Kernel panic in linux

Below are the steps to recover the kernal panic issue in Oracle Exadata machines

Take the diagnostics ISO from the working server and map it in faulty server then boot that server using that ISO and follow the below steps

mkdir /mnt/LVDbSys1
mkdir /mnt/LVDbSys2

mount /dev/mapper/VGExaDb-LVDbSys1 /mnt/LVDbSys1
mount /dev/mapper/VGExaDb-LVDbSys2 /mnt/LVDbSys2

vi   /mnt/LVDbSys1/var/log/cellos/dbnodeupdate.log
vi   /mnt/LVDbSys2/var/log/cellos/dbnodeupdate.log

go to end of the line - compare the time stamp. which is latest.

e2label /dev/mapper/VGExaDb-LVDbSys1 - check the label
e2label /dev/mapper/VGExaDb-LVDbSys2

ls -al /mnt/LVDbSys2

e2label /dev/mapper/VGExaDb-LVDbSys2 ''
e2label /dev/mapper/VGExaDb-LVDbSys1 DBSYS

e2label /dev/mapper/VGExaDb-LVDbSys1- check the label
e2label /dev/mapper/VGExaDb-LVDbSys2

mkdir /mnt/boot
ls -lrt /mnt/boot
mount /dev/sda1 /mnt/boot
df -h
tar cvf /mnt/LVDbSys1/failed_boot.tar /mnt/boot
ls -lrt /mnt/LVDbSys1

rm -rf /mnt/boot/*
tar -pjxvf /mnt/LVDbSys1/boot_backup.tbz -C /mnt
echo "(hd0) /dev/sda" > /tmp/grubmap

/sbin/grub --device-map=/tmp/grubmap

find /I_am_hd_boot

root (hd0,0)

setup (hd0)

quit

sync

umount /mnt/LVDbSys1

umount /mnt/LVDbSys2

umount /mnt/boot
disconnect the CD-ROM image from the Devices menu

Gather hardware information through command line

Commands to get CPU info:

1. /proc/cpuinfo

The /proc/cpuinfo file contains details about individual cpu cores. Output its contents with less or cat.

$ less /proc/cpuinfo
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 23
model name      : Intel(R) Core(TM)2 Quad CPU    Q8400  @ 2.66GHz
stepping        : 10
microcode       : 0xa07
cpu MHz         : 1998.000
cache size      : 2048 KB
physical id     : 0
siblings        : 4
core id         : 0
cpu cores       : 4
apicid          : 0
initial apicid  : 0
fpu             : yes
fpu_exception   : yes
cpuid level     : 13
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx lm constant_tsc arch_perfmon pebs bts rep_good nopl aperfmperf pni dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm sse4_1 xsave lahf_lm dtherm tpr_shadow vnmi flexpriority
bogomips        : 5303.14
clflush size    : 64
cache_alignment : 64
address sizes   : 36 bits physical, 48 bits virtual
power management:

Every processor or core is listed separately the various details about speed, cache size and model name are included in the description.

To count the number of processing units use grep with wc

$ cat /proc/cpuinfo | grep processor | wc -l
4

The number of processors shown by /proc/cpuinfo might not be the actual number of cores on the processor. For example a processor with 2 cores and hyperthreading would be reported as a processor with 4 cores.

To get the actual number of cores, check the core id for unique values

$ cat /proc/cpuinfo | grep 'core id'
core id         : 0
core id         : 2
core id         : 1
core id         : 3

So there are 4 different core ids. This indicates that there are 4 actual cores.

2. lscpu

lscpu is a small and quick command that does not need any options. It would simply print the cpu hardware details in a user-friendly format.

$ lscpu
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                4
On-line CPU(s) list:   0-3
Thread(s) per core:    1
Core(s) per socket:    4
Socket(s):             1
NUMA node(s):          1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 23
Stepping:              10
CPU MHz:               1998.000
BogoMIPS:              5303.14
Virtualization:        VT-x
L1d cache:             32K
L1i cache:             32K
L2 cache:              2048K
NUMA node0 CPU(s):     0-3

3. hardinfo

Hardinfo is a gtk based gui tool that generates reports about various hardware components. But it can also run from the command line only if there is no gui display available.

