Basic Linux Commands (by Nelson Padua-Perez)

Commands

Linux has tons of commands, but the ones described below are all you need for CMSC216.

Linux uses a tree structure where you have files and folders (directories). In the description below, we use % as command prompt.
After logging on you will be in "your home directory".

To list (display) the contents of a directory use the ls command. Some examples:

                % ls         /* Files that start with a period (e.g., .aliases) are not listed */ 
                % ls -al     /* Long listing that also displays hidden files (those starting with a period) */
                % ls -F      /* Adds forward slash to identify directories */

To change to a directory use the cd command. Three important directories:

. (single period) → Represents the current directory.
.. (two periods) → Represents the parent directory.
~ (tilde) → Represents your home directory.

Examples:

                % cd data_dir
                % cd             /* Takes you back to your home directory; great when you are lost */
                % cd /           /* Takes you to the root directory (under which all files/folder reside) */
                % cd ..          /* Takes you to the parent directory */
                % cd ~/216public /* Takes you to the 216public directory that resides in the home directory */

To create a directory use the mkdir command. For example:

                % mkdir data_dir

To display the directory where you find yourself in use the pwd command. For example:
```
                % pwd
            
```
There are several text editors (emacs, vim, etc.) A very simple one is nano. Once you start nano, look at the bottom of the window to see a summary of commands.
```
                % nano my_prog.c
            
```

To copy files use the cp command. For example:

                % cp my_prog.c my_copy.c
                % cp my_prog.c data_dir     /* Copies my_prog.c to the data_dir directory */
                % cp my_prog.c ..          /* Copies my_prog.c to the parent directory */
                % cp /tmp/e.txt .          /* Copies e.txt to current directory. Notice the period after e.txt */
                % cp -r Week1 ~/216        /* Copies the directory Week1 to the folder 216 in the home directory */

To remove a file use the rm command. For example:

                % rm my_prog.c

To remove a directory (it must not have any files) use the rmdir command. For example:
```
                % rmdir data_dir
            
```

To remove a directory (with files) use the rm command. For example:

                % rm -f -r Week1

To rename a file use the mv command. For example:

                % mv my_prog.c my_prog_new_name.c

To move a file to a directory use the mv command. For example:

                % mv my_prog.c data_dir

To display the contents of a file without opening with an editor use the less command. For example:
```
                % less my_prog.c
            
```

There are several ways to change file permissions using the chmod. For example:

                % chmod 400 data.txt   /* Only the owner (you) can read it */
                % chmod 600 data.txt   /* Only the owner (you) can read and write it */
                % chmod 700 data.txt   /* Only the owner (you) can read, write, and execute it */

Additional information about chmod can be found at File Directory Permissions.

To log out use the logout command. For example:
```
                % logout
            
```
To access Unix manual pages use the man command. For example to find information about the ls command:
```
                % man ls
            
```

To compare file use the diff command. For example:

                % diff file1 file2

To create a compressed zip archive file use the zip command. For example, for compressing a set of files:

                % zip mydata.zip data1.txt data2.txt

For compressing a directory (e.g., material):

                % zip -r material.zip material

To extract files:

                % unzip mydata.zip

To create a compressed gzip archive file use the tar command. For example, creating .tar.gz file for directory grep_example:
```
                % tar czf grep_example.tar.gz grep_example  
	    
```
To uncompress a .tar.gz file use the tar command.
```
		% tar xzf grep_example.tar.gz 
	    
```
To create a symbolic link to a file or directory use the ln command. For example to create a symbolic link in the current directory to the directory tasks that is in the tmp directory:
```
		% ln -s tmp/tasks 
	    
```
To find a file use the find command. After find, specify the directory and after -name the filename or matching expression (e.g., *.txt).
```
                find . -name a.out -print  
            
```
To find lines with a pattern in a file(s) you can use the grep command. For example, to find lines having the word TODO in all .c files of the current directory, you can execute the command:
```
                  grep TODO *.c
               
```
You can transfer files between computers by using the scp command. For example, to transfer to your computer a file named test.txt present in the home directory of a user with user id terpito (in the computer/system/host named grace.umd.edu) you can execute the command below. In a Mac you can open a terminal in order to access scp. In a PC, search for the cmd application, and once started it you will have a command line terminal with access to scp.
```
                                             scp terpito@grace.umd.edu:~/test.txt . 
                
```
Here is another example where we want to transfer the folder ~216public/projects/project1 present in the account of user terpito (in the grace.umd.edu host) to your current folder (notice the period at the end):
```
					scp -r terpito@grace.umd.edu:~/216public/projects/project1 .
				
```
In the following example, we are transfering a folder to the ~/216 folder of the user terpito in the grace.umd.edu host:
```
					scp -r sample_folder nelson@grace.umd.edu:~/216
				
```
In both examples above, you can drop the -r option if you are transferring a single file.

There are graphical programs that allow you to transfer files. See https://www.cs.umd.edu/~nelson/classes/resources/file_transfer/ for additional information.
You can find additional information about a command by using the man command. For example, to find additional information about grep:
```
                   man grep
                
```
If you want to transfer files from grace.umd.edu to your computer you can use https://dav.terpconnect.umd.edu/. Use your directory id and password to connect.

Additional Commands


                % date              /* Current date time */
                    
                % who               /* Current users */
                    
                % ps                /* Shows processes */
                    
                % emacs my_prog.c   /* Popular editor */
                    
                % ls -al | less     /* Using | to pipe output of one command as input to another */
                
                % cal               /* Calendar */
                
                % ls *.txt          /* Lists all files ending in .txt */

                % cat data.txt      /* Displays contents of a file (in this case data.txt) */

Miscellaneous

References

Web Accessibility