Linux – fast file system search with locate and updatedb

Typically

find

command is the most commonly used search utility in Linux. GNU find searches the directory tree rooted at each given starting-point by evaluating the given expression from left to right, according to the rules of precedence.

There is an alternative and fast way of searching for files and directories in Linux though and that is the

locate

command, and it goes hand in had with the

updatedb

utility which keeps an indexed database of the files in your system. The locate tools simply reads the database created by updatedb.

Installation –

sudo apt-get -y install mlocate           [Debian/Ubuntu]
sudo yum -y install mlocate               [CentOS/Redhat]

updatedb is usually has a daily cron job to update the default database(‘/var/lib/mlocate/mlocate.db’). To manually update the database, you can manually run the ‘updatedb’ command first. That will take a while depending on the number of files you have on your system, the last time updatedb ran or other file related changes.

First time – update the default database, run any of the below command depending on your requirements. Most likely, the first and/or third command is what you need.

updatedb
updatedb -U /some/path      # if interested in indexing specific directory that you will search frequently.
updatedb -v                 # verbose mode

Time to search
locate command is the utility to search for entries in a mlocate database.

Some examples –

locate cron         # any file or directory with cron in its name
locate -i cron      # case insensitive
locate -c cron      # only print number of found entries
locate -r 'cron$'   # regex - only files or directories with names ending in cron.
locate -r '/usr/.*ipaddress.*whl$'   # regex for eg. /usr/share/python-wheels/ipaddress-0.0.0-py2.py3-none-any.whl

locate can also print the statistics on count of files, directories, size used by updatedb default directory.

root@cloudclient:/tmp# locate -S
Database /var/lib/mlocate/mlocate.db:
	28,339 directories
	185,661 files
	11,616,040 bytes in file names
	4,481,938 bytes used to store database

Customizing updatedb
updatedb can be customized to output the search database to a different file than the default db, in addition to this we can change the directory to index other than the default root tree. We can then tell locate to use the custom db.

In the below example, I am indexing the files under home directory in /tmp/home.db database, and then run locate to use this custom DB. As you can see the number of files and directories is way lower and thus the search much faster although since it has to scan specific directory.

$ updatedb -U ~ -o /tmp/home.db
$ locate -d /tmp/home.db cron
$ locate -d /tmp/home.db -S
Database /tmp/home.db:
	3,530 directories
	29,943 files
	2,635,675 bytes in file names
	762,621 bytes used to store database

References –

https://linux.die.net/man/8/updatedb

https://linux.die.net/man/1/locate

C programming Language – Code snippets

C Programming Language, 2nd Edition

Compiling and running the sample codes using gcc :

gcc sample.c -o sample
./sample

Chapter 1 – Introductory tutorial : Input/output, characters, strings

0. Hello World!

#include<stdio.h>

int main()

{

  printf("Hello World\n");
  return 0;
}

1. Word counter

/* word counter */
#include<stdio.h>

#define  IN 1  //inside a word
#define  OUT   0  //outside a word

int main()

{
  int c,nl,nw,nc,state;
  state=OUT;
  nl=nw=nc=0;

  while((c=getchar())!=EOF)
   {
      ++nc;
      if(c=='\n') ++nl;
      if (c==' ' || c == '\n' || c=='\t') state=OUT;
      else if (state==OUT)
         {
           state=IN;
           ++nw;
          }
        }
      printf("lines=%d words=%d characters=%d\n",nl,nw,nc);
 return 0;
}

2. Convert Fahrenheit to Celsius

#include<stdio.h>

#define  LOWER 0
#define  UPPER 300
#define  STEP  20

int main()

  {
     int fahr;
     for (fahr = LOWER; fahr <= UPPER; fahr=fahr + STEP)
         printf("%3d %6.1f\n", fahr, (5.0/9.0)*(fahr-32));
     return 0;
 }

3. Count characters

/* character counter */
#include<stdio.h>

int main()

{
   short int nc;
   for(nc=0; getchar()!=EOF ; ++nc);
   printf("%d\n",nc);
   return 0;
}

4. Count characters by type – digits, white spaces etc.

/* count digits, white space, others */
#include<stdio.h>

int main()
 {
   int c,i,nwhite,nother;
   int ndigit[10];

   nwhite=nother=0;
   for(i=0; i < 10; ++i)  ndigit[i]=0;

   while((c=getchar())!=EOF)
     if(c>='0' && c<='9') ++ndigit[c-'0'];
     else if (c==' ' || c=='\n' || c=='\t') ++nwhite;
     else  ++nother;

   printf("digits=");
   for(i=0;i<10;++i) printf(" %d",ndigit[i]);
   printf(",white space=%d, other=%d\n",nwhite,nother);

return 0;
}

5. Copy input to output terminal

/* copy input to output 1 */
#include<stdio.h>

int main()

