======Linux Quick Reference======
This Linux quick reference has been compiled by current and former members of the EECS IT Support staff as a guide for the most basic Linux commands.

=====Unix Fundamentals=====

====Pathnames====
In a Linux shell, there are two types of paths. There are absolute paths (beginning in ''/'') that define a full path from the root directory and relative paths (paths that don't begin with ''/'') that are relative to your current directory. Example:

^ Example  ^ Path Description  ^
| ''foo.c''  | The **relative path** to the file ''foo.c'' in the current working directory  |
| ''pgms/foo.c''  | The **relative path** to the file ''foo.c'' in the ''pgms'' directory within the current working directory  |
| ''/home/jruser/pgms/foo.c''  | The **absolute path** to the file ''foo.c'' based on the root of the file system  |

====Directory Abbreviations====
^ Abbreviation  ^ Meaning  ^
| ''~''  | Your home (login) directory  |
| ''~username''  | The home directory for the ''username''  |
| ''.''  | The current working directory  |
| ''..''  | The parent directory of the current one  |
| ''../..''  | The parent directory of the parent directory  |

====Wildcards====
^ Character  ^ Meaning  ^
| ''?''  | Match a single character  |
| ''*''  | Match zero or more characters  |

===Examples===
^ Expression  ^ Description  ^ Examples  ^
| ''fo?.c''  | Matches ''fo'' followed by any single character, followed by ''.c''  | ''foa.c'', ''fob.c'', ''foc.c'', ''fo1.c''  |
| ''foo.*''  | Matches ''foo.'' followed by zero or more characters  | ''foo.txt'', ''foo.exe'', ''foo.'', ''foo.png''  |

====Redirection====
^ Command  ^ Effect  ^
| ''command > myfile''  | Redirects the output of ''command'' to the file ''myfile'' instead of ''stdout'' (the terminal output). If ''myfile'' exists, its contents will be replaced.  |
| ''command >> myfile''  | Similar to ''>'' except that it appends to the end of the file rather than overwriting.  |
| ''command < myfile''  | Redirects the contents of ''myfile'' to the input of ''command'' rather than ''stdin'' (keyboard terminal input).  |
| ''cmd1 | cmd2''  | Redirects ("pipes") the output of ''cmd1'' to the input of ''cmd2''.  |
| ''command | tee myfile''  | Duplicates the output of ''command'' to both ''stdout'' and ''myfile''.  |
| ''script myfile''  | Logs everything displayed on the terminal to the file ''myfile''. The logging is terminated with ''exit''.  |

=====Filesystem=====

====Listing/Creating Directories, Removing Files/Directories, Etc.====
See [[knowledge-base:linux-topics:basic-commands|]].

====Creating a File====
^ Command  ^ Effect  ^
| ''cat > myfile''  | Allows you to enter text with the keyboard to be stored in the file named ''myfile''. After entering the desired text, press ''Crtl+D''.  |
| ''vim myfile''  | Launches vim text editor on the file ''myfile''  |

====Displaying File Contents====

^ Command  ^ Effects  ^
| ''cat myfile''  | Displays entire contents of the file ''myfile'' at once. Note that to scroll this content, your terminal pragram must be able to handle scrolling.  |
| ''less myfile''  | Displays contents of the file ''myfile'' in a full screen, scrollable view.  |

====Comparing Files====
^ Command  ^ Effect  ^
| ''diff file1 file2''  | Performs a line-by-line comparison of the files ''file1'' and ''file2''. The differences are displayed in the shell output.  |
| ''cmp file1 file2''  | Performs a byte-by-byte comparison of the files ''file1'' and ''file2''. The differences are displayed in the shell window.  |

====Compressing Files====
Compressing files can reduce the size of files and help conserve space.

^ Command  ^ Effect  ^
| ''gzip myfile''  | Compresses the file ''myfile'', replacing it with the file name ''myfile.gz''  |
| ''gunzip myfile.gz''  | Uncompresses the file ''myfile.gz'', replacing it with the file named ''myfile''  |

