Basic Linux Shell Commands

The following is a breakdown of the most commonly used commands and the most commonly used arguments for them.

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 /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 filename.txt : cat the contents of filename.txt to your screen

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 editor. A clone of it is "nano"

pico /home/burst/public_html/index.html : edit the index page for the user's website.

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.

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

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/

ln : creates "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.

rm : delete (remove) 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 CAREFUL WITH THIS COMMAND!!!

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.

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.

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.

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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 data from one program and pipes it to another.)

> means create a new file, overwriting any content already there.

>> means to 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

sudo -- allows users to run programs with the security privileges of another user