrecv 0, timewait 1/0), ports 0

Transport Total     IP        IPv6
*	  0         -         -        
RAW	  1         0         1        
UDP	  17        15        2        
TCP	  21        19        2        
INET	  39        34        5        
FRAG	  0         0         0        

$