Linux Tutorial : Working with Files and Directories Part II

Listing Files in a Directory

Now that you are able to move from one directory to another, you will want to start displaying the contents of these directories. The ls command (ls is short for list) can be used to display the contents of a directory as well as detailed information about the files that are within a directory.

By itself, the ls command will list the files in the current directory:

sysadmin@localhost:~$ ls                                               

Desktop  Documents  Downloads  Music  Pictures  Public  Templates  

Videos      

sysadmin@localhost:~$

1 Listing Colors

There are many different types of files in Linux. As you learn more about Linux, you will discover many of these types. The following is a brief summary of some of the more common file types:

Type	Description
plain file	A file that isn't a special file type; also called a normal file
directory	A directory file (contains other files)
executable	A file that can be run like a program
symbolic link	A file that points to another file

On many Linux distributions, regular user accounts are modified so that the lscommand displays filenames, color-coded by file type. For example, directories may be displayed in blue, executable files may be displayed in green, and symbolic links may be displayed in cyan (light blue).

This is not a normal behavior for the ls command, but rather something that happens when you use the --color option to the ls command. The reason why ls seems to automatically perform this coloring, is that there is an alias for the ls command so it runs with the --color option:

sysadmin@localhost:~$ alias                                           

alias egrep='egrep --color=auto'                                       
alias fgrep='fgrep --color=auto'                                      

alias l='ls -CF'                                                    
alias grep='grep --color=auto'                                         

alias ls='ls --color=auto'                                             
alias la='ls -A'                                                       
alias ll='ls -alF'                                                    

sysadmin@localhost:~$

  

As you can see from the output above, when the ls command is executed, it really runs the command ls --color=auto.

In some cases, you might not want to see all of the colors (they can be a bit distracting sometimes). To avoid using the alias, place a backslash character \ in front of your command:

sysadmin@localhost:~$ ls                                               

Desktop  Documents  Downloads  Music  Pictures  Public  Templates  

Videos       

sysadmin@localhost:~$ \ls                                             

Desktop  Documents  Downloads  Music  Pictures  Public  Templates  
Videos       

sysadmin@localhost:~$

2 Listing Hidden Files

When you use the ls command to display the contents of a directory, not all files are shown automatically. The ls command doesn't display hidden files by default. A hidden file is any file (or directory) that begins with a dot . character.

To display all files, including hidden files, use the -a option to the ls command:

sysadmin@localhost:~$ ls -a                                            

.             .bashrc   .selected_editor  Downloads  Public           

..            .cache    Desktop           Music      Templates         

.bash_logout  .profile  Documents         Pictures   Videos

Why are files hidden in the first place? Most of the hidden files are customization files, designed to customize how Linux, your shell or programs work. For example, the .bashrc file in your home directory customizes features of the shell, such as creating or modifying variables and aliases.

These customization files are not ones that you work with on a regular basis. There are also many of them, as you can see, and having them displayed will make it more difficult to find the files that you do regularly work with. So, the fact that they are hidden is to your benefit.

3 Long Display Listing

There is information about each file, called metadata that is sometimes helpful to display. This may include who owns a file, the size of a file and the last time the contents of a file were modified. You can display this information by using the -loption to the ls command:

sysadmin@localhost:~$ ls -l                                           

total 0                                                                

drwxr-xr-x 1 sysadmin sysadmin 0 Jan 29  2015 Desktop                  

drwxr-xr-x 1 sysadmin sysadmin 0 Jan 29  2015 Documents                

drwxr-xr-x 1 sysadmin sysadmin 0 Jan 29  2015 Downloads                

drwxr-xr-x 1 sysadmin sysadmin 0 Jan 29  2015 Music                   

drwxr-xr-x 1 sysadmin sysadmin 0 Jan 29  2015 Pictures                 

drwxr-xr-x 1 sysadmin sysadmin 0 Jan 29  2015 Public                   

drwxr-xr-x 1 sysadmin sysadmin 0 Jan 29  2015 Templates                

drwxr-xr-x 1 sysadmin sysadmin 0 Jan 29  2015 Videos                   

sysadmin@localhost:~$

In the output above, each line describes metadata about a single file. The following describes each of the fields of data that you will see in the output of the ls -l command:

3.1 Human Readable Sizes

When you display file sizes with the -l option to the ls command, you end up with file sizes in bytes. For text files, a byte is 1 character.