{
   int c;
   c = getchar();
   while ( c!=EOF)
     {
       putchar(c);
       c = getchar();
     }
 return 0;
}

6. Copy input to output terminal (shorter version)

/* copy input to output */
#include<stdio.h>

int main()
{
   int c;
   while((c=getchar())!=EOF)
     putchar(c);
   return 0;
}

7. Print longest line

#include<stdio.h>

#define  MAXLINE  1000  /* maximum input line length */

int getline(char line[], int maxline);
void copy(char to[], char from[]);

/* print the longest input line */

int main()

{
  int len;  /* current line length */
  int max;  /* maximum length seen so far */
  char line[MAXLINE];   /* current input line */
  char longest[MAXLINE];   /*longest line saved here */

  max=0;
  while((len=getline(line,MAXLINE))>0)
    if(len>max)
       { max=len;
         copy(longest,line);
       }
 if(max>0)  /*there was a line*/
  printf("Longest Line from input is:\n%s\n",longest);
  printf("Line length is: %d characters\n",max);

return 0;
}


/*getline: read a line into s, return length */

int getline(char s[], int lim)
{
  int c,i;
  for(i=0; i<lim-1 && (c=getchar())!=EOF && c!='\n'; ++i)
   s[i]=c;
 if(c=='\n') {
  s[i]=c;
  ++i;
  }
 s[i]='\0';
 return i;
}

/*copy: copy 'from' into 'to'; assume 'to' is big enough */

void copy(char to[], char from[])
{
  int i;
  i=0;
  while((to[i]=from[i]) != '\0')
    ++i;
}

8. Print longest line – improved

#include<stdio.h>

#define  MAXLINE  1000  /* maximum input line length */

int max;
char line[MAXLINE];
char longest[MAXLINE];

int getline(char line[], int maxline);
void copy(char to[], char from[]);

/* print the longest input line */

int main()

{
  int len;  /* current line length */
  extern int max; /* maximum length seen so far */
  extern char longest[MAXLINE];  /*longest line saved here */

  max=0;
  while((len=getline(line,MAXLINE))>0)
    if(len>max)
       { max=len;
         copy(longest,line);
       }
 if(max>0)  /*there was a line*/
  printf("Longest Line from input is:\n%s\n",longest);
  printf("Line length is: %d characters\n",max);

return 0;
}


/*getline: read a line into s, return length */

int getline(char s[], int lim)
{
  int c,i;
  extern char line[];
  for(i=0; i<lim-1 && (c=getchar())!=EOF && c!='\n'; ++i)
   s[i]=c;
 if(c=='\n') {
  s[i]=c;
  ++i;
  }
 s[i]='\0';
 return i;
}

/*copy: copy 'from' into 'to'; assume 'to' is big enough */

void copy(char to[], char from[])
{
  int i;
  extern char line[], longest[];
  i=0;
  while((to[i]=from[i]) != '\0')
    ++i;
}

9. Power function

#include<stdio.h>

int power(int m, int n);

/* test power function */

int main()

{
  int i;
  for(i=0; i<10; ++i)
   printf("%d %d %d\n",i,power(2,i),power(-3,i));
 return 0;
}

int power(int base, int n)
 {
   int i,p=1;
   for(i=1;i<=n;++i)
     p*=base;
   return p;
 }

10. Reverse string

#include<stdio.h>

#define  MAXLINE  1000

int size=0;
void reverse(char line[], int lim);
int main()

{ extern int size;
  char line[MAXLINE];
  reverse(line,MAXLINE);
  while(size) putchar(line[--size]);
  printf("\n");
  return 0;
}

void reverse(char line[], int lim)

{
 extern int size;
 int c;
 while((c=getchar())!=EOF ) line[size++]=c;
 line[size]='\0';

}

11. Get digits only

#include<stdio.h>

int atoi(char s[]);

int main()

{
  char str[]="12a4c5 ";
  int i=0,n=0;

 while(str[i]!='\0')
   {
      if(str[i]>='0' && str[i]<='9') { n=10*n+(str[i]-'0'); i++; }
      else
       { i++; continue; }
   }
 printf("number=%d\n",n);
 return 0;

 }

12. Fahrenheit to Celsius table

/* print Fahrenheit-Celsius table for fahr =0,20,40,....,300 */

#include<stdio.h>

int main()

{
  int fahr, celsius;
  int lower, upper, step;
  lower = 0;
  upper = 300;
  step = 20;

  fahr = lower;
  while( fahr <= upper)
   {
      celsius = 5 * (fahr - 32) / 9;
      printf("%d\t%d\n", fahr, celsius);
      fahr = fahr + step;
   }

return 0;
}

Reference –

https://www.amazon.com/Programming-Language-2nd-Brian-Kernighan/dp/0131103628#reader_0131103628

Ansible : rolling upgrades or updates.

