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Thursday, September 16, 2010

Common SSH Commands or Linux Shell Commands

SSH is a necessary tool in any server admin’s arsenal. It is the main way you will be interacting with your servers whenever you are performing system tasks such as install, updates, etc. SSH allows you to remotely connect to your server and command it via the shell. Below is a list of commands you will use commonly while working with your server.
I prefer the popular SSH client Putty, but any SSH client will do.

Common SSH Commands or Linux Shell Commands,
 
ls : list files/directories in a directory, comparable to dir in windows/dos.
ls -al : shows all files (including ones that start with a period), directories, and details attributes for each file.

cd : change directory · · “cd /home/username/” will navigate to /home/username/ directory
cd ~ : go to your home directory
cd - : go to the last directory you were in
cd .. : go up a directory cat : print file contents to the screen
cat filename.txt : cat the contents of filename.txt to your screen

chmod: changes file access permissions
The set of 3 go in this order from left to right:
USER – GROUP – EVERONE
0 = —  No permission
1 = –X  Execute only
2 = -W-  Write only
3 = -WX  Write and execute
4 = R–  Read only
5 = R-X  Read and execute
6 = RW-  Read and write
7 = RWX  Read, write and execute
Usage:
chmod numberpermissions filename
chmod 000 : No one can access
chmod 644: Usually for HTML pages
chmod 755: Usually for CGI scripts

chown: changes file ownership permissions
The set of 2 go in this order from left to right:
USER – GROUP
chown root myfile.txt : Changes the owner of the file to root
chown root.root myfile.txt : Changes the owner and group of the file to root

tail : like cat, but only reads the end of the file
tail /var/log/messages : see the last 20 (by default) lines of /var/log/messages
tail -f /var/log/messages : watch the file continuously, while it’s being updated
tail -200 /var/log/messages : print the last 200 lines of the file to the screen

more : like cat, but opens the file one screen at a time rather than all at once
more /etc/userdomains : browse through the userdomains file. hit Spaceto go to the next page, q to quit

pico : friendly, easy to use file editor
pico /home/burst/public_html/index.html : edit the index page for the user’s website.


File Editing with VI ssh commands
 
vi : another editor, tons of features, harder to use at first than pico
vi /home/burst/public_html/index.html : edit the index page for the user’s website.
Whie in the vi program you can use the following useful commands, you will need to hit SHIFT + : to go into command mode
:q! : This force quits the file without saving and exits vi
:w : This writes the file to disk, saves it
:wq : This saves the file to disk and exists vi
:LINENUMBER : EG :25 : Takes you to line 25 within the file
:$ : Takes you to the last line of the file
:0 : Takes you to the first line of the file

grep : looks for patterns in files
grep root /etc/passwd : shows all matches of root in /etc/passwd
grep -v root /etc/passwd : shows all lines that do not match root

ln : create’s “links” between files and directories
ln -s /usr/local/apache/conf/httpd.conf /etc/httpd.conf : Now you can edit /etc/httpd.conf rather than the original. changes will affect the orginal, however you can delete the link and it will not delete the original.

last : shows who logged in and when
last -20 : shows only the last 20 logins
last -20 -a : shows last 20 logins, with the hostname in the last field

w : shows who is currently logged in and where they are logged in from.
who : This also shows who is on the server in an shell.

netstat : shows all current network connections.
netstat -an : shows all connections to the server, the source and destination ips and ports.
netstat -rn : shows routing table for all ips bound to the server.

top : shows live system processes in a nice table, memory information, uptime and other useful info. This is excellent for managing your system processes, resources and ensure everything is working fine and your server isn’t bogged down.
top then type Shift + M to sort by memory usage or Shift + P to sort by CPU usage

ps: ps is short for process status, which is similar to the top command. It’s used to show currently running processes and their PID.
A process ID is a unique number that identifies a process, with that you can kill or terminate a running program on your server (see kill command).
ps U username : shows processes for a certain user
ps aux : shows all system processes
ps aux –forest : shows all system processes like the above but organizes in a hierarchy that’s very useful!

touch : create an empty file
touch /home/burst/public_html/404.html : create an empty file called 404.html in the directory /home/burst/public_html/

file : attempts to guess what type of file a file is by looking at it’s content.
file * : prints out a list of all files/directories in a directory

du : shows disk usage.
du -sh : shows a summary, in human-readble form, of total disk space used in the current directory, including subdirectories.
du -sh * : same thing, but for each file and directory. helpful when finding large files taking up space.

