Navigating the Linux File System via CLI

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It’s said that everything on a computer is either a file or process.  In a strictly binary way, all computer data is just a combination of 1s and 0s; therefore, files are simply data that’s stored, typically on the hard disk or in memory (RAM), while processes are the manipulation of that data – usually in the CPU.  Here, we’ll be reviewing different ways to view those files on a Linux system.

OVERVIEW

Today, we’ll be covering basic Linux filesystem navigation: abstraction, location, viewing, and finding.  This will enable us to understand how files are organized & structured, how to list & parse files, and how to find what we’re looking for.

Outline

  • File System Structure
    • Manila Folders
    • System Directories
  • File Paths
    • The Root Directory
    • Absolute Paths
    • Relative Paths
    • “Dot” Paths
    • Tab Completion
  • Viewing
    • Files & Folders
    • Long Listing
    • Hidden Files
    • Home Directory
  • Finding
    • Which
    • Find
    • Locate

Prerequisites

  • Basic Linux CLI Operations

FILE SYSTEM STRUCTURE

Manila Folders

First, the file system’s structure needs to be understood conceptually. Luckily, file systems are a lot like manila folders that hold more folders or sheets of paper.

So, imagine a folder labeled “Income Taxes”. In it, you have 3 more folders labeled by year: “2001”, “2002”, and “2003”. In each of those folders, you have a mix of paperwork and folders for each employer you’ve had: “job1” and “job2”. The “job” folders hold most of your paperwork: W2s, paycheck stubs, and such. Altogether, the folder hierarchy might look like this (folders are bolded and underlined):

  • Income Taxes (folder)
    • 2001 (folder)
      • 1040-EZ Form (paperwork)
      • Job 1 (folder)
        • Paycheck 1 (paperwork)
        • W2 (paperwork)
    • 2002 (folder)
      • 1099-MISC Form (paperwork)
      • Job 1 (folder)
        • Paycheck 2 (paperwork)
        • W2 (paperwork)
      • Job 2 (folder)
        • Paycheck 3 (paperwork)
        • W2 (paperwork)
      • W4 Form (paperwork)
    • 2003 (folder)
      • Job 2 (folder)
        • Paycheck 4 (paperwork)
        • W2 (paperwork)

System Directories

Now, imagine (1) each folder as a Linux directory and (2) each sheet of paper as a Linux file. The file system equivalent might look like this (directories are bolded, underlined, and have a trailing slash):

  • income_taxes/
    • 2001/
      • 1040-ex.pdf
      • job1/
        • paycheck_1.xlsx
        • w2.pdf
    • 2002/
      • 1099-misc.pdf
      • job1/
        • paycheck_2.xlsx
        • w2.pdf
      • job2/
        • paycheck_3.xlsx
        • w2.pdf
      • w4.pdf
    • 2003/
      • job2/
        • paycheck_4.xlsx
        • w2.pdf
Here’s what the same files & folders would look like on a Linux system.

FILE PATHS

A file path – or just “path” for short – is the location of a file/directory from another directory in the file system hierarchy.

The Root Directory

The beginning of the file system hierarchy is the top-level directory, often referred to as the “root directory” or “slash”.

NOTE: The root directory vs the root user’s directory
Be careful not to confuse the root directory with the “root” user’s home directory.
– The root directory: /
– The root user’s home directory: /root/

Absolute Paths

An absolute path always starts from the root directory to a file/folder’s location in the hierarchy. Using the above example, if the “income_taxes” directory is in root directory, here are what all the absolute paths would look like for each file and directory (directories are bolded and highlighted):

  • /income_taxes/
  • /income_taxes/2001/
  • /income_taxes/2001/1040-ex.pdf
  • /income_taxes/2001/job1/
  • /income_taxes/2001/job1/paycheck_1.xlsx
  • /income_taxes/2001/job1/w2.pdf
  • /income_taxes/2002/
  • /income_taxes/2002/1099-misc.pdf
  • /income_taxes/2002/job1/
  • /income_taxes/2002/job1/paycheck_2.xlsx
  • /income_taxes/2002/job1/w2.pdf
  • /income_taxes/2002/job2/
  • /income_taxes/2002/job2/paycheck_3.xlsx
  • /income_taxes/2002/job2/w2.pdf
  • /income_taxes/2002/w4.pdf
  • /income_taxes/2003/
  • /income_taxes/2003/job2/
  • /income_taxes/2003/job2/paycheck_4.xlsx
  • /income_taxes/2003/job2/w2.pdf

