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);

Developer Instincts