Files & Directories

Navigating the File System

Accessing Directories

When you first log into a system or open a terminal, the default directory should be your home directory. You can print the exact path of this by typing echo $HOME. Many Linux distributions actually open new graphical terminals in $HOME/Desktop. The following commands are useful for directory navigation:

Command	Result
`pwd`	Displays the present working directory
`cd ~` or `cd`	Change to your home directory (shortcut name is ~ (tilde))
`cd ..`	Change to parent directory (..)
`cd -`	Change to previous directory (- (minus))

Understanding Absolute and Relative Paths

relative and absolute path

For example, suppose you are currently working in your home directory and wish to move to the /usr/bin directory. The following two ways will bring you to the same directory from your home directory:
- Absolute path name method
  Terminal window
```
cd /usr/bin
```
- Relative path name method
  Terminal window
```
cd ../../usr/bin
```

There are two ways to identify paths:

Absolute pathname: An absolute path name begins with the root directory and follows the tree, branch by branch, until it reaches the desired directory or file. Absolute paths always start with /.
Relative pathname: A relative path name starts from the present working directory. Relative paths never start with /.
Multiple slashes (/) between directories and files are allowed, but all but one slash between elements in the path name is ignored by the system. ////usr//bin is valid, but seen as /usr/bin by the system.
Most of the time, it is most convenient to use relative paths, which require less typing. Usually, you take advantage of the shortcuts provided by:
- . (present directory),
- .. (parent directory) and
- ~ (your home directory).

Exploring the File System

Traversing up and down the file system tree can get tedious. The tree command is a good way to get a bird’s-eye view of the file system tree. Use tree -d to view just the directories and to suppress listing file names.

Command	Usage
`cd /`	Changes your current directory to the root (/) directory (or path you supply)
`ls`	List the contents of the present working directory
`ls –a`	List all files, including hidden files and directories (those whose name start with `.`)
`tree`	Displays a tree view of the file system

List (`ls`)

This is the list command. It allows us to list the content of the directory.

ls
File-01.txt  file-01  notmyfile  notmyfile2

ls -la
total 100
drwx------  7 root      root       4096 Jun 11 07:18 .
drwxr-xr-x 19 root      root       4096 Jun  8 05:40 ..
-rw-r--r--  1 root      root       3208 Jun  8 05:40 .bashrc
drwx------  2 root      root       4096 Jun  8 05:36 .ssh
drwxr-xr-x  2 root      root       4096 Jun  8 05:40 .theia
-rw-r--r--  1 root      root      42118 Jun 11 07:18 File-01.txt
-rw-r--r--  1 root      root          0 Jun 11 07:18 file-01
lrwxrwxrwx  1 root      root          1 Jun  8 05:40 filesystem -> /
-rw-r--r--  1 testuser  testuser     18 Jun 11 07:18 notmyfile
-rw-r--r--  1 otheruser otheruser    19 Jun 11 07:18 notmyfile2

Sorting

All lists can be sorted. Linux allows us to list files using multiple sorting options.

ls command has some options built-in. By default ls sorts the files alphabetically.

What is timestamp?

This is the numerical representation of the time. It is the number of seconds passed from Unix epoch which is midnight of 1st of January, 1970.

Example how it looks is visible below

$ date +%s
1635797690
$ date
Mon 01 Nov 2021 08:14:52 PM UTC

The first sort option will be -t. This argument sorts files with the last modification time, newest files come first.

ls -lt
total 68
-rw-r--r-- 1 root      root      42118 Jun 11 07:18 File-01.txt
-rw-r--r-- 1 root      root         18 Jun 11 07:18 file-01.txt
-rw-r--r-- 1 root      root          0 Jun 11 07:18 file-02
-rw-r--r-- 1 testuser  testuser     18 Jun 11 07:18 notmyfile
-rw-r--r-- 1 otheruser otheruser    19 Jun 11 07:18 notmyfile2
drwxr-xr-x 2 root      root       4096 Jun 11 07:18 testDir
drwxr-xr-x 3 root      root       4096 Jun 11 07:18 testdir
-rw-r--r-- 1 root      root          0 Jun 11 07:18 file-01
drwx------ 3 root      root       4096 Jun 11 07:15 snap
lrwxrwxrwx 1 root      root          1 Jun  8 05:40 filesystem -> /

We use two arguments, to observe things better. We can specify exactly the modification time by adding u to the argument list.

