In this article, RHCSA Part 2: File and directory management, we will review some essential skills that are required in the day-to-day tasks of a system administrator.
Create, Delete, Copy, and Move Files and Directories
File and directory management is a critical competence that every system administrator should possess. This includes the ability to create / delete text files from scratch (the core of each program’s configuration) and directories (where you will organize files and other directories), and to find out the type of existing files.
The touch command can be used not only to create empty files, but also to update the access and modification times of existing files.
You can use
file [filename] to determine a file’s type (this will come in handy before launching your preferred text editor to edit it).
rm [filename] to delete it.
As for directories, you can create directories inside existing paths with
mkdir [directory] or create a full path with
mkdir -p [/full/path/to/directory].
When it comes to removing directories, you need to make sure that they’re empty before issuing the
rmdir [directory] command, or use the more powerful (handle with care!)
rm -rf [directory]. This last option will force remove recursively the
[directory] and all its contents – so use it at your own risk.
Input and Output Redirection and Pipelining
The command line environment provides two very useful features that allows to redirect the input and output of commands from and to files, and to send the output of a command to another, called redirection and pipelining, respectively.
To understand those two important concepts, we must first understand the three most important types of I/O (Input and Output) streams (or sequences) of characters, which are in fact special files, in the *nix sense of the word.
- Standard input (aka stdin) is by default attached to the keyboard. In other words, the keyboard is the standard input device to enter commands to the command line.
- Standard output (aka stdout) is by default attached to the screen, the device that “receives” the output of commands and display them on the screen.
- Standard error (aka stderr), is where the status messages of a command is sent to by default, which is also the screen.
In the following example, the output of
ls /var is sent to stdout (the screen), as well as the result of ls /tecmint. But in the latter case, it is stderr that is shown.
To more easily identify these special files, they are each assigned a file descriptor, an abstract representation that is used to access them. The essential thing to understand is that these files, just like others, can be redirected. What this means is that you can capture the output from a file or script and send it as input to another file, command, or script. This will allow you to store on disk, for example, the output of commands for later processing or analysis.
To redirect stdin (fd 0), stdout (fd 1), or stderr (fd 2), the following operators are available.
|>||Redirects standard output to a file containing standard output. If the destination file exists, it will be overwritten.|
|>>||Appends standard output to a file.|
|2>||Redirects standard error to a file containing standard output. If the destination file exists, it will be overwritten.|
|2>>||Appends standard error to the existing file.|
|&>||Redirects both standard output and standard error to a file; if the specified file exists, it will be overwritten.|
|<||Uses the specified file as standard input.|
|<>||The specified file is used for both standard input and standard output.|
As opposed to redirection, pipelining is performed by adding a vertical bar
(|) after a command and before another one.
- Redirection is used to send the output of a command to a file, or to send a file as input to a command.
- Pipelining is used to send the output of a command to another command as input.
Examples Of Redirection and Pipelining
Example 1: Redirecting the output of a command to a file
There will be times when you will need to iterate over a list of files. To do that, you can first save that list to a file and then read that file line by line. While it is true that you can iterate over the output of ls directly, this example serves to illustrate redirection.
# ls -1 /var/mail > mail.txt
Example 2: Redirecting both stdout and stderr to /dev/null
In case we want to prevent both stdout and stderr to be displayed on the screen, we can redirect both file descriptors to
/dev/null. Note how the output changes when the redirection is implemented for the same command.
# ls /var /tecmint # ls /var/ /tecmint &> /dev/null
Example 3: Using a file as input to a command
While the classic syntax of the cat command is as follows.
# cat [file(s)]
You can also send a file as input, using the correct redirection operator.
# cat < mail.txt
Example 4: Sending the output of a command as input to another
If you have a large directory or process listing and want to be able to locate a certain file or process at a glance, you will want to pipeline the listing to grep.
Note that we use to pipelines in the following example. The first one looks for the required keyword, while the second one will eliminate the actual
grep command from the results. This example lists all the processes associated with the apache user.
# ps -ef | grep apache | grep -v grep