====Searching Within Files====

^ Command  ^ Effect  ^
| ''grep hello myfile''  | Displays the lines from ''myfile'' that contain the string ''hello''  |
| ''grep hello file1 file2''  | Displays the lines within files ''file1'' and ''file2'' that contain the string ''hello''  |
| ''grep "some string with spaces" myfile''  | Displays the lines within the file ''myfile'' that contain the entire string ''some string with spaces''  |
| ''grep hello *''  | Displays the lines within any file in the current directory that contain the string ''hello'' (Fun fact: the ''*'' is not parsed by grep but instead by the shell. ''*'' is a wildcard in the shell that expands to a space delimited list of all files in the current directory)  |
| ''grep -v hello *''  | Displays the lines within any file in the current directory that do not contain the string ''hello'' (''-v'' stands for invert-match)  |
| ''grep -i hello *''  | Displays the lines within any file in the current directory that contain the string ''hello'' in any letter-case. For example, lines containing ''hello'', ''HELLO'', and ''HeLlO'' would all be displayed  |

====Changing Access Modes (Permissions)====
See [[knowledge-base:linux-topics:file-storage|]]

=====Process and Job Control====
====Important Terms====
  * **Process**: any running program on a Linux system
  * **PID**: process identifier (a number that identifies the process)
  * **Job**: a process or group of processes that a shell is running
  * **Job-ID**: job identifier
  * **Background job**: a job that is running in the background (e.g., the process is not using terminal input and the terminal is still accepting commands or running another program in the foreground)
  * **Foreground job**: a job that is running in the foreground and consuming terminal input/output

====Listing Processes and Jobs====
^ Command  ^ Effect  ^
| ''ps''  | Displays a list of processes and corresponding PIDs  |
| ''ps gx''  | Displays a list of processes and corresponding PIDs, including "hidden" processes  |
| ''jobs''  | Displays a list of current jobs and corresponding job-IDs.  |

====Stopping (Suspending) a Job/Process====
To stop (suspend) a job/process, type ''CTRL+Z''. The suspended process will still be listed in the list of active processes, even though it has been suspended from active execution.

====Running a Job in the Background===
To run a job in the background, the easiest way is to start the process in the background. You can do this by adding a ''&'' anywhere after the command.
^ Command  ^ Effect  ^
| ''firefox &''  | Launches Firefox as a background process. The browser window will still appear; however, your shell will continue to accept commands.  |
| ''bg''  | Continues running the most recently suspended job in the background  |

You can also suspend a process running in the foreground by pressing ''CTRL+Z'' and move it to the background:
<code>
~> firefox
# firefox is now running in the foreground and the terminal is not available
# press CTRL+Z to suspend it and get the shell back
~> bg
~>
</code>

====Bringing a Job to the Foreground====
^ Command  ^ Effect  ^
| ''fg''  | Brings the last suspended job to the foreground  |
| ''fg id#''  | Brings the job corresponding to the job-ID ''id#'' to the foreground (You can find job IDs by running ''jobs'' as mentioned above)  |

====Terminating a Job/Process====
^ Command  ^ Effect  ^
| ''CTRL+C''  | Terminates the current program in the foreground, telling it to quickly finish  |
| ''kill id#''  | Terminates the job/process with the job-ID/PID ''id#'', telling it to quickly finish  |
| ''kill -KILL id#''  | Kills the job/process with the job-ID/PID ''id#'' (This is a stronger kill than the other two, which should essentially forces it to terminate without giving it time to finish)  |

=====Miscellaneous Commands=====

====Displaying the Current Date and Time====
^ Command  ^ Effect  ^
| ''date''  | Displays the current date and time  |

====Printing====
^ Command  ^ Effect  ^
| ''lpstat -p''  | Lists all available printers and their statuses  |
| ''lp myfile''  | Prints the file ''myfile'' on the default printer  |
| ''lp -d myprinter myfile''  | Prints the file ''myfile'' on the printer named ''myprinter''  |
| ''lp -n # myfile''  | Prints ''#'' copies of the file ''myfile''  |\
| ''lp -o sides=two-sided myfile''  | Prints the file ''myfile'' double-sided (if the printer supports it)  |
| ''lpstat -o myprinter''  | Displays the contents of the print queue.  |
| ''cancel id#''  | Cancels the print request identified by id number ''id#''  |

See [[knowledge-base:linux-topics:command-line-printing|]] for more info.

====Getting Information on Users====
^ Command  ^ Effect  ^
| ''who''  | Displays a list of the users logged into the current machine  |

====Using the Command History====
^ Command  ^ Effect  ^
| ''history''  | Displays the last 15 commands from the current session  |
| ''!num''  | Repeats the command with history number ''num''  |
| ''!str''  | Repeats the most recent command that begins with the string ''str''  |
| ''!!''  | Repeats the last command  |
| ''!$''  | Expands to the last argument of the last command  |
| ''grep command $HISTFILE''  | Searches your history file for the command. ''history'' only applies to the current shell session, but the log of all shells is saved to your histfile when you exit  |