For smaller files, byte sizes are fine. However, for larger files it is hard to comprehend how large the file is. For example, consider the output of the following command:

sysadmin@localhost:~$ ls -l /usr/bin/omshell                                 

-rwxr-xr-c 1 root root 1561400 Oct 9 2012 /usr/bin/omshell              

sysadmin@localhost:~$

As you can see, the file size is hard to determine in bytes. Is 1561400 a large file or small? It seems fairly large, but it is hard to determine using bytes.

Think of it this way: if someone were to give you the distance between Boston and New York using inches, that value would essentially be meaningless because for a distance like that, you think in terms of miles.

It would be better if the file size was presented in a more human readable size, like megabytes or gigabytes. To accomplish this, add the -h option to the lscommand:

sysadmin@localhost:~$ ls -lh /usr/bin/omshell                                 

-rwxr-xr-c 1 root root 1.5M Oct 9 2012 /usr/bin/omshell              

sysadmin@localhost:~$

Important: The -h option must be used with the -l option.

4 Listing Directories

When the command ls -d is used, it refers to the current directory, and not the contents within it. Without any other options, it is rather meaningless, although it is important to note that the current directory is always referred to with a single period (.):

sysadmin@localhost:~$ ls -d                                            

.                                                                      

To use the ls -d command in a meaningful way requires the addition of the -loption. In this case, note that the first command lists the details of the contents in the /home/sysadmin directory, while the second command lists the /home/sysadmin directory itself.

sysadmin@localhost:~$ ls -l                                            

total 0                                                                

drwxr-xr-x 1 sysadmin sysadmin   0 Apr 15  2015 Desktop                 

drwxr-xr-x 1 sysadmin sysadmin   0 Apr 15  2015 Documents              

drwxr-xr-x 1 sysadmin sysadmin   0 Apr 15  2015 Downloads              

drwxr-xr-x 1 sysadmin sysadmin   0 Apr 15  2015 Music                 

drwxr-xr-x 1 sysadmin sysadmin   0 Apr 15  2015 Pictures              

drwxr-xr-x 1 sysadmin sysadmin   0 Apr 15  2015 Public                 

drwxr-xr-x 1 sysadmin sysadmin   0 Apr 15  2015 Templates              

drwxr-xr-x 1 sysadmin sysadmin   0 Apr 15  2015 Videos                

drwxr-xr-x 1 sysadmin sysadmin 420 Apr 15  2015 test                   

sysadmin@localhost:~$ ls -ld                                           

drwxr-xr-x 1 sysadmin sysadmin 224 Nov  7 17:07 .                      

sysadmin@localhost:~$

Note the single period at the end of the second long listing. This indicates that the current directory is being listed, and not the contents.

5 Recursive Listing

There will be times when you want to display all of the files in a directory as well as all of the files in all subdirectories under a directory. This is called a recursive listing.

To perform a recursive listing, use the -R option to the ls command:

Note: The output shown below will vary from the results you will see if you execute the command within the virtual machine environment of this course.

sysadmin@localhost:~$ ls -R /etc/ppp                                         

/etc/ppp:                                                                     

chap-secrets   ip-down.ipv6to4    ip-up.ipv6to4    ipv6-up    pap-secrets

ip-down        ip-up              ipv6-down        options    peers


/etc/ppp/peers:

sysadmin@localhost:~$

Note that in the previous example, the files in the /etc/ppp directory were listed first. After that, the files in the /etc/ppp/peers directory were listed (there were no files in this case, but if any file had been in this directory, they would have been displayed).

Be careful with this option; for example, running the command ls -R / would list every file on the file system, including all files on any attached USB device and DVD in the system. Limit the use of the -R option to smaller directory structures.