But please remember, in order to print this information properly you have to use t with another argument (u in this case).

ls -ltu
total 68
-rw-r--r-- 1 root      root          0 Jun 11 07:18 file-02
-rw-r--r-- 1 testuser  testuser     18 Jun 11 07:18 notmyfile
-rw-r--r-- 1 otheruser otheruser    19 Jun 11 07:18 notmyfile2
-rw-r--r-- 1 root      root      42118 Jun 11 07:18 File-01.txt
-rw-r--r-- 1 root      root          0 Jun 11 07:18 file-01
-rw-r--r-- 1 root      root         18 Jun 11 07:18 file-01.txt
drwxr-xr-x 2 root      root       4096 Jun 11 07:18 testDir
drwxr-xr-x 3 root      root       4096 Jun 11 07:18 testdir
lrwxrwxrwx 1 root      root          1 Jun 11 07:15 filesystem -> /
drwx------ 3 root      root       4096 Jun 11 07:15 snap

List and order it by ctime - metadata change.

ls -ltc
total 68
-rw-r--r-- 1 root      root      42118 Jun 11 07:18 File-01.txt
-rw-r--r-- 1 root      root         18 Jun 11 07:18 file-01.txt
-rw-r--r-- 1 root      root          0 Jun 11 07:18 file-02
-rw-r--r-- 1 testuser  testuser     18 Jun 11 07:18 notmyfile
-rw-r--r-- 1 otheruser otheruser    19 Jun 11 07:18 notmyfile2
drwxr-xr-x 2 root      root       4096 Jun 11 07:18 testDir
drwxr-xr-x 3 root      root       4096 Jun 11 07:18 testdir
-rw-r--r-- 1 root      root          0 Jun 11 07:18 file-01
drwx------ 3 root      root       4096 Jun 11 07:15 snap
lrwxrwxrwx 1 root      root          1 Jun  8 05:40 filesystem -> /

Execute these commands and carefully observe the output

touch theNewestFile #(this creates a new file)

ls -ltu
total 68
-rw-r--r-- 1 root      root          0 Jun 11 07:43 theNewestFile
-rw-r--r-- 1 root      root          0 Jun 11 07:18 file-02
-rw-r--r-- 1 testuser  testuser     18 Jun 11 07:18 notmyfile
-rw-r--r-- 1 otheruser otheruser    19 Jun 11 07:18 notmyfile2
-rw-r--r-- 1 root      root      42118 Jun 11 07:18 File-01.txt
-rw-r--r-- 1 root      root          0 Jun 11 07:18 file-01
-rw-r--r-- 1 root      root         18 Jun 11 07:18 file-01.txt
drwxr-xr-x 2 root      root       4096 Jun 11 07:18 testDir
drwxr-xr-x 3 root      root       4096 Jun 11 07:18 testdir
lrwxrwxrwx 1 root      root          1 Jun 11 07:15 filesystem -> /
drwx------ 3 root      root       4096 Jun 11 07:15 snap

ls -ltc
total 68
-rw-r--r-- 1 root      root          0 Jun 11 07:43 theNewestFile
-rw-r--r-- 1 root      root      42118 Jun 11 07:18 File-01.txt
-rw-r--r-- 1 root      root         18 Jun 11 07:18 file-01.txt
-rw-r--r-- 1 root      root          0 Jun 11 07:18 file-02
-rw-r--r-- 1 testuser  testuser     18 Jun 11 07:18 notmyfile
-rw-r--r-- 1 otheruser otheruser    19 Jun 11 07:18 notmyfile2
drwxr-xr-x 2 root      root       4096 Jun 11 07:18 testDir
drwxr-xr-x 3 root      root       4096 Jun 11 07:18 testdir
-rw-r--r-- 1 root      root          0 Jun 11 07:18 file-01
drwx------ 3 root      root       4096 Jun 11 07:15 snap
lrwxrwxrwx 1 root      root          1 Jun  8 05:40 filesystem -> /

