Lesson 5: Files

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About Files

Everything in Linux is a file... except the things that aren’t.

Files have:

• Owners
• Permissions (what different people can do with it)
• An inode (a low-level description of the file)
• Size
• Filename

$ ls -il
total 8
2884381 drwxrwxr-x 5 test test 4096 Nov  6 11:46 Documents
2629156 -rw-rw-r-- 1 test test    0 Nov 13 14:09 file.txt
2884382 drwxrwxr-x 2 test test 4096 Nov  6 13:22 Pictures

Everything is a file!?

Yes. Except the things that aren’t..

int read_medical_device_data(int device_file_pointer) {
    // Open a connection to the device
    int * stream = open(device_file_pointer);
    // Write the stream of data to the screen
    write(STDOUT, stream);
    // Do some other stuff with that data
    // Close the data stream
    close(stream);

    return EXIT_SUCCESS;
}

File Extensions

.jpg, .txt, .py

Not necessary, more of a recommendation.

$ ls
some_text_file  squirrel

$ file some_text_file
some_text_file: ASCII text

$ file squirrel
squirrel: JPEG image data, JFIF standard 1.01

Hidden Files

Any file starting with . is called a hidden file and is not listed by default.

Adding the -a flag to ls command includes hidden files in your output.

$ ls
Documents  file.txt  Pictures

$ ls -a
.  ..  Documents  file.txt  .hidden_file  Pictures  .vimrc

Finding Metadata with ‘ls -l’

$ ls -l
drwxrwxr-x   5   test     test     4096   Nov  6 11:46 Documents
-rw-rw-r--   1   test     test        0   Nov 13 14:09 file.txt
drwxrwxr-x   2   test     test     4096   Nov  6 13:22 Pictures
----------     -------  -------  -------- ------------ -------------
    |             |        |         |         |             |
    |             |        |         |         |        File Name
    |             |        |         |         +--- Modification Time
    |             |        |         +-------------  Size (in bytes)
    |             |        +-----------------------       Group
    |             +--------------------------------       Owner
    +----------------------------------------------  File Permissions

Editing Metadata

$ chown root myfile
  # Change the owner of myfile to "root".

$ chown root:staff myfile
  # Change the owner of myfile to "root" and group to "staff".

$ chown -hR root /mydir
  # Change the owner of /mydir and subfiles to "root".

$ chgrp -R devops /home/$yourusername/bootcamp
  # Make the group devops own the bootcamp dir

chmod and Octal Permissions

+-----+--------+-------+
| rwx | Binary | Octal |
+-----+--------+-------+
| --- | 000    | 0     |
| --x | 001    | 1     |
| -w- | 010    | 2     |
| -wx | 011    | 3     |
| r-- | 100    | 4     |
| r-x | 101    | 5     |
| rw- | 110    | 6     |
| rwx | 111    | 7     |
+-----+--------+-------+

• u, g, o for user, group, other
• -, +, = for remove, add, set
• r, w, x for read, write, execute

Executing a File?

For instance:

$ ls -alh my-script
-r-xr-xr-x 1 username username 1.9K Sep 27 09:44 my-script

$ cat my-script
#!/bin/bash
# The above line tells Linux how to invoke the script on my behalf.
echo 'This is a script being run without using bash!'

$ ./my-script  # my-script is invoked just like a compiled binary!
This is a script being run without using bash!

Types of Files

• - is a normal file
• d is a directory
• b is a block device
• l is a symlink

Directories

Directories are also files!

• +r allows you to read the contents of the directory.
• +w allows you to add files to the directory.
• +x allows you to use the directory at all.

$ ls -alh | grep foobarbaz
drw-rw-rw-  2 voigte   voigte   4.0K Sep 29 10:47 foobarbaz

$ ls -alh foobarbaz   # Below is the literal output, not psuedo-output
ls: cannot access foobarbaz/.: Permission denied
ls: cannot access foobarbaz/..: Permission denied
total 0
d????????? ? ? ? ?            ? .
d????????? ? ? ? ?            ? ..

TODO: Messing with Files

$ touch foo # create empty file called foo

• Create an empty file in /home/$yourusername/bootcamp.
• Who can do what to the file?
• Change the group to devops.
• Make a file called allperms and give user, group, and world +rwx.
• Make more files and practice changing their permissions.

Further Reading

• Permission Mishaps
• Access the Linux kernel using the /proc filesytem