6 Sort a Listing

By default, the ls command sorts files alphabetically by file name. Sometimes, It may be useful to sort files using different criteria.

To sort files by size, we can use the -S option. Note the difference in the output of the following two commands:

sysadmin@localhost:~$ ls /etc/ssh                               

moduli           ssh_host_dsa_key.pub    ssh_host_rsa_key     sshd_confi
ssh_config        ssh_host_ecdsa_key      ssh_host_rsa_key.pub        

sysadmin@localhost:~$ ls -S /etc/ssh                                   
ssh_host_dsa_key  ssh_host_ecdsa_key.pub  ssh_import_id               
moduli            ssh_host_dsa_key      ssh_host_ecdsa_key            

sshd_config       ssh_host_dsa_key.pub  ssh_host_ecdsa_key.pub         
ssh_host_rsa_key  ssh_host_rsa_key.pub                                 

sysadmin@localhost:~$
ssh_config        ssh_import_id                                        

The same files and directories are listed, but in a different order. While the -Soption works by itself, you can't really tell that the output is sorted by size, so it is most useful when used with the -l option. The following command will list files from largest to smallest and display the actual size of the file.

sysadmin@localhost:~$ ls -lS /etc/ssh                                  

total 160                                                             

-rw-r--r-- 1 root root 125749 Apr 29  2014 moduli                      
-rw-r--r-- 1 root root   2489 Jan 29  2015 sshd_config                

-rw-r--r-- 1 root root   1669 Apr 29  2014 ssh_config                  
-rw------- 1 root root   1675 Jan 29  2015 ssh_host_rsa_key            
-rw------- 1 root root    668 Jan 29  2015 ssh_host_dsa_key           

-rw-r--r-- 1 root root    302 Jan 10  2011 ssh_import_id               
-rw-r--r-- 1 root root    607 Jan 29  2015 ssh_host_dsa_key.pub       
-rw-r--r-- 1 root root    399 Jan 29  2015 ssh_host_rsa_key.pub       
-rw------- 1 root root    227 Jan 29  2015 ssh_host_ecdsa_key         

sysadmin@localhost:~$
-rw-r--r-- 1 root root    179 Jan 29  2015 ssh_host_ecdsa_key.pub     

It may also be useful to use the -h option to display human-readable file sizes:

sysadmin@localhost:~$ ls -lSh /etc/ssh                                

total 160K                                                             
-rw-r--r-- 1 root root 123K Apr 29  2014 moduli                        

-rw------- 1 root root 1.7K Jan 29  2015 ssh_host_rsa_key              
-rw-r--r-- 1 root root 2.5K Jan 29  2015 sshd_config                   
-rw-r--r-- 1 root root 1.7K Apr 29  2014 ssh_config                   

-rw-r--r-- 1 root root  399 Jan 29  2015 ssh_host_rsa_key.pub          
-rw------- 1 root root  668 Jan 29  2015 ssh_host_dsa_key              
-rw-r--r-- 1 root root  607 Jan 29  2015 ssh_host_dsa_key.pub          
-rw-r--r-- 1 root root  302 Jan 10  2011 ssh_import_id           

sysadmin@localhost:~$
-rw------- 1 root root  227 Jan 29  2015 ssh_host_ecdsa_key           

-rw-r--r-- 1 root root  179 Jan 29  2015 ssh_host_ecdsa_key.pub      

It is also possible to sort files based on the time they were modified. You can do this by using the -t option.

The -t option will list the most recently modified files first. This option can be used alone, but again, is usually more helpful when paired with the -l option:

sysadmin@localhost:~$ ls -tl /etc/ssh                                 

total 160                                                             

-rw------- 1 root root    668 Jan 29  2015 ssh_host_dsa_key            
-rw-r--r-- 1 root root    607 Jan 29  2015 ssh_host_dsa_key.pub        

-rw-r--r-- 1 root root    179 Jan 29  2015 ssh_host_ecdsa_key.pub     
-rw------- 1 root root    227 Jan 29  2015 ssh_host_ecdsa_key          
-rw------- 1 root root   1675 Jan 29  2015 ssh_host_rsa_key            

-rw-r--r-- 1 root root 125749 Apr 29  2014 moduli                      
-rw-r--r-- 1 root root    399 Jan 29  2015 ssh_host_rsa_key.pub       
-rw-r--r-- 1 root root   2489 Jan 29  2015 sshd_config                
-rw-r--r-- 1 root root   1669 Apr 29  2014 ssh_config                  

sysadmin@localhost:~$
-rw-r--r-- 1 root root    302 Jan 10  2011 ssh_import_id               

It is important to remember that the modified date on directories represents the last time a file was added to or removed from the directory.