echo "hello world!" > file-02 #(this will add something to the file)

ls -ltu
total 72
-rw-r--r-- 1 root      root          0 Jun 11 07:43 theNewestFile
-rw-r--r-- 1 root      root         13 Jun 11 07:18 file-02
-rw-r--r-- 1 testuser  testuser     18 Jun 11 07:18 notmyfile
-rw-r--r-- 1 otheruser otheruser    19 Jun 11 07:18 notmyfile2
-rw-r--r-- 1 root      root      42118 Jun 11 07:18 File-01.txt
-rw-r--r-- 1 root      root          0 Jun 11 07:18 file-01
-rw-r--r-- 1 root      root         18 Jun 11 07:18 file-01.txt
drwxr-xr-x 2 root      root       4096 Jun 11 07:18 testDir
drwxr-xr-x 3 root      root       4096 Jun 11 07:18 testdir
lrwxrwxrwx 1 root      root          1 Jun 11 07:15 filesystem -> /
drwx------ 3 root      root       4096 Jun 11 07:15 snap

ls -ltc
total 72
-rw-r--r-- 1 root      root         13 Jun 11 07:45 file-02
-rw-r--r-- 1 root      root          0 Jun 11 07:43 theNewestFile
-rw-r--r-- 1 root      root      42118 Jun 11 07:18 File-01.txt
-rw-r--r-- 1 root      root         18 Jun 11 07:18 file-01.txt
-rw-r--r-- 1 testuser  testuser     18 Jun 11 07:18 notmyfile
-rw-r--r-- 1 otheruser otheruser    19 Jun 11 07:18 notmyfile2
drwxr-xr-x 2 root      root       4096 Jun 11 07:18 testDir
drwxr-xr-x 3 root      root       4096 Jun 11 07:18 testdir
-rw-r--r-- 1 root      root          0 Jun 11 07:18 file-01
drwx------ 3 root      root       4096 Jun 11 07:15 snap
lrwxrwxrwx 1 root      root          1 Jun  8 05:40 filesystem -> /

chmod 444 file-01 #(this will change the permissions of the file)

ls -ltu
total 72
-rw-r--r-- 1 root      root          0 Jun 11 07:43 theNewestFile
-rw-r--r-- 1 root      root         13 Jun 11 07:18 file-02
-rw-r--r-- 1 testuser  testuser     18 Jun 11 07:18 notmyfile
-rw-r--r-- 1 otheruser otheruser    19 Jun 11 07:18 notmyfile2
-rw-r--r-- 1 root      root      42118 Jun 11 07:18 File-01.txt
-r--r--r-- 1 root      root          0 Jun 11 07:18 file-01
-rw-r--r-- 1 root      root         18 Jun 11 07:18 file-01.txt
drwxr-xr-x 2 root      root       4096 Jun 11 07:18 testDir
drwxr-xr-x 3 root      root       4096 Jun 11 07:18 testdir
lrwxrwxrwx 1 root      root          1 Jun 11 07:15 filesystem -> /
drwx------ 3 root      root       4096 Jun 11 07:15 snap

ls -ltc
total 72
-r--r--r-- 1 root      root          0 Jun 11 07:48 file-01
-rw-r--r-- 1 root      root         13 Jun 11 07:45 file-02
-rw-r--r-- 1 root      root          0 Jun 11 07:43 theNewestFile
-rw-r--r-- 1 root      root      42118 Jun 11 07:18 File-01.txt
-rw-r--r-- 1 root      root         18 Jun 11 07:18 file-01.txt
-rw-r--r-- 1 testuser  testuser     18 Jun 11 07:18 notmyfile
-rw-r--r-- 1 otheruser otheruser    19 Jun 11 07:18 notmyfile2
drwxr-xr-x 2 root      root       4096 Jun 11 07:18 testDir
drwxr-xr-x 3 root      root       4096 Jun 11 07:18 testdir
drwx------ 3 root      root       4096 Jun 11 07:15 snap
lrwxrwxrwx 1 root      root          1 Jun  8 05:40 filesystem -> /

The format contains a few columns. Let’s try to understand them:

permissions: Kind of permission the object has.
number of hard links: By default every object has 1 hard link.
Owner: The owner of the object. It doesn’t mean who created it, but who owns it at this moment.
Group: Owner belongs to the group (of users). This information is here too. This means that our ‘groupmates’ have specific access to the file.
Size: File size in bytes.
date and time of last modification of the object.
file name.