$ hardinfo | less

It would produce a large report about many hardware parts, by reading files from the /proc directory. The cpu information is towards the beginning of the report. The report can also be written to a text file.

Hardinfo also performs a few benchmark tests taking a few minutes before the report is displayed.

4. lshw

The lshw command can display limited information about the cpu. lshw by default shows information about various hardware parts, and the '-class' option can be used to pickup information about a specific hardware part.

$ sudo lshw -class processor
  *-cpu                   
       description: CPU
       product: Intel(R) Core(TM)2 Quad CPU    Q8400  @ 2.66GHz
       vendor: Intel Corp.
       physical id: 0
       bus info: cpu@0
       version: Intel(R) Core(TM)2 Quad CPU    Q8400  @ 2.66GHz
       slot: LGA 775
       size: 1998MHz
       capacity: 4GHz
       width: 64 bits
       clock: 333MHz
       capabilities: fpu fpu_exception wp vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx x86-64 constant_tsc arch_perfmon pebs bts rep_good nopl aperfmperf pni dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm sse4_1 xsave lahf_lm dtherm tpr_shadow vnmi flexpriority cpufreq

The vendor, model and speed of the processor are being shown correctly. However it is not possible to deduce the number of cores on the processor from the above output.

5. nproc

The nproc command just prints out the number of processing units available. Note that the number of processing units might not always be the same as number of cores.

$ nproc
4

6. dmidecode

The dmidecode command displays some information about the cpu, which includes the socket type, vendor name and various flags.

$ sudo dmidecode -t 4
# dmidecode 2.12
SMBIOS 2.4 present.

Handle 0x0000, DMI type 4, 35 bytes
Processor Information
        Socket Designation: LGA 775
        Type: Central Processor
        Family: Pentium D
        Manufacturer: Intel(R) Corporation
        ID: 7A 06 01 00 FF FB EB BF
        Signature: Type 0, Family 6, Model 23, Stepping 10
        Flags:
                FPU (Floating-point unit on-chip)
                VME (Virtual mode extension)
                DE (Debugging extension)
                PSE (Page size extension)
                TSC (Time stamp counter)
                MSR (Model specific registers)
                PAE (Physical address extension)
                MCE (Machine check exception)
                CX8 (CMPXCHG8 instruction supported)
                APIC (On-chip APIC hardware supported)
                SEP (Fast system call)
                MTRR (Memory type range registers)
                PGE (Page global enable)
                MCA (Machine check architecture)
                CMOV (Conditional move instruction supported)
                PAT (Page attribute table)
                PSE-36 (36-bit page size extension)
                CLFSH (CLFLUSH instruction supported)
                DS (Debug store)
                ACPI (ACPI supported)
                MMX (MMX technology supported)
                FXSR (FXSAVE and FXSTOR instructions supported)
                SSE (Streaming SIMD extensions)
                SSE2 (Streaming SIMD extensions 2)
                SS (Self-snoop)
                HTT (Multi-threading)
                TM (Thermal monitor supported)
                PBE (Pending break enabled)
        Version: Intel(R) Core(TM)2 Quad CPU    Q8400  @ 2.66GHz
        Voltage: 1.6 V
        External Clock: 333 MHz
        Max Speed: 4000 MHz
        Current Speed: 2666 MHz
        Status: Populated, Enabled
        Upgrade: Socket LGA775
        L1 Cache Handle: 0x0003
        L2 Cache Handle: 0x0001
        L3 Cache Handle: Not Provided
        Serial Number: Not Specified
        Asset Tag: Not Specified
        Part Number: Not Specified

7. cpuid

The cpuid command fetches CPUID information about Intel and AMD x86 processors.

The program can be installed with apt on ubuntu

$ sudo apt-get install cpuid

And here is sample output

$ cpuid

.....

