After giving notice at my last job I found myself whipping together a lot of documentation for the person who would be taking over for me.
He really enjoyed the “Linux Basics” one I put together and said it would be a useful thing to stick on my blog… so here it is. 
Note: Please forgive any odd formating, it is taken from a wiki.
File System
/ : root of the file system contains all devices and directory’s
/root : the root users home directory
/home : all other users home dirs reside in here
/boot : All the kernels and boot specific info
/tmp : temporary files are stored here, is commonly world writable so keep an eye on it
/dev : on linux even hardware devices are part of the file system, they are stored here.
/bin : executables that should be safe for normal users to run
/var : the system writes data here during its operation, commonly contains /var/lib/mysql and /var/www
/opt : optional software, 3rd parties stick stuff here
/sbin : system executables that only root should need
/proc : the OS uses this to keep track of everything on the system in real time. No need to muck around in here
/mnt or /media: this ware new file systems get mounted (cds, floppys, flash drives)
/etc : all config files
FS NOTE: when tweaking configs do ‘cp something.conf something.conf.bk’ and tweak away. If you flub something up just ‘rm -f something.conf; mv something.conf.bk something.conf; service restart something’ and your back up and running with your original config.
Basic commands
- whoami : displays current user
- top : displays the top cpu/memory eaters and system load.. like task manager on windows
- ps : displays all processes running.. ps aux is the most useful way to run it
- wall “some text” : sends a broadcast message to all logged on users
- man program : displays the ‘man page’ or manual for a given program. Uber useful. Use space bar to page down and q to exit
- program -h : displays the help for a given program, briefer than man
- du -sh dirName : Displays the total size of a directory recursively
- df -kh : displays total and available storage on all partitions for the system
- locate filename : finds ware a program or file is located on the system
- w : displays who is ssh’ed or logged in.
- watch -n seconds filename : will execute a file every n seconds. Useful to watch who is online, watch -n 3 w
- wget http://somesite.com/somefile : gets a file via ftp, rsync, http, etc from a remote host.
- netstat : displays all listening ports and active connections
- ifconfig : used for listing network interface info and setting it
- clear : clears the terminal
- md5sum filename : displays the md5 checksum of the given file
additional command operators
| the pipe is used to send one command through another. ps | more -- pauses ps ps | grep ssh -- only display lines that contain ssh
; used to "stack commands" or issue multiple commands on 1 line. cd ..; ls
& puts a command in the background. Will let you know when the command is finished
> write what is displayed on the screen from a given command to a text file ls -alh /root > /root/myRoot.txt
>> appends screen output to an existing file
File Permissions
Listing Permissions
ls -al will display all files in a list with their owners and permissions
-rw-r--r-- 1 irq13 irq13 1006 Jan 24 10:16 .bashrc
Now to break down the above example…
-rw-r--r-- is the permissions area. The first - would be d if the item is directory, otherwise it will be -. The second 3 dashes indicate read/write/execute for the owner, the second is r/w/x for the group and third is r/w/x for everyone else.
The next number is the inodes associated with the file. This isn’t important for you to know the basics
Next when it says irq13 irq13 that indicates the owner of the files name group
Changing ownership of a file
chown username:groupname file
Changing permissions of a file
chmod XXX filename
- chmod uses a numeric system for assigning ownership. XXX represents 3 numbers. The first is the permissions applied to the owning user, 2nd is group, 3rd is everyone else.
1: execute 2: write 3: write & execute 4: read 5: read & execute 6: read & write 7: read, write & execute
Remember that 777 is only to be used as a trouble shooting step to rule fs permissions out. NEVER leave a dir as 777. Its useful to do ‘ls -alh * > perm_capture.txt’ before messing with a file. That way you can restore its original permissions.
Attributes
Files also have attributes, similar to the ones found in the windows world.
lsattr filename : Lists the attributes of a file or directory
chattr +-=[ASacDdIijsTtu] filename
- to add an attribute use + to remove use –
File Attributes
append only (a) compressed (c) no dump (d) immutable (i) data journaling (j) secure deletion (s) no tail-merging (t) undeletable (u) no atime updates (A) synchronous directory updates (D) syn-chronous updates (S) top of directory hierarchy (T)
Use man chattr for an explanation of each attribute
launching scripts and bins
- If an executable file is in your path you may simply type its name from anywhere on the system and it will execute.
- To see what your path is type ‘path’
- To execute a file in the current directory type “./filename
- To execute a file it must have execute permissions for either your username or a group you belong to.
User Management
useradd
useradd userName
- then run “passwd userName” to set the new users pw
passwd
passwd username
- will ask for the new pw twice
Service/Daemon Management
restarting/stopping/starting a service
On any init.d based linux distro you can restart a service with the following…
- /etc/init.d/serviceName restart
You may replace ‘restart’ with ‘stop’ or ‘start’ (and in some cases ‘status’).
Forcefully stopping a service
killall processName
Killing on instance of a service
kill pid
- The pid can be gathered by either top or ps
Disabling/adding/listing services
chkconfig –list
- displays all the services and if they are set to run in different runlevels
- use the –del daemonName to remove a service or –add daemonName to add one
setting a program to run at startup
Add a line executing the command at the end of /etc/rc.local
File Manipulation
Editing Text Files
vi is by far the best text editor but has a learning curve to it. If you want simplicity use nano
display a text file from the command line
cat filename
- or
more filename
Display the last few lines of a text file
tail filename
- or you can display the last 50 lines of a file with…
tail -50 filename
- or you can display lines as they are written to a file (or follow) with the following: (UBER useful for log files)
tail -f filename
copy a file
cp filename destination
move a file
mv filename destination
delete a file
rm -f filename : removes the file. -f makes it so it doesn’t ask you if you are sure
Displaying the differences between two files
diff file1 file2
Installing crap
On redhat derived systems (RedHat, Fedora, CentOS, Rocks, Mandrake, etc) yum is your package manager.
yum install appname : installs the application from the remote yum repository
yum search appname : does a search on the repository for a given program
yum remove appname : uninstalls an app
use ‘man yum’ for a complete list
archives
tar.gz or .tgz is the most common compression found in the linux world. that is tared (Tape ARchive) and gziped. Sometimes called “tar balls”.
tar -xzf file.tgz : will X’trackt a tar/gzip file.
tar -czf myfile.tgz someDir : will create a tar and gziped archive of the given directory
gunzip : un gzips a file
unzip : unzips a .zip file
Linux Security
Read these this SANs Checklist (www) (pdf) and install Bastille Linux.