Owner and group: Names of the users and groups → System keeps and translates them from numerical representation. These are UID for user identifier and GID for group identifier. We can list this information in numeric way, by entering the ls -n command. This works like ls -l, but it changes the user-friendly names to UIDs and GIDs.

ls -n
total 68
-rw-r--r-- 1    0    0 42118 Jun 11 07:18 File-01.txt
-rw-r--r-- 1    0    0     0 Jun 11 07:18 file-01
-rw-r--r-- 1    0    0    18 Jun 11 07:18 file-01.txt
-rw-r--r-- 1    0    0     0 Jun 11 07:18 file-02
lrwxrwxrwx 1    0    0     1 Jun  8 05:40 filesystem -> /
-rw-r--r-- 1 1001 1001    18 Jun 11 07:18 notmyfile
-rw-r--r-- 1 1002 1002    19 Jun 11 07:18 notmyfile2
drwx------ 3    0    0  4096 Jun 11 07:15 snap
drwxr-xr-x 2    0    0  4096 Jun 11 07:18 testDir
drwxr-xr-x 3    0    0  4096 Jun 11 07:18 testdir

A few more useful commands:

# print the username of the creator of the file
ls -al --author

# prints directories only
ls -ald

# prints inodes (there will be a lesson about inodes)
ls -ali

# recursively prints all subdirectories
ls -alR

# prints list in the reversed order
ls -alr

# prints the version of the binary
ls --version

- indicates that we will pass arguments. We have two ways here
- - one dash informs the system that we will pass one letter argument, like l
- -- two dashes means that argument will contain more than one letter. Most commonly it will be an english word.
l means long listing format.

Sort Content by Size

ls -s
total 72
44 File-01.txt   0 file-01   4 file-01.txt   4 file-02   0 filesystem   4 notmyfile   4 notmyfile2   4 snap   4 testDir   4 testdir   0 theNewestFile

This shows the short list of files and allocated space. As we already know, we can combine this argument - s - with others. Let’s do it.

ls -ls
total 72
44 -rw-r--r-- 1 root      root      42118 Jun 11 07:18 File-01.txt
 0 -r--r--r-- 1 root      root          0 Jun 11 07:18 file-01
 4 -rw-r--r-- 1 root      root         18 Jun 11 07:18 file-01.txt
 4 -rw-r--r-- 1 root      root         13 Jun 11 07:45 file-02
 0 lrwxrwxrwx 1 root      root          1 Jun  8 05:40 filesystem -> /
 4 -rw-r--r-- 1 testuser  testuser     18 Jun 11 07:18 notmyfile
 4 -rw-r--r-- 1 otheruser otheruser    19 Jun 11 07:18 notmyfile2
 4 drwx------ 3 root      root       4096 Jun 11 07:15 snap
 4 drwxr-xr-x 2 root      root       4096 Jun 11 07:18 testDir
 4 drwxr-xr-x 3 root      root       4096 Jun 11 07:18 testdir
 0 -rw-r--r-- 1 root      root          0 Jun 11 07:43 theNewestFile

But this is what you have by default, using ls -l, right? No? You are correct, the answer is no. Take a look on the beginning of each line, this is where you can find, what was added by -s.

Why we used s? When capital S is used, this means sort.

ls -lS sorts files by size, largest going first.

