Useful Linux commands

***********NETWORK COMMANDS***********
#To know if a host/ip is reachable

ping hostname/IP

#To see the network route how a host is reachable. Packet flow path.

traceroute hostname/IP 

or

tracepath hostname

# To add route to reach through network when you have multiple network/ethernet NIC cards in a server
route add -net 192.168.49.0 netmask 255.255.255.0 gw 192.168.200.1
        - Here any ip in 192.168.49.XX network is routed via the gateway 192.168.200.1
        - when there are more than 1 NIC, this can be helpful.

#To delete the route
route del -net 192.168.49.0 netmask 255.255.255.0 

#To check if a particular port is open or reachable in a remote server

telnet IP port

or 

nc -zvw10 IPaddress portnumner --> in case of tcp port

nc -zuw10 IPaddress portnumber --> in case of udp port

You will get "succeeded" as output if port is reachable

"Failed: no route to host" when port is not open

"failed: Connection refused" when port is open and no service available on the port.

10 is the time in seconds to wait for the response.

#Quick look on IP addresses available

ip addr

# detailed look on ip address

ifconfig

***********PACKET CAPTURE**************
#To capture packets in the network or ethernet card

tcpdump -s packetsize -i ethernetinterface -w writetofile.pcap host <ipaddress> port <number> or port <number2> 

# Quick read from pcap file one liner.

tcpdump -r filetoread.pcap

*************SYSTEM INFO****************
#To know if a linux machine is 64bit or 32 bit

uname -a 

this will tell u the kernel version, and all.

if u see x86_64, it's a 64bit or i386,i586,etc, it's 32bit

#To know the cpu info

cat /proc/cpuinfo

#To know the memory info

cat /proc/meminfo

#To know the OS name

cat /etc/*release

#To find the disk usage of a directory

du -sh <directory>

try du -sh * for summary of each sub directories and files in current directory.

***************FILE COMMANDS************
# To see top n lines of a file

head -n number filename

# To see botton n lines of a file

tail -n number filename

# To see a particular line of a file like 25th line

head -n 25 filename | tail -n1

# To cut a line into multiple fields

cut -d'<delimiter>' -f<field#> filename

where delimiter is one character and field# is the field number

#To sort above output and print unique values with the count

cut -d'<delimiter>' -f<field#> filename | sort | uniq -c

To be Continued.. 

MySQL useful functions

#To know the current database used
select database();

#To know the version of the database server
select version();

#To know the current logged in user
select user();

#To diff two dates in days,years,etc.
TimeStampDiff(YEAR, date1, date2);
   Param1- const, like YEAR,MONTH,DAY,HOUR,etc
   param2 & 3 - actual dates

#To fetch the year part of a date
select year(<date>);

#To fetch the month part of a date
select month(<date>);

#To fetch the day part of date
select dayofmonth(<date>);

#To get the mod i.e. 5mod3
select mod(5,3);

# Ifnull
select ifnull(exp1,exp2) - this will output exp1 if it's true or else exp2
select ifnull(null,10) - will return 10


# _ represents a character
To find names containing exactly five characters, use five instances of the “_” pattern character:
 SELECT * FROM pet WHERE name LIKE '_____';

# regular expressions

regexp 'w' -> containing w
regexp '^b' -> starting with b
regexp 'fy$' -> ending with fy
regexp '^…..$' -> containing 5 letters
regexp '^.{5}$' -> containing 5 letters

“.” matches any single character.

A character class “[...]” matches any character within the brackets. For example, “[abc]” matches “a”, “b”, or “c”. To name a range of characters, use a dash. “[a-z]” matches any letter, whereas “[0-9]” matches any digit.

“*” matches zero or more instances of the thing preceding it. For example, “x*” matches any number of “x” characters, “[0-9]*” matches any number of digits, and “.*” matches any number of anything.

A REGEXP pattern match succeeds if the pattern matches anywhere in the value being tested. (This differs from a LIKE pattern match, which succeeds only if the pattern matches the entire value.)

To anchor a pattern so that it must match the beginning or end of the value being tested, use “^” at the beginning or “$” at the end of the pattern

# To fetch total records from all tables of a database.
mysqlshow --count databasename -uroot -p


#User defined variable
mysql> SELECT @min_price:=MIN(price),@max_price:=MAX(price) FROM shop;
mysql> SELECT * FROM shop WHERE price=@min_price OR price=@max_price;


To be continued..

MySQL errors and resolutions.

Common Errors & Resolutions

Error 113 - MySQL server/host is not reachable. Check the iptables if MySQL port (default 3306) is open

Error 13 - Permission denied. MySQL service is not able access the file, check the owner/group and the mode of the required file.

Definer Error:

The user specified as a definer ('user1'@'10.10.26.%') does not exist

Reason: The functions,triggers,procedures which was created by some other user than the current user. Mostly happens in dump restoration.

Solution: give grant all privileges on the database to the user you are using.