Making a change to live servers in production is something which has to be done with extreme care and planning. Several deployment types such as blue/green, canary, rolling update are in use today to minimize user impact. Ansible can be used to orchestrate a zero-downtime rolling change to a service.

A typical upgrade of an application, such as patching, might go like this –

1. disable monitoring alerts for a node
2. disable or pull out from load balancer
3. make changes to server
4. Reboot node
5. wait for node to be UP and do sanity check
6. put node back to load balancer
7. turn on monitoring of node

Rinse and repeat.

Ansible would be a great choice in orchestrating above steps. Let us start with an inventory of web servers, a load balancer and a monitoring node with nagios –

[webservers]
web1.example.net
web2.example.net
web3.example.net
web4.example.net
web5.example.net

[balancer]
haproxy.example.net

[monitoring]
nagios.example.net

The web servers are running apache2, and we will patch apache and the kernel. For the patch to take effect, the servers need to be recycled. We will perform the patching one node at a time, wait for the node to be healthy and go to the next. The first portion of our playbook would be something like this –

---
- hosts: webservers
  serial: 1

  pre_tasks:
  - name: Stop apache service
    service: name=httpd state=stopped

  tasks:
  - name: update apache
    yum: name=httpd state=latest
  - name: Update Kernel
    yum: name=kernel state=latest
  - name: Reboot server
    shell: /sbin/reboot -r +1

  post_tasks:
  - name: Wait for webserver to come up
    wait_for: host={{ inventory_hostname }} port=80 state=started delay=65 timeout=300
    delegate_to: 127.0.0.1

I haven’t included the playbook tasks for disabling/enabling monitoring as well as removing/adding node to the load balancer. The procedures might differ depending on what type of monitoring system or load balancer technology you are using. In addition to this, the sanity check show is a simple port 80 probing, in reality a much more sophisticated validation can be done.

References –

http://docs.ansible.com/ansible/latest/playbooks_delegation.html

http://docs.ansible.com/ansible/latest/guide_rolling_upgrade.html

How to terminate or cancel an unresponsive ssh session.

While connected to a remote host through an ssh connection using Putty or other ssh clients, your client might freeze and not respond to any keyboard activity. In order to force an exit, there is a “secret” keyboard shortcut – Enter~. [ Enter followed by ~ followed by .(dot) ]

[daniel@kauai ~]$ ssh daniel@linuxfreelancer.com

Hostname..........: svm1010.xenvps-server.net
Uptime............: 21:25:34 up 14654 days, 15:00,  2 users,  load average: 0.09, 0.19, 0.17
Server IP.........: 173.230.241.181
Operating System..: Ubuntu 10.04.4 LTS
Username..........: daniel

daniel@svm1010:~$ uname
Linux

***Press*** Enter~.  {Enter key, followed by '~', followed by '.'}

daniel@svm1010:~$ Connection to linuxfreelancer.com closed.

[daniel@kauai ~]$ 

After you press the Enter~. the connection will be aborted and your session will return to the connection originating client shell.

Alternatively, you can background the ssh session with Enter~Ctrl+Z and then foreground it with fg command.

References –

https://serverfault.com/questions/283129/why-do-consoles-sometimes-hang-forever-when-ssh-connection-breaks

ipython tutorial and how to delete sensitive data from history

ipython is program which allows you to run python code in an interactive shell. Although Python itself when run from CLI opens an interactive shell as well, ipython is much more powerful and greatly improves your productivity. Some of the things you can do with ipython but not the default python shell is command or code and file name completion, view history, copy/paste a single or multiline code, nicely colored help with in the shell, run Linux commands such as ls or cat, scroll up/down to previous commands, automatically adds spaces after you press enter, etc.

Installation

pip install ipython

Quick demo
Start ipython by typing the

ipython

command in your CLI –

daniel@lindell:/tmp$ ipython
Python 2.7.12 (default, Nov 19 2016, 06:48:10) 
Type "copyright", "credits" or "license" for more information.