So, arguments are case-sensitive, like… everything in Linux :)

--human-readable, or better - -h

ls -lh prints the size of the files not in bytes, but in more readable form, with K, M, or G, that sort of things.

ls -lh
total 72K
-rw-r--r-- 1 root      root       42K Jun 11 07:18 File-01.txt
-r--r--r-- 1 root      root         0 Jun 11 07:18 file-01
-rw-r--r-- 1 root      root        18 Jun 11 07:18 file-01.txt
-rw-r--r-- 1 root      root        13 Jun 11 07:45 file-02
lrwxrwxrwx 1 root      root         1 Jun  8 05:40 filesystem -> /
-rw-r--r-- 1 testuser  testuser    18 Jun 11 07:18 notmyfile
-rw-r--r-- 1 otheruser otheruser   19 Jun 11 07:18 notmyfile2
drwx------ 3 root      root      4.0K Jun 11 07:15 snap
drwxr-xr-x 2 root      root      4.0K Jun 11 07:18 testDir
drwxr-xr-x 3 root      root      4.0K Jun 11 07:18 testdir
-rw-r--r-- 1 root      root         0 Jun 11 07:43 theNewestFile

h uses the powers of 1024. So, 1K is 1 powered by 1024. We have another option which uses powers of 1000.

ls -l --si
total 74k
-rw-r--r-- 1 root      root       43k Jun 11 07:18 File-01.txt
-r--r--r-- 1 root      root         0 Jun 11 07:18 file-01
-rw-r--r-- 1 root      root        18 Jun 11 07:18 file-01.txt
-rw-r--r-- 1 root      root        13 Jun 11 07:45 file-02
lrwxrwxrwx 1 root      root         1 Jun  8 05:40 filesystem -> /
-rw-r--r-- 1 testuser  testuser    18 Jun 11 07:18 notmyfile
-rw-r--r-- 1 otheruser otheruser   19 Jun 11 07:18 notmyfile2
drwx------ 3 root      root      4.1k Jun 11 07:15 snap
drwxr-xr-x 2 root      root      4.1k Jun 11 07:18 testDir
drwxr-xr-x 3 root      root      4.1k Jun 11 07:18 testdir
-rw-r--r-- 1 root      root         0 Jun 11 07:43 theNewestFile

ls -lSh
total 72K
-rw-r--r-- 1 root      root       42K Jun 11 07:18 File-01.txt
drwx------ 3 root      root      4.0K Jun 11 07:15 snap
drwxr-xr-x 2 root      root      4.0K Jun 11 07:18 testDir
drwxr-xr-x 3 root      root      4.0K Jun 11 07:18 testdir
-rw-r--r-- 1 otheruser otheruser   19 Jun 11 07:18 notmyfile2
-rw-r--r-- 1 root      root        18 Jun 11 07:18 file-01.txt
-rw-r--r-- 1 testuser  testuser    18 Jun 11 07:18 notmyfile
-rw-r--r-- 1 root      root        13 Jun 11 07:45 file-02
lrwxrwxrwx 1 root      root         1 Jun  8 05:40 filesystem -> /
-r--r--r-- 1 root      root         0 Jun 11 07:18 file-01
-rw-r--r-- 1 root      root         0 Jun 11 07:43 theNewestFile

Navigating the Directory History

The cd command remembers where you were last, and lets you get back there with cd -.
For remembering more than just the last directory visited, use pushd to change the directory instead of cd; this pushes your starting directory onto a list.
Using popd will then send you back to those directories, walking in reverse order (the most recent directory will be the first one retrieved with popd).
The list of directories is displayed with the dirs command.

Working with Files and Directories

Locating Applications

Depending on the specifics of particular distribution’s policy, programs and software packages can be installed in various directories.
In general, executable programs and scripts should live in the
- /bin, /usr/bin, /sbin, /usr/sbin directories, or
- somewhere under /opt.
- They can also appear in /usr/local/bin and /usr/local/sbin, or
- in a directory in a user’s account space, such as /home/student/bin.
One way to locate programs is to employ the which utility. For example, to find out exactly where the diff program resides on the file system:

which diff
/usr/bin/diff

If which does not find the program, whereis is a good alternative because it looks for packages in a broader range of system directories:

whereis diff
diff: /usr/bin/diff /usr/share/man/man1/diff.1.gz /usr/share/man/man1p/diff.1p.gz

as well as locating source and man files packaged with the program.