Relative Paths

Conversely, a relative path starts from a specific directory to a file/folder’s location in the hierarchy. Continuing with the same “income_taxes” model as before, here are a few examples of different relative paths:

  • From the “income_taxes” directory: ./2001/1040-ex.pdf
  • From the “2002” directory: ./1099-misc.pdf

Although rare, a relative path can also be defined from the / directory, which would make it look similar to an absolute path:

  • From the “/” directory: ./income_taxes/2003/job2/paycheck_4.xlsx

“Dot” Paths

When a dot (.) is used where a path is expected, it represents the CWD (Current Working Directory). For example, if you execute ls, ls ., or ls ./, they should all display the same output.

Now, if you use a double-dot (..) it means “up” one directory in the hierarchy. So, if you are in the “2002” folder and execute ls .. or ls ../, you’ll see the contents of the “income_taxes” directory.

You can use as many dots as you’d like to go up the directory tree; for example:

[[email protected] job2]$ pwd
/income_taxes/2003/job2
[[email protected] openssl]$ cd ../../..
[[email protected] /]$ pwd
/

Tab Completion

In addition to commands, tab completion works for file paths as well and can be used to navigate through a file system quickly.  To use tab completion, begin typing a path and hit “Tab” (perhaps twice).  All the possible path options will be displayed for you (much like executing the `ls` command):

[[email protected] ~]$ /
bin/   dev/   home/  lib64/ mnt/   proc/  run/   srv/   tmp/   var/
boot/  etc/   lib/   media/ opt/   root/  sbin/  sys/   usr/

Now, to narrow the possibilities down, begin typing part of the path.  For example, typing `/b` and hitting “Tab” will display all the paths that start with `/b`:

[[email protected] ~]$ /b
bin/  boot/

Next, type /bi and hit “Tab”. Since there is now only one possible file/directory that starts with “/bi”, the rest of the path will be filled in for you:

[[email protected] ~]$ /bin/

From here, you can repeat the process: press “Tab” to see the available options under /bin/. However, since there are many possible paths to display, you’ll be prompted to continue:

[[email protected] ~]$ /bin/
Display all 734 possibilities? (y or n)

NOTE: y for “yes”
If you hit y here, you’ll see a long list of files. From here, press Enter to scroll down until you reach the bottom of the list or hit q to exit.

Enter n for now and narrow the list down by typing /bin/whoa and hit “Tab”. Again, since there is only one possibility, the remainder will be automatically filled in for you:

[[email protected] ~]$ /bin/whoami

Tab completion for paths is often used in conjunction with commands as well. For example, typing cat /home/penguin/ and hitting “Tab” could result in the following:

[[email protected] ~]$ cat /home/penguin/
.bash_history  .bash_logout   .bash_profile  .bashrc        Directory/     Folder/        .lesshst

From here, continue to type part of the directory or file you want to specify and hit “Tab” as needed until you reach the desired full path:

[[email protected] ~]$ cat /home/penguin/
.bash_history  .bash_logout   .bash_profile  .bashrc        Directory/     Folder/        .lesshst
[[email protected] ~]$ cat /home/penguin/Directory/
AnotherDirectory/ duplicate.png     MyNewFolder/      picture.png
[[email protected] ~]$ cat /home/penguin/Directory/AnotherDirectory/
bar.txt  foo.txt
[[email protected] ~]$ cat /home/penguin/Directory/AnotherDirectory/foo.txt

Finally, you can hit “Enter” to actually execute the full cat command:

[[email protected] ~]$ cat /home/penguin/Directory/AnotherDirectory/foo.txt
Hello World!