# perror 28

OS error code  28:  No space left on device

Here’s the complete list:

OS error code   1:  Operation not permitted

OS error code   2:  No such file or directory

OS error code   3:  No such process

OS error code   4:  Interrupted system call

OS error code   5:  Input/output error

OS error code   6:  No such device or address

OS error code   7:  Argument list too long

OS error code   8:  Exec format error

OS error code   9:  Bad file descriptor

OS error code  10:  No child processes

OS error code  11:  Resource temporarily unavailable

OS error code  12:  Cannot allocate memory

OS error code  13:  Permission denied

OS error code  14:  Bad address

OS error code  15:  Block device required

OS error code  16:  Device or resource busy

OS error code  17:  File exists

OS error code  18:  Invalid cross-device link

OS error code  19:  No such device

OS error code  20:  Not a directory

OS error code  21:  Is a directory

OS error code  22:  Invalid argument

OS error code  23:  Too many open files in system

OS error code  24:  Too many open files

OS error code  25:  Inappropriate ioctl for device

OS error code  26:  Text file busy

OS error code  27:  File too large

OS error code  28:  No space left on device

OS error code  30:  Read-only file system

OS error code  31:  Too many links

OS error code  32:  Broken pipe

OS error code  33:  Numerical argument out of domain

OS error code  34:  Numerical result out of range

OS error code  35:  Resource deadlock avoided

OS error code  36:  File name too long

OS error code  37:  No locks available

OS error code  38:  Function not implemented

OS error code  39:  Directory not empty

OS error code  40:  Too many levels of symbolic links

OS error code  42:  No message of desired type

OS error code  43:  Identifier removed

OS error code  44:  Channel number out of range

OS error code  45:  Level 2 not synchronized

OS error code  46:  Level 3 halted

OS error code  47:  Level 3 reset

OS error code  48:  Link number out of range

OS error code  49:  Protocol driver not attached

OS error code  50:  No CSI structure available

OS error code  51:  Level 2 halted

OS error code  52:  Invalid exchange

OS error code  53:  Invalid request descriptor

OS error code  54:  Exchange full

OS error code  55:  No anode

OS error code  56:  Invalid request code

OS error code  57:  Invalid slot

OS error code  59:  Bad font file format

OS error code  60:  Device not a stream

OS error code  61:  No data available

OS error code  62:  Timer expired

OS error code  63:  Out of streams resources

OS error code  64:  Machine is not on the network

OS error code  65:  Package not installed

OS error code  66:  Object is remote

OS error code  67:  Link has been severed

OS error code  68:  Advertise error

OS error code  69:  Srmount error

OS error code  70:  Communication error on send

OS error code  71:  Protocol error

OS error code  72:  Multihop attempted

OS error code  73:  RFS specific error

OS error code  74:  Bad message

OS error code  75:  Value too large for defined data type

OS error code  76:  Name not unique on network

OS error code  77:  File descriptor in bad state

OS error code  78:  Remote address changed

OS error code  79:  Can not access a needed shared library

OS error code  80:  Accessing a corrupted shared library

OS error code  81:  .lib section in a.out corrupted

OS error code  82:  Attempting to link in too many shared libraries

OS error code  83:  Cannot exec a shared library directly

OS error code  84:  Invalid or incomplete multibyte or wide character

OS error code  85:  Interrupted system call should be restarted

OS error code  86:  Streams pipe error

OS error code  87:  Too many users

OS error code  88:  Socket operation on non-socket

OS error code  89:  Destination address required

OS error code  90:  Message too long

OS error code  91:  Protocol wrong type for socket

OS error code  92:  Protocol not available

OS error code  93:  Protocol not supported

OS error code  94:  Socket type not supported

OS error code  95:  Operation not supported

OS error code  96:  Protocol family not supported

OS error code  97:  Address family not supported by protocol

OS error code  98:  Address already in use

OS error code  99:  Cannot assign requested address

OS error code 100:  Network is down

OS error code 101:  Network is unreachable

OS error code 102:  Network dropped connection on reset

OS error code 103:  Software caused connection abort

OS error code 104:  Connection reset by peer

OS error code 105:  No buffer space available

OS error code 106:  Transport endpoint is already connected

OS error code 107:  Transport endpoint is not connected

OS error code 108:  Cannot send after transport endpoint shutdown

OS error code 109:  Too many references: cannot splice

OS error code 110:  Connection timed out

OS error code 111:  Connection refused

OS error code 112:  Host is down

OS error code 113:  No route to host

OS error code 114:  Operation already in progress

OS error code 115:  Operation now in progress

OS error code 116:  Stale NFS file handle

OS error code 117:  Structure needs cleaning

OS error code 118:  Not a XENIX named type file

OS error code 119:  No XENIX semaphores available

OS error code 120:  Is a named type file

OS error code 121:  Remote I/O error

OS error code 122:  Disk quota exceeded

OS error code 123:  No medium found

OS error code 124:  Wrong medium type

OS error code 125:  Operation canceled

MySQL error code 126: Index file is crashed

MySQL error code 127: Record-file is crashed

MySQL error code 128: Out of memory

MySQL error code 130: Incorrect file format

MySQL error code 131: Command not supported by database

MySQL error code 132: Old database file

MySQL error code 133: No record read before update

MySQL error code 134: Record was already deleted (or record file crashed)

MySQL error code 135: No more room in record file

MySQL error code 136: No more room in index file

MySQL error code 137: No more records (read after end of file)

MySQL error code 138: Unsupported extension used for table

MySQL error code 139: Too big row

MySQL error code 140: Wrong create options

MySQL error code 141: Duplicate unique key or constraint on write or update

MySQL error code 142: Unknown character set used

MySQL error code 143: Conflicting table definitions in sub-tables of MERGE table

MySQL error code 144: Table is crashed and last repair failed

MySQL error code 145: Table was marked as crashed and should be repaired

MySQL error code 146: Lock timed out; Retry transaction

MySQL error code 147: Lock table is full;  Restart program with a larger locktable

MySQL error code 148: Updates are not allowed under a read only transactions

MySQL error code 149: Lock deadlock; Retry transaction

MySQL error code 150: Foreign key constraint is incorrectly formed

MySQL error code 151: Cannot add a child row

MySQL error code 152: Cannot delete a parent row

Reference:

https://dev.mysql.com/doc/refman/5.6/en/error-messages-server.html