IPython 5.4.1 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: print('ipython')
ipython

In [2]: 

With in the ipython interactive shell you can run any python code, let us walk through some examples –

  In [1]: x=2

In [2]: x
Out[2]: 2

In [3]: mylist=[1,2,3,4,5]

In [4]: [i**3 for i in mylist]
Out[4]: [1, 8, 27, 64, 125]

In [5]: with open('/etc/hosts') as fp:
   ...:     for line in fp:
   ...:         if 'localhost' in line:
   ...:             print line
   ...:             
127.0.0.1	localhost

::1     ip6-localhost ip6-loopback


In [6]: ls /opt/
ansible/  google/  vagrant/

In [7]: 

Go back to previously typed commands / History
With ipython, you can either press the UP arrow key or type

 history 

command to view history. ipython keeps session history as well as all input and output lines in a SQLite file which is located in

~/.ipython/profile_default/history.sqlite 

You can view and modify this file using

sqlite3

tool –

daniel@lindell:/tmp$ sqlite3 ~/.ipython/profile_default/history.sqlite
SQLite version 3.11.0 2016-02-15 17:29:24
Enter ".help" for usage hints.
sqlite> .schema
CREATE TABLE sessions (session integer
                        primary key autoincrement, start timestamp,
                        end timestamp, num_cmds integer, remark text);
CREATE TABLE history
                (session integer, line integer, source text, source_raw text,
                PRIMARY KEY (session, line));
CREATE TABLE output_history
                        (session integer, line integer, output text,
                        PRIMARY KEY (session, line));
sqlite> 

Deleting sensitive data from history
You can delete any line from history by using SQL. First use SELECT statement to find the line number and then use DELETE statement to delete it. In this example, we are deleting line number 10 from the history table –

sqlite> select * from history;
sqlite> .schema history
CREATE TABLE history
                (session integer, line integer, source text, source_raw text,
                PRIMARY KEY (session, line));
sqlite> delete from history where line=10;

References –
https://ipython.org/
http://www.sqlitetutorial.net/sqlite-delete/

Mount partition from a raw disk image

In this post, I will share how you can mount a raw disk image such as an image generated with dd. Raw disk image or RAW Image Format is a bit-for-bit copy of disk data, without any metadata information on files. In Linux, dd is a popular tool for data transfer by duplicating entire disk for instance. Let us create a disk image of a mount with an EXT3 file system –

[root@kauai src]# dd if=/dev/sdb of=disk.img 

7233761+0 records in
7233760+0 records out
3703685120 bytes (3.7 GB) copied, 236.166 s, 15.7 MB/s

[root@kauai src]# ls -alh disk.img 
-rw-r--r--. 1 root root 3.5G Jan 15 18:44 disk.img

We have copied a mount with multiple files into a single disk.img file which we can copy to another system. Now let us examine the raw disk layout, that we can use to mount as a file system –

[root@kauai src]# fdisk -lu disk.img 
You must set cylinders.
You can do this from the extra functions menu.

Disk disk.img: 0 MB, 0 bytes
124 heads, 62 sectors/track, 0 cylinders, total 0 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xdebbbd93

   Device Boot      Start         End      Blocks   Id  System
disk.img          630416      945623      157604   83  Linux

As we can see the raw disk has 512 byte size sectors and it starts at offset 630416, given this information we can use mount command to mount the disk image –

[root@kauai src]# mount -o loop,offset=$((630416*512)) disk.img /mnt/hdisk/
[root@kauai src]# ls -al /mnt/hdisk/
total 37
drwxr-xr-x. 3 root root  1024 Jan 15 18:39 .
drwxr-xr-x. 4 root root  4096 Nov 17 20:04 ..
-rw-r--r--. 1 root root    15 Jan 15 18:39 file21
-rw-r--r--. 1 root root    15 Jan 15 18:39 file22
-rw-r--r--. 1 root root    15 Jan 15 18:39 file23
-rw-r--r--. 1 root root    15 Jan 15 18:39 file24
-rw-r--r--. 1 root root    15 Jan 15 18:39 file25
-rw-r--r--. 1 root root    15 Jan 15 18:39 file26
-rw-r--r--. 1 root root    15 Jan 15 18:39 file27
-rw-r--r--. 1 root root    15 Jan 15 18:39 file28
-rw-r--r--. 1 root root    15 Jan 15 18:39 file29
-rw-r--r--. 1 root root    15 Jan 15 18:39 file30
drwx------. 2 root root 12288 Jan 15 18:37 lost+found

[root@kauai src]# cat /mnt/hdisk/file26 
File number 26

Here we were able to mount the disk image and be able to read the content of one of the text files.

References –

https://en.wikipedia.org/wiki/Dd_(Unix)

https://linux.die.net/man/8/mount