On the other hand, observe the following example in which there is only one file/folder in each directory:

[[email protected] TestFolder]$ pwd
/home/brian/TestFolder
[[email protected] TestFolder]$ ls
OneDirectory
[[email protected] TestFolder]$ ls OneDirectory/
OneFolder
[[email protected] TestFolder]$ ls OneDirectory/OneFolder/
OnlyDir
[[email protected] TestFolder]$ ls OneDirectory/OneFolder/OnlyDir/
only_file.txt

When tab completing, since there is only one possible file/folder per directory, they will be filled in automatically – no need to enter a partial path at all. Simply typing cat (notice the space after cat here) and hitting “Tab” four times will result in the following path:

[[email protected] ~]$ cat OneDirectory/OneFolder/OnlyDir/only_file.txt

NOTE: Avoid Typos
Another benefit from tab completion is confirmation that the path you’re using is without any typos.

VIEWING

Files & Folders

When browsing a file system, since folders appear to contain files within them, it might seem that folders themselves are not files. However, recall that all data on a computer is either a file or process – This means that folders, a.k.a. directories, are just a special type of file.

NOTE: Special Files
Some other special types of files:
– links
– sockets
– devices

Long Listing

To view files, and some info about them, use the long listing command: ls -l.

[[email protected] somedir]$ ls -l
total 0
drwxrwxr-x. 2 root    bear    6 Jul 18 16:20 bar
-rw-r--r--. 1 penguin root    0 Jul 18 16:20 foo.cfg
lrwxrwxrwx. 1 penguin penguin 9 Jul 18 16:21 spam -> /tmp/eggs

The 1st field of each line: the first letter/symbol tells you what kind of file it is (the remaining letters and dashes describe the permissions)

NOTE: Common File Types
Here are the most common file types you’re likely to see:
- = normal file
d = directory
l = link

The 2nd field: the number of links to the file
The 3rd field: represents the user associated with the file
The 4th field: represents the group associated with the file
The 5th field: the size of the file (default is in bytes)
The 6th – 8th fields: when the file was last modified
The 9th field: the file/directory name

Hidden Files

Often, there are “hidden” files on the system – these are files/folders that are preceded with a . in the name. When files are hidden, they can only be seen when explicitly searched for.

NOTE: Why Hide Files?
Hidden files have multiple purposes, but it’s mainly to (1) deter accidental deletion/modification of important files and (2) obscure static files from cluttering up the screen when browsing.

To view these files, use the -a flag to view “all” files: ls -al

[[email protected] ~]$ ls -al
total 20
drwx------. 3 penguin penguin  111 Jul 18 16:24 .
drwxr-xr-x. 7 root    root      70 Jul 18 16:10 ..
-rw-------. 1 penguin penguin 1497 Jul 18 16:21 .bash_history
-rw-r--r--. 1 penguin penguin   18 Aug  8  2019 .bash_logout
-rw-r--r--. 1 penguin penguin  193 Aug  8  2019 .bash_profile
-rw-r--r--. 1 penguin penguin  231 Aug  8  2019 .bashrc
drwx------. 2 penguin penguin   29 Jul 16 17:08 .ssh
-rw-------. 1 penguin penguin  670 Jul 18 16:06 .viminfo

Here’s an example of how hidden files can prevent accidental deletion:

[[email protected] ~]$ pwd
/home/penguin
[[email protected] ~]$ ls -a
.  ..  bar  .bash_history  .bash_logout  .bash_profile  .bashrc  foo.txt  .ssh  .viminfo
[[email protected] ~]$ rm -rf ./*
[[email protected] ~]$ ls -a
.  ..  .bash_history  .bash_logout  .bash_profile  .bashrc  .ssh  .viminfo

Only the bar directory and foo.txt file were deleted while the hidden files remain.

Home Directory

Each Linux system has a directory, called “home”, located here: `/home/`.  By default, most users have a home directory within `/home/` that is named after the user:

[[email protected] ~]$ ls -ald /home/*
drwx------. 2 anna    anna     62 Jul 18 15:57 /home/anna
drwx------. 2 bear    bear     62 Jul 18 16:10 /home/bear
drwx------. 2 brian   brian    62 Jul 18 15:58 /home/brian
drwx------. 3 penguin penguin 111 Jul 18 16:24 /home/penguin
drwx------. 2 zeta    zeta     62 Jul 18 15:58 /home/zeta

Since the home directory is often referenced, it has a shortcut: `~`.  So, `~` is equivalent to `/home/<UserName>/`.  For example, if you are logged in as the user `anna` and execute the `ls ~` command, you’ll see the contents of your home directory: `/home/anna/`.

