Red Hat Linux cheat sheet commands examples RHEL

October 30, 2010

Useful Linux Commands (Red Hat)

Revised 3/1/2000

Getting information

man commandname
display the manual page for a particular command named commandname
man -S sectionnumber commandname
display the manual page under a specific section numbered sectionnumber for the command named commandname. Sometimes the same command will exist in more than one section. man alone will display the lowest-numbered section only. To see other sections, you’ll need to use the -s flag. The different sections are:

  • 1 – user commands
  • 2 – system calls
  • 3 – libc calls
  • 4 – devices
  • 5 – file formats and protocols
  • 6 – games
  • 7 – conventions, macro packages, etc.
  • 8 – system administration
man -l commandname
list all the man pages available for commandname
man -l intro
interesting reading– when you have the time
man -K keyword
searches all man pages for keywords
the other major way to find information about commands. Somewhat obscure to navigate, but quite useful. Basically, navigate using tab key to select a node, return key to enter a node, ‘l’ key to go back, ‘q’ to quit. Useful information available through info include:

  • libg++
  • iostream
  • gdb
  • make
  • tar
  • gzip
which commandname
find the path to a commandname
whatis commandname
tells you what a command is and does. works sometimes.

File Manipulation

Some file notations
. (current directory)
./myfile.txt (a file in current directory)
./mydirectory/myfile.txt (a file in a subdirectory of current directory)
mydirectory/myfile.txt (same as above, a file in a subdirectory of current directory)
.. (parent directory of current directory)
../otherdirectory (a sibling subdirectory of current directory [go back one level and down another directory])
/ (the ultimate root directory)
~ (my home directory)
~/mydirectory (a subdirectory of my home directory)
~otheruser (another user’s home directory)
filename*.txt (“*” is the wildcard character)
cp fromfilename tofilename
copy from fromfilename tofilename
mv fromfilename tofilename
Move from fromfilename to tofilename.
remove a file (DANGER: this is permanent!)
rm -i
asks you before removing a file
rm -R
recursively remove a directory and every file in it (DANGER)
> filename
redirect output to a file named filename. Overwrites that file. Example: ls > directory.txt
>> filename
redirect output to a file named filename. Appends to the end of that file.
< redirect in
redirect input to a program. Example: myprogram < myfile.txt
| mycommand
pipe output so it goes through a certain command. Example: ls -l | less
| grep myword
pipe output through grep. grep searches for “myword” and displays only those lines containing that word. Example (to search for all files containing “.cc”: ls -l | grep .cc
find mydirectory -name “nametolookfor”
searches mydirectory and all its subdirectories for a file named “nametolookfor”
find . -name “proj3*”
searches the current directory and all its subdirectories for a file begining with “proj3”.

Moving Around

shows which directory you are currently in
cd newdirectory
changes to a new directory
cd ~
change to your home directory.
cd [tab key]
change to your home directory.
cd –
changes to the last directory you were in.
mkdir directoryname
Make a new directory.

Executing commands (some are tcsh-specific)

^Z (Control-Z)
This suspends a currently running command. You can then use “bg” or “fg” on it.
Sends your suspended command to run in the background.
Resumes execution of your currently suspended command.
fg jobnumber
Resumes execution of your suspended command with job number jobnumber.. Use “sps” or “jobs” to see job numbers.
mycommand &
runs the command named mycommand in the background
repeats the last command you ran beginning with the word “foo”
shows background jobs that were spawned from this current shell.
nice +nicelevel commandname
runs a command with a lower priority. Priorities range from 0 to +19, +19 being the lowest priority available. If you are going to run a cpu-intensive command, it is nice to nice it.
renice +nicelevel jobnumber
resets a particular command to a different priority level
shows your current processes attached to this terminal. Use “ps -aux” to see everything.
show the processes that are using the most resources
top -Uusername
show a user’s processes in order of resource usage.
kill jobnumber
kill off a particular process (BE CAREFUL)
kill -9 jobnumber
kill off a particularly troublesome process if the above doesn’t work (BE VERY CAREFUL)

Using Text Files

cat filename
outputs content of a file to the screen
cat filename1 filename2 > filename3
concatenates filename1 and filename2 into a new file named filename3
more filename
displays a file page by page
less filename
does the same thing as more, but a little cooler
head filename
shows the first 10 lines of a file.
head -numberoflines filename
shows the first specified number of lines of a file. Ex. tail -50 filename
tail filename
shows the last 10 lines of a file.
tail -numberoflines filename
shows the last specified number of lines of a file. Ex. tail -50 filename
nl filename
sticks line numbers in front of each line in your file and redirects output to the screen

Using Other Files

file filename
find out what type of file filename is
gzip filename
compress a file
gunzip filename
uncompress a file
tar cvf tarfilename file1 file2 file3 etc.
archive a bunch of files or directories into one tar file.
tar xvf tarfilename
un-archive a tar file into its constituent files
zcat filename
like gunzip, but sends output to stdout.
zcat filename | tar -xvf –
unzips a compressed, tar’ed file, and untars it directly without creating an intermediate file
eject the cd tray
play audio CD’s in the cdrom drive.
cat .au > /dev/audio
play an au sound file.
play MP3 files.
view pdf files
view ps files
view dvi files
view gif, jpeg, png files

Spying and Surveillance on Other Users

Check to see if someone is logged into your system.
.plan files
If you put text into a file named “.plan” in your home directory, people will see it when they finger you.
in case you forget.
who is on
who is on
who is on, and what they’re doing
this directory has files that show the biggest user of the /scratch directory
talk username
start chat session with another user
ytalk username
start chat session with multiple users

Learning about the Machine

see the system load
quota -v
How much of your disk and file quota you are using up.
How much disk space is free on various drives.
How much space files are taking up in this current directory.

Miscellaneous Fun

display a clock
log into a different machine securely. We recommend using “ssh” over telnet and rlogin. May interfere with Java Swing GUI stuff.
set autolist
Auto-suggest commands you type on the command line.
mesg n
Turns messages off so you won’t be disturbed.
Tells you who your new mail is from.
A text-based program which will show a message when you receive new mail.
shows a picture of a mailbox on your screen. The flag goes up and it beeps when you get new mail.
set nobeep = 1
Turns off that annoying beep.
text-based web browser

Administrative Commands

(most of these commands must be executed as root)

view and play with your ethernet and tcp/ip settings
su username
change your current shell to one owned by username
mount and unmount file systems
brings up the Red Hat control panel
shutdown time
shuts down the system at time
manually edit routing table (warning: careful)
init #
manually tell init to go to runlevel #
check the hard disk for errors
can greatly increase the performance of your hard disk (careful)
utility to help configure Plug’N’Play devices