If the files in a directory were modified many days or months ago, it may be harder to tell exactly when they were modified, as only the date is provided for older files. For more detailed modification time information you can use the --full-time option to display the complete timestamp (including hours, seconds, minutes...):

sysadmin@localhost:~$ ls -t --full-time /etc/ssh                       

total 160                                                             

-rw------- 1 root root    668 2015-01-29 03:17:33.000000000 +0000 ssh_host_dsa_key                                                            

-rw-r--r-- 1 root root    607 2015-01-29 03:17:33.000000000 +0000 ssh_host_dsa_key.pub                                                         
-rw------- 1 root root    227 2015-01-29 03:17:33.000000000 +0000 ssh_host_ecdsa_key                                                           

-rw------- 1 root root   1675 2015-01-29 03:17:33.000000000 +0000 ssh_host_rsa_key                                                             
-rw-r--r-- 1 root root    179 2015-01-29 03:17:33.000000000 +0000 ssh_host_ecdsa_key.pub                                                      
-rw-r--r-- 1 root root    399 2015-01-29 03:17:33.000000000 +0000 ssh_host_rsa_key.pub                                                         

-rw-r--r-- 1 root root    302 2011-01-10 18:48:29.000000000 +0000 ssh_import_id 
-rw-r--r-- 1 root root   2489 2015-01-29 03:17:33.000000000 +0000 sshd_config   
-rw-r--r-- 1 root root 125749 2014-04-29 23:58:51.000000000 +0000 moduli-rw-r--r-- 1 root root   1669 2014-04-29 23:58:51.000000000 +0000 ssh_config    

sysadmin@localhost:~$

The --full-time option will assume the -l option automatically.

It is possible to perform a reverse sort with either the -S or -t options by using the -r option. The following command will sort files by size, smallest to largest:

sysadmin@localhost:~$ ls -lrS /etc/ssh                                 

total 160                                                              
-rw-r--r-- 1 root root    179 Jan 29  2015 ssh_host_ecdsa_key.pub      

-rw-r--r-- 1 root root    302 Jan 10  2011 ssh_import_id               
-rw------- 1 root root    227 Jan 29  2015 ssh_host_ecdsa_key          
-rw-r--r-- 1 root root    399 Jan 29  2015 ssh_host_rsa_key.pub        

-rw-r--r-- 1 root root   1669 Apr 29  2014 ssh_config                  
-rw-r--r-- 1 root root    607 Jan 29  2015 ssh_host_dsa_key.pub        
-rw------- 1 root root    668 Jan 29  2015 ssh_host_dsa_key            
-rw------- 1 root root   1675 Jan 29  2015 ssh_host_rsa_key            

sysadmin@localhost:~$
-rw-r--r-- 1 root root   2489 Jan 29  2015 sshd_config                 

-rw-r--r-- 1 root root 125749 Apr 29  2014 moduli                      

The following command will list files by modification date, oldest to newest:

sysadmin@localhost:~$ ls -lrt /etc/ssh                                 

total 160                                                              
-rw-r--r-- 1 root root    302 Jan 10  2011 ssh_import_id               

-rw-r--r-- 1 root root 125749 Apr 29  2014 moduli                      
-rw-r--r-- 1 root root   1669 Apr 29  2014 ssh_config                  
-rw-r--r-- 1 root root   2489 Jan 29  2015 sshd_config                 