Vendor ID: "GenuineIntel"; CPUID level 13

Intel-specific functions:
Version 0001067a:
Type 0 - Original OEM
Family 6 - Pentium Pro
Model 7 - Pentium III/Pentium III Xeon - external L2 cache
Stepping 10
Reserved 4

Extended brand string: "Intel(R) Core(TM)2 Quad CPU    Q8400  @ 2.66GHz"
CLFLUSH instruction cache line size: 8
Initial APIC ID: 2
Hyper threading siblings: 4

Feature flags bfebfbff:
FPU    Floating Point Unit
VME    Virtual 8086 Mode Enhancements
DE     Debugging Extensions
PSE    Page Size Extensions
TSC    Time Stamp Counter
MSR    Model Specific Registers
PAE    Physical Address Extension
MCE    Machine Check Exception
CX8    COMPXCHG8B Instruction
APIC   On-chip Advanced Programmable Interrupt Controller present and enabled
SEP    Fast System Call
MTRR   Memory Type Range Registers
PGE    PTE Global Flag
MCA    Machine Check Architecture
CMOV   Conditional Move and Compare Instructions
FGPAT  Page Attribute Table
PSE-36 36-bit Page Size Extension
CLFSH  CFLUSH instruction
DS     Debug store
ACPI   Thermal Monitor and Clock Ctrl
MMX    MMX instruction set
FXSR   Fast FP/MMX Streaming SIMD Extensions save/restore
SSE    Streaming SIMD Extensions instruction set
SSE2   SSE2 extensions
SS     Self Snoop
HT     Hyper Threading
TM     Thermal monitor
31     reserved

.....

8. inxi

Inxi is a script that uses other programs to generate a well structured easy to read report about various hardware components on the system. Check out the full tutorial on inxi.

$ sudo apt-get install inxi

Print out cpu/processor related information

$ inxi -C
CPU:       Quad core Intel Core2 Quad CPU Q8400 (-MCP-) cache: 2048 KB flags: (lm nx sse sse2 sse3 sse4_1 ssse3 vmx) 
           Clock Speeds: 1: 1998.00 MHz 2: 1998.00 MHz 3: 1998.00 MHz 4: 1998.00 MHz

9.Hwloc / lstopo

Hwloc (Portable hardware locality) is a small utility that reports the structure of the processor in a neat visual diagram. The diagram shows the number of cores, hyperthreads and cache size. A single diagram tells it all.

$ sudo apt-get install hwloc
$ hwloc

The above diagram clearly shows -

Total L2 Cache - 4096 KB - 4MB
Total Cores - 4
Processing unit per core - 1

Hyper-threaded processor
For a hyperthreaded processor, the hwloc output diagram could look like this

The diagram indicates

Total L3 Cache - 8MB
Total Cores - 4
Processing units per Core - 2 [hyper threading]

Awk Command in Unix:

Awk tool:

Awk is one of the most powerful tools in Unix used for processing the rows and columns in a file. Awk has built in string functions and associative arrays. Awk supports most of the operators, conditional blocks, and loops available in C language.

One of the good things is that you can convert Awk scripts into Perl scripts using a2p utility.

The basic syntax of AWK:

awk 'BEGIN {start_action} {action} END {stop_action}' filename

cat /etc/passwd | awk -F : '{ if($3 >= '500') print $1;}' | more

Above command will only show normal users, not systems user

Here the actions in the begin block are performed before processing the file and the actions in the end block are performed after processing the file. The rest of the actions are performed while processing the file.

Examples:

Create a file input_file with the following data. This file can be easily created using the output of ls -l.

-rw-r--r-- 1 center center  0 Dec  8 21:39 p1
-rw-r--r-- 1 center center 17 Dec  8 21:15 t1
-rw-r--r-- 1 center center 26 Dec  8 21:38 t2
-rw-r--r-- 1 center center 25 Dec  8 21:38 t3
-rw-r--r-- 1 center center 43 Dec  8 21:39 t4
-rw-r--r-- 1 center center 48 Dec  8 21:39 t5

From the data, you can observe that this file has rows and columns. The rows are separated by a new line character and the columns are separated by a space characters. We will use this file as the input for the examples discussed here. 

1. awk '{print $1}' input_file

Here $1 has a meaning. $1, $2, $3... represents the first, second, third columns... in a row respectively. This awk command will print the first column in each row as shown below.