[[email protected] ~]$ pwd
/home/anna
[[email protected] ~]$ ls
bar.txt  foo.txt
[[email protected] ~]$ ls ~
bar.txt  foo.txt
[[email protected] ~]$ ls ~/
bar.txt  foo.txt

The `~` also works for changing directories:


[[email protected] ~]$ cd /tmp
[[email protected] tmp]$ pwd
/tmp
[[email protected] tmp]$ cd ~
[[email protected] ~]$ pwd
/home/anna

FINDING

Which

If you want to find where a command is located in the files system, use the which command, followed by the command you want to search for: which <SomeCommand>. For example, if you want to find where the hostname command is:

[[email protected] ~]$ which hostname
/bin/hostname

Find

To list all files and directories – including hidden files – from your present working directory, you can use the find command. By default, this action is recursive, meaning that the find command will list everything from all subdirectories as well:

[[email protected] ~]$ find
.
./.bash_logout
./.bash_profile
./.bashrc
./.bash_history
./.ssh
./.ssh/authorized_keys
./.viminfo

Now, to specify a different directory, simply type find followed by the directory you want: find /<PathToDirectory>/.

[[email protected] ~]$ find /etc/
/etc/
/etc/fstab
/etc/crypttab
/etc/mtab
/etc/resolv.conf
/etc/grub.d
find: ‘/etc/grub.d’: Permission denied
/etc/terminfo
...

If you want to list the files only, use the -type f option:

[[email protected] ~]$ find /etc/ -type f
/etc/fstab
/etc/crypttab
/etc/resolv.conf
find: ‘/etc/grub.d’: Permission denied
/etc/magic
...

Similarly, if you just want to list directories, use the -type d option:

[[email protected] ~]$ find /etc/ -type d
/etc/
/etc/grub.d
find: ‘/etc/grub.d’: Permission denied
/etc/terminfo
...

NOTE: Options for find
There are many useful options for the find command: finding files by modification time (older or newer), user, and permissions; also, executing commands against the found files/directories. Use the manual page to see more details: man find.

Locate

To search for a specific file/directory that you already know the name of, you can use the locate command (from the mlocate package) followed by the name, or part of the name:

[[email protected] ~]$ locate fstab
/etc/fstab
/usr/lib/dracut/modules.d/95fstab-sys
/usr/lib/dracut/modules.d/95fstab-sys/module-setup.sh
/usr/lib/dracut/modules.d/95fstab-sys/mount-sys.sh
/usr/lib/systemd/system-generators/systemd-fstab-generator
/usr/share/man/man5/fstab.5.gz
/usr/share/man/man8/systemd-fstab-generator.8.gz
/usr/share/vim/vim74/syntax/fstab.vim

However, if a known file/directory cannot be found, it’s likely that updatedb needs to be run (with sudo privileges).

[[email protected] ~]$ sudo updatedb

updatedb registers all the current files on the system so the locate command can find them. So, if you’ve just created a new file, updatedb would need to be run before locate could find that file:

[[email protected] ~]$ ls -l
total 0
[[email protected] ~]$ touch findme.txt
[[email protected] ~]$ ls -l
total 0
-rw-rw-r--. 1 penguin penguin 0 Jul 19 19:40 findme.txt
[[email protected] ~]$ locate findme
[[email protected] ~]$ sudo updatedb
[[email protected] ~]$ locate findme
/home/penguin/findme.txt

In contrast, if a file has just been deleted, locate will “think” that file still exists until updatedb is run again:

[[email protected] ~]$ ls -l
total 0
-rw-rw-r--. 1 penguin penguin 0 Jul 19 19:40 findme.txt
[[email protected] ~]$ rm -f findme.txt
[[email protected] ~]$ ls -l
total 0
[[email protected] ~]$ locate findme
/home/penguin/findme.txt
[[email protected] ~]$ sudo updatedb
[[email protected] ~]$ locate findme

CONCLUSION

These are the basics of file system navigation. Now, we can move around, understand where files are, and how to find them.

  • System directories are like manila folders
  • Absolute and relative file paths
  • Listing hidden files
  • Finding files and directories

To learn more about file systems, see Linux top-level directories.

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