wc : word count
wc -l filename.txt : tells how many lines are in filename.txt

cp : copy a file
cp filename filename.backup : copies filename to filename.backup
cp -a /home/burst/new_design/* /home/burst/public_html/ : copies all files, retaining permissions form one directory to another.
cp -av * ../newdir : Copies all files and directories recurrsively in the current directory INTO newdir

mv : Move a file command
mv oldfilename newfilename : Move a file or directory from oldfilename to newfilename

rm : delete a file
rm filename.txt : deletes filename.txt, will more than likely ask if you really want to delete it
rm -f filename.txt : deletes filename.txt, will not ask for confirmation before deleting.
rm -rf tmp/ : recursively deletes the directory tmp, and all files in it, including subdirectories. BE VERY CAREFULL WITH THIS COMMAND!!!

TAR
: Creating and Extracting .tar.gz and .tar files
tar -zxvf file.tar.gz : Extracts the file
tar -xvf file.tar : Extracts the file
tar -cf archive.tar contents/ : Takes everything from contents/ and puts it into archive.tar
gzip -d filename.gz : Decompress the file, extract it

ZIP Files:  Extracting .zip files shell command
unzip file.zip


Firewall - iptables commands
iptables -I INPUT -s IPADDRESSHERE -j DROP : This command stops any connections from the IP address
iptables -L : List all rules in iptables
iptables -F : Flushes all iptables rules (clears the firewall)
iptables –save : Saves the currenty ruleset in memory to disk
service iptables restart : Restarts iptables


Apache Shell Commands
httpd -v : Outputs the build date and version of the Apache server.
httpd -l : Lists compiled in Apache modules
httpd status : Only works if mod_status is enabled and shows a page of active connections
service httpd restart : Restarted Apache web server


MySQL Shell Commands
mysqladmin processlist : Shows active mysql connections and queries
mysqladmin drop databasenamehere : Drops/deletes the selected database
mysqladmin create databasenamehere : Creates a mysql database
Restore MySQL Database Shell Command
mysql -u username -p password databasename < databasefile.sql : Restores a MySQL database from databasefile.sql
Backup MySQL Database Shell Command
mysqldump -u username -p password databasename > databasefile.sql : Backup MySQL database to databasefile.sql

kill: terminate a system process
kill -9 PID EG: kill -9 431
kill PID
EG: kill 10550
Use top or ps ux to get system PIDs (Process IDs)
EG:
PID TTY TIME COMMAND
10550 pts/3 0:01 /bin/csh
10574 pts/4 0:02 /bin/csh
10590 pts/4 0:09 APP
Each line represents one process, with a process being loosely defined as a running instance of a program. The column headed PID (process ID) shows the assigned process numbers of the processes. The heading COMMAND shows the location of the executed process.


Putting commands together
Often you will find you need to use different commands on the same line. Here are some examples. Note that the | character is called a pipe, it takes date from one program and pipes it to another.
> means create a new file, overwriting any content already there.
>> means tp append data to a file, creating a newone if it doesn not already exist.
< send input from a file back into a command.
grep User /usr/local/apache/conf/httpd.conf |more
This will dump all lines that match User from the httpd.conf, then print the results to your screen one page at a time.
last -a > /root/lastlogins.tmp
This will print all the current login history to a file called lastlogins.tmp in /root/
tail -10000 /var/log/exim_mainlog |grep domain.com |more
This will grab the last 10,000 lines from /var/log/exim_mainlog, find all occurances of domain.com (the period represents ‘anything’,
– comment it out with a so it will be interpretted literally), then send it to your screen page by page.
netstat -an |grep :80 |wc -l
Show how many active connections there are to apache (httpd runs on port 80)
mysqladmin processlist |wc -l
Show how many current open connections there are to mysql

Original: http://www.apache.com/ssh-cheat-sheet/

Wednesday, September 15, 2010

All about CRONTAB


What is Crontab?

Crontab (CRON TABle)is a program that manipulates the CRON daemon, making it easy for users to schedule task and run programs/commands at pre determined periods of time. Crontab can also be considered a file witch contains commands that will be run by the system as the user that owns the crontab file.

What is the purpose of Crontab?