Working with Files

Viewing Files

Command	Usage
`cat`	Used for viewing files that are not very long; it does not provide any scroll-back.
`tac`	Used to look at a file backwards, starting with the last line.
`less`	Used to view larger files because it is a paging program. It pauses at each screen full of text, provides scroll-back capabilities, and lets you search and navigate within the file. Use `/` to search forward and `?` to search backward. An older program named `more` is still used, but has fewer capabilities: “less is more”.
`tail`	Used to print the last 10 lines of a file by default. You can change the number of lines by doing `-n 15` or just `-15` if you wanted to look at the last 15 lines instead of the default.
`head`	The opposite of tail; by default, it prints the first 10 lines of a file.

Creating, Updating and Modifying Files

touch is often used to set or update the access, change, and modify times of files.
By default, it resets a file’s timestamp to match the current time. However, you can also create an empty file using touch:

touch <filename> — This is normally done to create an empty file as a placeholder for a later purpose.
touch provides several useful options.
- For example, the -t option allows you to set the date and timestamp of the file to a specific value, as in:
  Terminal window
```
touch -t 12091600 myfile
```
  This sets the myfile file’s timestamp to 4 p.m., December 9th (12 09 1600).

`mkdir` and `rmdir`

mkdir is used to create a directory:
mkdir sampdir — creates a sample directory named sampdir under the current directory.
mkdir /usr/sampdir — creates a sample directory called sampdir under /usr.

Create multiple directories with similar names:

mkdir testdir{1..10}

mkdir mydirectory anotherdirectory thirddirectory

In order to create deeper structure, we have to use -p argument. This allows us to create the whole structure, without creating parent directory as first step.
Terminal window
```
mkdir -p parentdir/childdir{01..100}
```
with -p we allowed the system to create parent directory and the second part we already know. Under the parentdir we created 100 files, from childdir001 to childdir100. Please notice, how nice it is formatted by the system! We said 01, but system created directories to keep three digits as it is in childdir100.
Removing a directory is done with rmdir.
The directory must be empty or the command will fail.
To remove a directory and all of its contents you have to do rm -rf.

Moving, Renaming or Removing a File

mv does double duty, in that it can:
- Simply rename a file
- Move a file to another location, while possibly changing its name at the same time.
If you are not certain about removing files that match a pattern you supply, it is always good to run rm interactively (rm –i) to prompt before every removal.

Command	Usage
`mv`	Rename a file
`rm`	Remove a file
`rm –f`	Forcefully remove a file
`rm –i`	Interactively remove a file

Renaming or Removing a Directory

rmdir works only on empty directories; otherwise you get an error.
While typing rm –rf is a fast and easy way to remove a whole file system tree recursively, it is extremely dangerous and should be used with the utmost care, especially when used by root (recall that recursive means drilling down through all sub-directories, all the way down a tree).

Command	Usage
`mv`	Rename a directory
`rmdir`	Remove an empty directory
`rm -rf`	Forcefully remove a directory recursively

Searching for Files

The main tools for doing this are the locate and find utilities.

locate

The locate utility program performs a search taking advantage of a previously constructed database of files and directories on your system, matching all entries that contain a specified character string. This can sometimes result in a very long list.
To get a shorter (and possibly more relevant) list, we can use the grep program as a filter. grep will print only the lines that contain one or more specified strings, as in:
Terminal window
```
locate zip | grep bin
```
which will list all the files and directories with both zip and bin in their name.
locate utilizes a database created by a related utility, updatedb.
Most Linux systems run this automatically once a day. However, you can update it at any time by just running updatedb from the command line as the root user.

The find Program

find recurses down the filesystem tree from any particular directory (or set of directories) and locates files that match specified conditions. The default pathname is always the present working directory.
For example, administrators sometimes scan for potentially large core files (which contain diagnostic information after a program fails) that are more than several weeks old in order to remove them.
It is also common to remove files in inessential or outdated files in /tmp (and other volatile directories, such as those containing cached files) that have not been accessed recently.

