Debugging your VPS or Dedicated Server - Common Commands to use with Putty

Common SSH Commands or Linux Shell Commands

cd : change directory · · cd /usr/local/apache : go to /usr/local/apache/ 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 filename52.txt : cat the contents of filename52.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

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. 


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 space to 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. 
While 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 original, 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.

netstat commands are useful if you are currently experiencing a DOS or denial of service attack.

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/yourdomain/new_design/* /home/yourdomain/public_html/ : copies all files, retaining permissions form one directory to another. 
cp -av * ../newdir : Copies all files and directories recursively 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 Cautious 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 current ruleset in memory to disk
service iptables restart : Restarts iptables 

How do I block an IP with iptables:
The following command will drop all incoming connections from ip xx.xx.xx.xx
iptables -I INPUT -s xx.xx.xx.xx -j DROP
to block a range, such as xx.xx.xx.* specify the CIDR block as follows:
iptables -I INPUT -s xx.xx.xx.0/24 -j DROP
If you wanted to later remove this entry you would replace the -I with -D as follows:
iptables -D INPUT -s xx.xx.xx.xx -j DROP
iptables -D INPUT -s xx.xx.xx.0/24 -j DROP

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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 new one if it does 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 occurrences of domain.com (the period represents 'anything', 
-- comment it out with a so it will be interpreted 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