Cron is designed to maintain a list of commands that the system needs to run at a given time interval.
For example if you have a script that generates statistics and needs to be run every couple of hours or everyday cron can do it for you. Or for example if you have a script that sends a newsletter every month you can use cron to run the script that sends the newsletter every month or week.

Crontab commands

When your logged in to your server you can use program cron using the following commands:
  • crontab -l
    Lists the current cron jobs
  • crontab -e
    Edit your current crontab file and ad/remove/edit crontab tasks.
  • crontab -r
    Remove the crontab file.
  • crontab -v
    Displays the last time you edited your crontab file.

The crontab file – components of crontab

When you enter the edit mode (crontab -e) and start adding tasks to your cron file you need to consider the following syntax:
crontab syntax
Cron Structure

The asterisk (*) symbolizes that every instance of that field (i.e. every minute, hour, day, month, weekday) will be used in the command.

Example on how to setup your first crontab

Lets say you have a script named run-me.sh located in /home/your_username you want to run every day at 13:30. You will need to login your server console and input the following commands:
  • crontab -e
    This will open the crontab file and let you edit it. By default this file will be opened with the VI editor and you will need to press the “Insert” key on your keyboard to be able to write in that file.
  • 30 13 * * * /home/your_username/run-me.sh >/dev/null 2>&1
    The first character you see is “30” this means that crontab will run the script every  time the clock hits the 30 minutes mark. Next “13” this means that crontab will run the script when the clock hits 13. The next three * tell crontab to run the script every day, of every month of every weekday. Combining these fields crontab will run the script every day at exactly 13:30. You may notice that we added the “>/dev/null 2>&1” string at the end of the command. The default cron job will always send and e-mail to the root account when ever a command is executed. Now you don't want to be notified every day that your crontab job has been executed. If you don't want to receive e-mails every day notifying you about your job's execution place this “>/dev/null 2>&1” at the end of every instance of every crontab command.
When you are finished adding your commands to the crontab file you need to save and exit. If you are using VI as your editor you need to issue the following commands:
  • Press the Esc (Escape key) on your keyboard to enter the command mode of VI
  • After you pressed Escape then type the following characters :wq! and press Enter. Remember you have to type this characters (remove the quotes): “:wq!”.
 

Now to list your crontab job just issue the following command: crontab -l

 
If you need to ad another crontab job or even more all you need to do is follow the same steps as above and just ad another line to the file.
REMEMBER: Every crontab job needs to be placed on its own line in the file and after the last line you need to insert a non-braking character (press Enter).
The crontab syntax goes a little beyond its boundaries and has more advance meaning for some users.
For example if you wish to use more then one instance of one column you will separate those instances by a comma “,” or if you wish to use a time period lets say for example from Sunday till Tuesday you will use dash “-”.

10,20,30 13 * * 0-2 /home/your_username/run-me.sh >/dev/null 2>&1

This crontab job will run your scrip “run-me.sh” on from Sunday until Tuesday13:10, 13:20 and 13:30. Remember there are no spaces between values separated by commas “,” and neither in dashes “-”. There is also an operator that some versions of cron support called the slash operator “/” that can be used to skip a given number of values in your jobs. (Sunday, Monday, Tuesday) at

Crontab examples - cron examples

The syntax of crontab is not very easy to understand from the start and the best way of understanding is from examples:

Run the script every day at 12:00.

0 12 * * * /home/your_username/run-me.sh >/dev/null 2>&1
 

Run the script every day at 12:00, 14:00 and 16:00.

0 12,14,16 * * * /home/your_username/run-me.sh >/dev/null 2>&1
 

Run the script every Sunday at 13:30.

30 13 * * 0 /home/your_username/run-me.sh >/dev/null 2>&1
 

Run the script every Saturday at 12:00, 14:00 and 16:00.

0 12,14,16 * * 6 /home/your_username/run-me.sh >/dev/null 2>&1
 

Run the script on the first, fifteenth and twentieth of every month.


0 0 1,15,20 * * /home/your_username/run-me.sh >/dev/null 2>&1
 

Run the script every day from 3 to 3 hours: 00:00, 03:00, 06:00 etc.