-rw-r--r-- 1 root root    179 Jan 29  2015 ssh_host_ecdsa_key.pub      
-rw-r--r-- 1 root root    399 Jan 29  2015 ssh_host_rsa_key.pub        
-rw------- 1 root root   1675 Jan 29  2015 ssh_host_rsa_key            
-rw------- 1 root root    227 Jan 29  2015 ssh_host_ecdsa_key          

sysadmin@localhost:~$
-rw-r--r-- 1 root root    607 Jan 29  2015 ssh_host_dsa_key.pub        

-rw------- 1 root root    668 Jan 29  2015 ssh_host_dsa_key            

7 Listing With Globs

In a previous chapter, we discussed the use of file globs to match filenames using wildcard characters. For example, we demonstrated that you can list all of the files in the /etc directory that begin with the letter e with the following command:

sysadmin@localhost:~$ echo /etc/e*                                    

/etc/encript.cfg /etc/environment /etc/ethers /etc/event.d /etc/exports

sysadmin@localhost:~$

Now that you know that the ls command is normally used to list files in a directory, using the echo command may seem to have been a strange choice. However, there is something about the ls command that might have caused confusion while we were discussing globs. This "feature" might also cause problems when you try to list files using glob patterns.

Keep in mind that it is the shell, not the echo or ls command, that expands the glob pattern into corresponding file names. In other words, when you typed the echo /etc/e* command, what the shell did before executing the echocommand was replace e* with all of the files and directories within the /etcdirectory that match the pattern.

So, if you were to run the ls /etc/e* command, what the shell would really run would be this:

ls /etc/encript.cfg /etc/environment /etc/ethers /etc/event.d /etc/exports

When the ls command sees multiple arguments, it performs a list operation on each item separately. In other words, the command ls /etc/encript.cfg /etc/environment is essentially the same as ls /etc/encript.cfg; ls /etc/environment.

Now consider what happens when you run the ls command on a file, such as encript.cfg:

sysadmin@localhost:~$ ls /etc/enscript.cfg                             
/etc/enscript.cfg        

sysadmin@localhost:~$

As you can see, running the ls command on a single file results in the name of the file being printed. Typically this is useful if you want to see details about a specific file by using the -l option to the ls command:

sysadmin@localhost:~$ ls -l /etc/enscript.cfg                         

-r--r--r--. 1 root root 4843 Nov 11 2010 /etc/enscript.cfg      

sysadmin@localhost:~$

However, what if the ls command is given a directory name as an argument? In this case, the output of the command is different than if the argument was a file name:

sysadmin@localhost:~$ ls /etc/event.d                                    

ck-log-system-restart  ck-log-system-start  ck-log-system-stop            

sysadmin@localhost:~$

If you give a directory name as an argument to the ls command, the command will display the contents of the directory (the names of the files in the directory), not just provide the directory name. The filenames you see in the example above are the names of the files in the /etc/event.d directory.

Why is this a problem when using globs? Consider the following output:

sysadmin@localhost:~$ ls /etc/e*                                          

/etc/encript.cfg /etc/environment /etc/ethers /etc/event.d /etc/exports       
/etc/event.d:

sysadmin@localhost:~$
ck-log-system-restart  ck-log-system-start  ck-log-system-stop            

As you can see, when the ls command sees a filename as an argument, it just displays the filename. However, for any directory, it will display the contents of the directory, not just the directory name.

This becomes even more confusing in a situation like the following:

sysadmin@localhost:~$ ls /etc/ev*                                        

ck-log-system-restart  ck-log-system-start  ck-log-system-stop            

sysadmin@localhost:~$

In the previous example, it seems like the ls command is just plain wrong. But what really happened is that the only thing that matches the glob /etc/ev* is the /etc/event.d directory. So, the ls command only displayed the files in that directory!

There is a simple solution to this problem: when you use glob arguments with the ls command, always use the -d option. When you use the -d option, then the ls command won't display the contents of a directory, but rather the name of the directory:

sysadmin@localhost:~$ ls -d /etc/e*                                      

/etc/encript.cfg /etc/environment /etc/ethers /etc/event.d /etc/exports   

sysadmin@localhost:~$