-rw-r--r--
-rw-r--r--
-rw-r--r--
-rw-r--r--
-rw-r--r--
-rw-r--r--

To print the 4th and 6th columns in a file use awk '{print $4,$5}' input_file

Here the Begin and End blocks are not used in awk. So, the print command will be executed for each row it reads from the file. In the next example we will see how to use the Begin and End blocks.

2. awk 'BEGIN {sum=0} {sum=sum+$5} END {print sum}' input_file

This will prints the sum of the value in the 5th column. In the Begin block the variable sum is assigned with value 0. In the next block the value of 5th column is added to the sum variable. This addition of the 5th column to the sum variable repeats for every row it processed. When all the rows are processed the sum variable will hold the sum of the values in the 5th column. This value is printed in the End block.

3. In this example we will see how to execute the awk script written in a file. Create a file sum_column and paste the below script in that file

#!/usr/bin/awk -f
BEGIN {sum=0} 
{sum=sum+$5} 
END {print sum}

Now execute the the script using awk command as 

awk -f sum_column input_file.

This will run the script in sum_column file and displays the sum of the 5th column in the input_file.

4. awk '{ if($9 == "t4") print $0;}' input_file

This awk command checks for the string "t4" in the 9th column and if it finds a match then it will print the entire line. The output of this awk command is

-rw-r--r-- 1 pcenter pcenter 43 Dec  8 21:39 t4

5. awk 'BEGIN { for(i=1;i<=5;i++) print "square of", i, "is",i*i; }'

This will print the squares of first numbers from 1 to 5. The output of the command is

square of 1 is 1
square of 2 is 4
square of 3 is 9
square of 4 is 16
square of 5 is 25

Notice that the syntax of “if” and “for” are similar to the C language.

Awk Built in Variables: 

You have already seen $0, $1, $2... which prints the entire line, first column, second column... respectively. Now we will see other built in variables with examples. 

FS - Input field separator variable:

So far, we have seen the fields separted by a space character. By default Awk assumes that fields in a file are separted by space characters. If the fields in the file are separted by any other character, we can use the FS variable to tell about the delimiter. 

6. awk 'BEGIN {FS=":"} {print $2}' input_file
OR
awk -F: '{print $2} input_file

This will print the result as 

39 p1
15 t1
38 t2
38 t3
39 t4
39 t5

OFS - Output field separator variable: 

By default whenever we printed the fields using the print statement the fields are displayed with space character as delimiter. For example

7. awk '{print $4,$5}' input_file

The output of this command will be

center 0
center 17
center 26
center 25
center 43
center 48

We can change this default behavior using the OFS variable as

awk 'BEGIN {OFS=":"} {print $4,$5}' input_file

center:0
center:17
center:26
center:25
center:43
center:48

Note: print $4,$5 and print $4$5 will not work the same way. The first one displays the output with space as delimiter. The second one displays the output without any delimiter. 

NF - Number of fileds variable:

The NF can be used to know the number of fields in line

8. awk '{print NF}' input_file
This will display the number of columns in each row.

NR - number of records variable: 
The NR can be used to know the line number or count of lines in a file.

9. awk '{print NR}' input_file
This will display the line numbers from 1.

10. awk 'END {print NR}' input_file
This will display the total number of lines in the file.

Files and folders with ls

This article aims at deep insight of file listing command in Linux with relevant examples.

$ls
$echo *

above commands will list the files and folders

$echo */

will list only directories

list all the files within a directory including hidden files aka (.) dot files

# ls -a

 list all the files within a directory including hidden files, but do not list implied ‘.’ and ‘..’

#ls -A

Print the content of a directory in long format listing

#ls -l

Example:

drwxr-xr-x  5 muthu linuxinterest      4096 Sep 30 11:31 Binary

Here, drwxr-xr-x is file permission for owner, group and world. Owner has Read(r), Write(w) and Execute(x) permission. The group to which this file belongs has Read(r) and Execute(x) permission but not Write(w) permission, same permission implies for the world that have access to this file.