0 */3 * * * /home/your_username/run-me.sh >/dev/null 2>&1


Original: http://www.tutorial5.com/content/view/95/51/

Show all errors in PHP

<?php
ini_set('error_reporting', E_ALL);
ini_set('display_errors', 'On');
ini_set('display_startup_errors', 'On');
?>

For MySQL errors: echo mysql_errno($conn) . ": " . mysql_error($conn);

Thursday, September 2, 2010

Random Float in PHP

<?php
function random_float ($min,$max) {
   return (
$min+lcg_value()*(abs($max-$min)));
}
?>



Random numbers with Gauss distribution (normal distribution)
<?php

function gauss()
{  
// N(0,1)
    // returns random number with normal distribution:
    //   mean=0
    //   std dev=1
   
    // auxilary vars
   
$x=random_0_1();
   
$y=random_0_1();
   
   
// two independent variables with normal distribution N(0,1)
   
$u=sqrt(-2*log($x))*cos(2*pi()*$y);
   
$v=sqrt(-2*log($x))*sin(2*pi()*$y);
   
   
// i will return only one, couse only one needed
   
return $u;
}

function
gauss_ms($m=0.0,$s=1.0)
{  
// N(m,s)
    // returns random number with normal distribution:
    //   mean=m
    //   std dev=s
   
   
return gauss()*$s+$m;
}

function
random_0_1()
{  
// auxiliary function
    // returns random number with flat distribution from 0 to 1
   
return (float)rand()/(float)getrandmax();
}

?>



Other versions of Gauss Distribution
<?php
function gauss($algorithm = "polar") {
   
$randmax = 9999;
   
    switch(
$algorithm) {
       
       
//polar-methode by marsaglia
       
case "polar":
           
$v = 2;
            while (
$v > 1) {
               
$u1 = rand(0, $randmax) / $randmax;
               
$u2 = rand(0, $randmax) / $randmax;

               
$v = (2 * $u1 - 1) * (2 * $u1 - 1) + (2 * $u2 - 1) * (2 * $u2 - 1);
            }
           
            return (
2* $u1 - 1) * (( -2 * log($v) / $v) ^ 0.5);
       
       
// box-muller-method
       
case "boxmuller":
            do {
               
$u1 = rand(0, $randmax) / $randmax;
               
$u2 = rand(0, $randmax) / $randmax;                   
               
               
$x = sqrt(-2 * log($u1)) * cos(2 * pi() * $u2);
            } while (
strval($x) == "1.#INF" or strval($x) == "-1.#INF");
           
           
// the check has to be done cause sometimes (1:10000)
            // values such as "1.#INF" occur and i dont know why
           
           
return $x;

       
// twelve random numbers  
       
case "zwoelfer":
           
$sum = 0;
            for (
$i = 0; $i < 12; $i++) {
               
$sum += rand(0, $randmax) / $randmax;
            }
            return
$sum;
     }      
}
?>

Thursday, August 5, 2010

PHP isEmail Function


Code to validate email in PHP using Preg Match.
This will cover emails of format:
asdfasdf@asdfasdf.com

function isEmail($email)
{
      if(preg_match("~([a-zA-Z0-9!#$%&amp;'*+-/=?^_`{|}~])@([a-zA-Z0-9-]).([a-zA-Z0-9]{2,4})~",$email)) 
        return true;
   else
        return false;
}

Tuesday, June 22, 2010

CSS Hacks Compiled


A compiled version of mostly used CSS hacks/filters used for Cross Browser Compatibility issues. (Ahh and yes the issues are always because of IE, more specifically a poor level of Standard Support in IE  :P )


In the list below first 4 points are preferred. By preferred it doesn't mean to always use these. These hacks are only to be used when needed. For some of hacks it will not be a Valid CSS. So better use these with caution. 


1. Conditional

SYNTAX:::
Positive
<!--[if condition]> HTML <![endif]-->
Negative
<!--[if !condition]><![IGNORE[--><![IGNORE[]]> HTML <!--<![endif]-->
condition is one of the following:

IE
For ALL versions of IE
lt IE version
IE versions less than version
lte IE version
IE versions less than or equal to version
IE version
Only specified version of IE
gte IE version
IE versions greater than or equal to version
gt IE version
IE versions greater than version

version is the version of Internet Explorer 5, 5.5, 6, or 7

NOTE: Versions of IE older than IE 5 don't support conditional comments, you may get unexpected results in those browsers.
NOTE: Versions of IE for MAC don't support conditional comments.