Using find

When no arguments are given, find lists all files in the current directory and all of its sub directories. Commonly used options to shorten the list include:

-name (only list files with a certain pattern in their name), -iname (also ignore the case of file names), and -type (which will restrict the results to files of a certain specified type, such as d for directory, l for symbolic link, or f for a regular file, etc.).
Searching for files and directories named gcc:
Terminal window
```
find /usr -name gcc
```
Searching only for directories named gcc:
Terminal window
```
find /usr -type d -name gcc
```
Searching only for regular files named gcc:
Terminal window
```
find /usr -type f -name gcc
```

Using Advanced find Options

Another good use of find is being able to run commands on the files that match your search criteria. The -exec option is used for this purpose.
To find and remove all files that end with .swp:
Terminal window
```
find -name "*.swp" -exec rm {} ';'
```
The {} (squiggly brackets) is a placeholder that will be filled with all the file names that result from the find expression, and the preceding command will be run on each one individually.
Please note that you have to end the command with either ';' (including the single-quotes) or \;. Both forms are fine.
One can also use the -ok option, which behaves the same as -exec, except that find will prompt you for permission before executing the command. This makes it a good way to test your results before blindly executing any potentially dangerous commands.

Finding Files Based on Time and Size

To find files based on time:
Terminal window
```
find / -ctime 3
```
Here, -ctime is when the inode metadata (i.e. file ownership, permissions, etc.) last changed; it is often, but not necessarily, when the file was first created.
You can also search for accessed/last read (-atime) or modified/last written (-mtime) times.
The number is the number of days and can be expressed as either a number (n) that means exactly that value, +n, which means greater than that number, or -n, which means less than that number. There are similar options for times in minutes (as in -cmin, -amin, and -mmin).
To find files based on sizes:
Terminal window
```
find / -size 0
```
Note the size here is in 512-byte blocks, by default; you can also specify bytes (c), kilobytes (k), megabytes (M), gigabytes (G), etc.
As with the time numbers above, file sizes can also be exact numbers (n), +n or -n.
For example, to find files greater than 10 MB in size and running a command on those files:
Terminal window
```
find / -size +10M -exec command {} ';'
```
To search for files using the ? wildcard, replace each unknown character with ?.
- For example, if you know only the first two letters are ‘ba’ of a three-letter filename with an extension of .out, type ls ba?.out.

Wildcards and Matching File Names

You can search for a filename containing specific characters using wildcards.

Wildcard	Result
`?`	Matches any single character
`*`	Matches any string of characters
`[set]`	Matches any character in the set of characters, for example `[adf]` will match any occurrence of a, d, or f
`[!set]`	Matches any character not in the set of characters

To search for files using the * wildcard, replace the unknown string with *.
- For example, if you remember only that the extension was .out, type ls *.out

Hard and Soft Links

Hard Links

The ln utility is used to create hard links and (with the -s option) soft links, also known as symbolic links or symlinks. These two kinds of links are very useful in UNIX-based operating systems.
Suppose that file1 already exists. A hard link, called file2, is created with the command:

ln file1 file2

ls -li file1 file2
49302296 -rw-r--r-- 2 student student 0 Oct 24 2024 file1
49302296 -rw-r--r-- 2 student student 0 Oct 24 2024 file2

The -i option to ls prints out in the first column the inode number, which is a unique quantity for each file object. This field is the same for both of these files; what is really going on here is that it is only one file, but it has more than one name associated with it, as is indicated by the 2 that appears in the ls output. Thus, there was already another object linked to file1 before the command was executed.
Hard links are very useful and they save space. For one thing, if you remove either file1 or file2 in the example, the inode object (and the remaining file name) will remain, which might be undesirable, as it may lead to subtle errors later if you recreate a file of that name.
If you edit one of the files, exactly what happens depends on your editor; most editors, including vi and gedit, will retain the link by default, but it is possible that modifying one of the names may break the link and result in the creation of two objects.

Soft (Symbolic) Links

Soft (or Symbolic) links are created with the -s option, as in:

ln -s file1 file3

ls -li file1 file3
49302296 -rw-r--r-- 2 student student 0 Oct 24 2024 file1
49302397 lrwxrwxrwx 1 student student 5 Oct 24 2024 file3 -> file1

Symbolic links take no extra space on the file system (unless their names are very long). They are extremely convenient, as they can easily be modified to point to different places. An easy way to create a shortcut from your home directory to long path names is to create a symbolic link.
Unlike hard links, soft links can point to objects even on different file systems, partitions, and/or disks and other media, which may or may not be currently available or even exist. In the case where the link does not point to a currently available or existing object, you obtain a dangling link.