The Initial ‘d‘ means its a Directory.
Number ‘5‘ represents Symbolic Link.
The File Binary belongs to user muthu and group linuxinterest      .
Sep 30 11:31 represents the date and time it was last modified

print the content of directory in long format listing, showing hidden/dot files

#ls -la

Setting up the alias for 'ls -l' as 'll'

Add alias ll='ls -l'to .bashrc

logout and login then try by issueing the commad ll

listing the name of the files without the name of its owner, when used with switch (-g)

#ls -g

listing the name of files in long listing format without the name of group it belongs, when used with switch (-G) along with switch (-l).

#ls -Gl

Print the size of files and folders in the current directory, in human readable format

#ls -hl or #ls -hs (small h and small s)

if switch (-h) output size in power of 1024, that is standard, What else power values are supported in ls command

There exist a switch -si which is similar to switch -h. The only difference is switch -si uses power of 1000 unlike switch -h which uses the power of 1024.

#ls -si

print the contents of a directory separated with comma

#ls -m

Example:

 muthu, kumar, linux, test

Print the contect in reverse order

#ls -rl

print the sub-directories recursively

#ls -R

sort the files based upon the size

#ls -S

To sort the files based upon size in descending order with the smallest file listed at first and largest at last

#ls -Sr

 List the contents of a directory with no additional information appearing one file per line

acheieved this by using one 1

#ls -l

Example:

muthu
kumar
test
linux

Find with -exec option

Difference between {} \; and {} \+ and | xargs

Below 3 commands run and output same result but the first command takes a little time and the format is also little different.

find . -type f -exec file {} \;
find . -type f -exec file {} \+
find . -type f | xargs file

It's because 1st one runs the file command for every file coming from the find command. So, basically it runs as:

file file1.txt
file file2.txt

But latter 2 find with -exec commands run file command once for all files like below:

file file1.txt file2.txt

Following command will move found files to another location.

find . -type f -iname '*.cpp' -exec mv {} ./test/ \;

To find the files in the current directory that contain the text "chrome"

find . -exec grep chrome {} \;

Delete Files Older Than x Days on Linux

find /path/to/files* -mtime +5 -exec rm {} \;

find /path/to/files* -mtime +5 -exec ls -lrt {} \;

cd /var/tmp && find stuff -mtime +90 -exec /bin/rm {} \+

Note that there are spaces between rm, {}, and \;

• -mtime +60 means you are looking for a file modified 60 days ago.
• -mtime -60 means less than 60 days.
• -mtime 60 If you skip + or - it means exactly 60 days.

Display content of file on screen that were last modified 60 days ago, use
$ find /home/you -iname "*.txt" -mtime -60 -exec cat {} \;

Count total number of files using wc command
$ find /home/you -iname "*.txt" -mtime -60 | wc -l

You can also use access time to find out pdf files. Following command will print the list of all pdf file that were accessed in exactly last 60th  day,
$ find /home/you -iname "*.pdf" -mtime 60 -print

Print the file names which contains bills:

find . -name '*bills*' -print

this prints all the files

./may/batch_bills_123.log
./april/batch_bills_456.log

List the files based on the size

ls -lS /path/to/folder/

Capital S.

This will sort files in size

To exclude directories:

ls -lS | grep -v '^d'

List only regular files 

ls -lS | grep '^-'

Filter with specified size:

find . -type f -size +100k | grep '.txt.'

find . -type f -size +100k | grep '.png\|.jpg'

The above could also be rewritten as

find . -type f -size +100k -name "*.png" -o -name "*.jpg"

To list all files over 20MB in the current directory 

find . -size +20M

To get top 10 largest files:

ls -halt |head

grep search options

Description

grep searches the named input FILEs (or standard input if no files are named, or if a single dash ("-") is given as the file name) for lines containing a match to the given PATTERN. By default, grep prints the matching lines.

In addition, three variant programs egrep, fgrep and rgrep are available:

egrep is the same as grep -E.
fgrep is the same as grep -F.
rgrep is the same as grep -r.

Search 2 different words by using grep:

$ egrep -w 'word1|word2' /path/to/file

search more words by using below commands

ls -lrt | grep -ir "fillter\|new\|php"

Options in the grep;

^ – beginning of the line
c – letter c
c – letter c
: – colon
$ – end of line
v, --invert-match Invert the sense of matching, to select non-matching lines
-y The same as -i.