<head>
<title>Test</title>
<link href="allBrowsers.css" rel="stylesheet" type="text/css">
<!--[if IE]> <link href="onlyIE.css" rel="stylesheet" type="text/css"> <![endif]-->
<!--[if lt IE 7]> <link href="belowIE6.css" rel="stylesheet" type="text/css"> <![endif]-->
<!--[if !lt IE 7]><![IGNORE[--><![IGNORE[]]> <link href="new.css" rel="stylesheet" type="text/css"> <!--<![endif]-->
<!--[if !IE]>--> <link href="gudBrowsers_notIE.css" rel="stylesheet" type="text/css"> <!--<![endif]-->
</head>

2. !important

background: green !important; /* Major browsers other than IE 6 and below respect the importance immediately */
background: red; /* IE 6 and below use this value instead, even though the above was marked as important */

3. @import

@import "styles.css" all; imports the stylesheet in all major browsers except IE 7 and below. It may or may not work in future versions of IE.

4. body[class|="page-body"] {} selects the body element with the class page-body in IE 7 and all modern browsers except Opera 9 and below. It may or may not work in future versions.

5. _property: value and -property: value apply the property value in IE 6 and below. Warning: this uses invalid CSS.

6. *property: value applies the property value in IE 7 and below. It may or may not work in future versions. Warning: this uses invalid CSS

7. body:empty {} selects the body element in Firefox 1.5 and 2.0 only. It may or may not work in future versions. Warning: this uses invalid CSS 2.x but valid CSS 3 according to recent drafts.

8. >body {} selects the body element in IE 7 only. It may or may not work in future versions. Warning: this uses invalid CSS!

9. html* {} selects all descendants of the html element in IE 7 and below. It may or may not work in future versions. Warning: this uses invalid CSS!

10. The !ie identifier allows the property to be applied in IE 7 and below. It may or may not work in future versions. Warning: this uses invalid CSS!

11. The !important! identifier allows the property to be applied with importance in IE 7 and below and the property is not applied in other browsers. It may or may not work in future versions. Warning: this uses invalid CSS!

Thanks to http://www.webdevout.net/css-hacks. Please do visit it for details. I have changed a few things for easy understanding and quick reference.

Wednesday, May 5, 2010

iPhone Hanged? Freezed? Reset it...

In my case, my iphone just freezed. I got a message and iphone showed its preview and then suddenly hanged. The clock was running, so as other processes. Tried to turn it off, but of no use. Pressed home button alot of times, nothing happened. It wasn't even going to standby mode.
So what is the solution? 
We have to manually restart the iphone now. In my case pressing the power button for long time didn't work too. So to reset you just have to press both HOME button and POWER/WAKE/SLEEP button together for for some time. This way iphone will restart. 

Saturday, March 27, 2010

Installing PHP 5.3.x on RedHat ES5, CentOS 5, etc

Some cases arise when we want to upgrade php and we do not want to affect any dependencies it has.

Obviously we can use YUM for this ;)
The step below will upgrade your php with all it dependencies to latest version. We have 5.3.1 at the moment.

To install PHP 5.3.1 (latest version) you can make use of a RPM repository maintained by Remi. We will cover here for ES5 stuff.
So first of all we will need to get the latest remi release
wget http://download.fedora.redhat.com/pub/epel/5/i386/epel-release-5-3.noarch.rpm
wget http://rpms.famillecollet.com/enterprise/remi-release-5.rpm 
rpm -Uvh remi-release-5*.rpm epel-release-5*.rpm
You now have the Remi repository on your system, however it is disabled by default. Obviously you don’t want all of your packages been effected by this repository, however to enable it for a specific package, run the following:yum --enablerepo=remi update php
Now you can just run 
php -v
This will give you the version of php.
You can now restart your server.

