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.
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)
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:
~> 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
~>
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 Command-Line Printing for more info.
| 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 |