Lesson 6: Files

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Overview

What are files?

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

Files have:

Owner atime, ctime, mtime
Group POSIX ACLs
Permissions Spinlock
Inode i_ino
Size read, write and link count
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...

This functionality isn't just limited to the shell! Let's say you're programming an interface for a device that streams data from a sensor. Using the "Everything is a file" philosophy, we could read data from the device like so:

int read_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;
}

More file metadata

$ ls -l
crw-rw-rw- 1 root  tty   5, 0 Jan  6 13:45 /dev/tty
brw-rw---- 1 root  disk  8, 0 Dec 21 14:12 /dev/sda
srw-rw-rw- 1 root  root  0    Dec 21 14:13 /var/run/acpid.socket
prw------- 1 lance lance 0    Jan  5 17:44 /var/run/screen/S-lance/12138.ramereth
lrwxrwxrwx 1 root  root  4    Nov 25 09:26 /var/run -> /run

$ stat /etc/services
  File: `/etc/services'
  Size: 19303       Blocks: 40         IO Block: 4096   regular file
Device: fc00h/64512d  Inode: 525111      Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2015-01-07 08:22:43.768316048 -0800
Modify: 2012-05-03 09:01:30.934310452 -0700
Change: 2012-05-03 09:01:30.982310456 -0700
 Birth: -

File Extensions

.jpg, .txt, .py

Not necessary, more of a recommendation.

File contains information about its encoding

$ 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
    |        +-------------------------------------  References Count
    +----------------------------------------------  File Permissions
                                                         & Type

Editing Metadata

You can edit the metadata of a file with various commands, but some of the most useful commands are chown, chmod, and chgrp commands. These commands allow you to edit the owner, the read/write/execute, and the group permissions of a file respectively.

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

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

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

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

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     |
+-----+--------+-------+

chmod and Octal Permissions

Example:

$ chmod ug+x my_script.sh
# Adds the permission to execute the file to its
# owner user and owner group.

$ chmod o-w myfile.txt
# Removes the permission to write to the file
# from users other than its owners.

Executing a File?

When a file has the +x permission set, it can be run as a program.

For instance:

$ ls -lh 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

Directories

Directories are also files!

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

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

Exercise: Messing with Files

# create empty file called foo
$ touch foo

Exercise Answer Key

$ touch bootcamp/emptyfile
$ ls -alh bootcamp/emptyfile
-rw-rw-r-- 1 dobc dobc 0 Nov  3 22:38 bootcamp/emptyfile
# You may need to create the devops group.
$ sudo chown dobc:devops bootcamp/emptyfile
# Alternatively, you can also do the following
$ sudo chgrp devops bootcamp/emptyfile
$ touch allperms
$ chmod ugo+rwx allperms
$ ls -l allperms
-rwxrwxrwx 1 dobc dobc 0 Nov  3 22:39 allperms

Bonus: What's another way of giving a file all permissions?

Further Reading

Next: Lesson 7: Packages, Software, Libraries