Upgrade to PHP 5.2.10 - CentOS5 using YUM


There is a version of PHP (5.2.10) in CentOS’s Testing repository. You could either download each rpm and install, or set up your yum to install from the testing repository.
Install the testing repository’s GPG key:
rpm --import http://dev.centos.org/centos/RPM-GPG-KEY-CentOS-testing
And download the CentOS-Testing repo file:
cd /etc/yum.repos.d
wget http://dev.centos.org/centos/5/CentOS-Testing.repo
Now you can install/update to PHP 5.2.10 by running
yum --disablerepo=* --enablerepo=c5-testing update php

Friday, February 19, 2010

vim Cheat Sheet


Working with files
Vim commandAction
:e filenameOpen a new file. You can use the Tab key for automatic file name completion, just like at the shell command prompt.
:w filenameSave changes to a file. If you don't specify a file name, Vim saves as the file name you were editing. For saving the file under a different name, specify the file name.
:qQuit Vim. If you have unsaved changes, Vim refuses to exit.
:q!Exit Vim without saving changes.
:wqWrite the file and exit.
:xAlmost the same as :wq, write the file and exit if you've made changes to the file. If you haven't made any changes to the file, Vim exits without writing the file.
These Vim commands and keys work both in command mode and visual mode.
Vim commandAction
j or Up ArrowMove the cursor up one line.
k or Down ArrowDown one line.
h or Left ArrowLeft one character.
l or Right ArrowRight one character.
eTo the end of a word.
ETo the end of a whitespace-delimited word.
bTo the beginning of a word.
BTo the beginning of a whitespace-delimited word.
0To the beginning of a line.
^To the first non-whitespace character of a line.
$To the end of a line.
HTo the first line of the screen.
MTo the middle line of the screen.
LTo the the last line of the screen.
:nJump to line number n. For example, to jump to line 42, you'd type :42
Inserting and overwriting text
Vim commandAction
iInsert before cursor.
IInsert to the start of the current line.
aAppend after cursor.
AAppend to the end of the current line.
oOpen a new line below and insert.
OOpen a new line above and insert.
CChange the rest of the current line.
rOverwrite one character. After overwriting the single character, go back to command mode.
REnter insert mode but replace characters rather than inserting.
The ESC keyExit insert/overwrite mode and go back to command mode.
Deleting text
Vim commandAction
xDelete characters under the cursor.
XDelete characters before the cursor.
dd or :dDelete the current line.
Entering visual mode
Vim commandAction
vStart highlighting characters. Use the normal movement keys and commands to select text for highlighting.
VStart highlighting lines.
The ESC keyExit visual mode and return to command mode.
Editing blocks of text
Note: the Vim commands marked with (V) work in visual mode, when you've selected some text. The other commands work in the command mode, when you haven't selected any text.
Vim commandAction
~Change the case of characters. This works both in visual and command mode. In visual mode, change the case of highlighted characters. In command mode, change the case of the character uder cursor.
> (V)Shift right (indent).
< (V)Shift left (de-indent).
c (V)Change the highlighted text.
y (V)Yank the highlighted text. In Windows terms, "copy the selected text to clipboard."
d (V)Delete the highlighted text. In Windows terms, "cut the selected text to clipboard."
yy or :y or YYank the current line. You don't need to highlight it first.
dd or :dDelete the current line. Again, you don't need to highlight it first.
pPut the text you yanked or deleted. In Windows terms, "paste the contents of the clipboard". Put characters after the cursor. Put lines below the current line.
PPut characters before the cursor. Put lines above the current line.
Undo and redo
Vim commandAction
uUndo the last action.
UUndo all the latest changes that were made to the current line.
Ctrl + rRedo.
Vim commandAction
/patternSearch the file for pattern.
nScan for next search match in the same direction.
NScan for next search match but opposite direction.
Replace
Vim commandAction
:rs/foo/bar/aSubstitute foo with barr determines the range and a determines the arguments.
The range (r) can be
nothingWork on current line only.
numberWork on the line whose number you give.
%The whole file.
Arguments (a) can be
gReplace all occurrences in the line. Without this, Vim replaces only the first occurrences in each line.
iIgnore case for the search pattern.
IDon't ignore case.
cConfirm each substitution. You can type y to substitute this match, n to skip this match, a to substitute this and all the remaining matches ("Yes to all"), and q to quit substitution.
Examples
:452s/foo/bar/Replace the first occurrence of the word foo with bar on line number 452.
:s/foo/bar/gReplace every occurrence of the word foo with bar on current line.
:%s/foo/bar/gReplace every occurrence of the word foo with bar in the whole file.
:%s/foo/bar/giThe same as above, but ignore the case of the pattern you want to substitute. This replaces fooFOOFoo, and so on.
:%s/foo/bar/gcConfirm every substitution.
:%s/foo/bar/cFor each line on the file, replace the first occurrence of foo with bar and confirm every substitution.

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