HTB Academy - Fundamentals - File Descriptors and Redirections

File Descriptors

A file descriptor (FD) in Unix/Linux operating systems is a connection maintained by the kernel to perform input/output (I/O) operations.

In Windows-based operating systems, this is commonly called a file handle.

By default, the first three file descriptors in Linux are:

STDIN and STDOUT

The cat command provides a simple example of how standard input and standard output work.

When running cat, the program receives input through standard input:

STDIN - FD 0

After entering text and pressing [ENTER], the input is returned to the terminal through standard output:

STDOUT - FD 1

In simple terms:

• STDIN - FD 0 is input.
• STDOUT - FD 1 is output.
• STDERR - FD 2 is error output.

STDOUT and STDERR

The find command can produce both standard output and standard error output.

Run the following command:

find /etc/ -name shadow

Valid results are returned through STDOUT, while errors such as Permission denied are returned through STDERR.

Redirect STDERR to /dev/null

We can redirect standard errors to /dev/null using file descriptor 2:

find /etc/ -name shadow 2>/dev/null

The following part performs the redirection:

2>/dev/null

• 2 represents STDERR.
• > redirects the stream.
• /dev/null discards anything written to it.

After redirecting STDERR, errors such as Permission denied are no longer displayed. Only standard output remains visible.

Redirect STDOUT to a File

Standard output can be redirected to a file using the greater-than operator:

find /etc/ -name shadow 2>/dev/null > results.txt

This command:

1. Redirects errors to /dev/null.
2. Redirects standard output to results.txt.

The following two redirections are equivalent:

> results.txt

1> results.txt

File descriptor 1 does not have to be specified because STDOUT is the default stream redirected by >.

If results.txt does not exist, it is created automatically. If it already exists, its contents are overwritten.

Redirect STDOUT and STDERR to Separate Files

Standard output and standard error can be redirected to different files:

find /etc/ -name shadow 2> stderr.txt 1> stdout.txt

This command creates two files:

Append STDOUT to a File

The single greater-than operator (>) overwrites the destination file:

command > output.txt

The double greater-than operator (>>) appends output to the destination file without deleting its existing contents:

find /etc/ -name passwd >> stdout.txt 2>/dev/null

This command appends the results to stdout.txt and redirects errors to /dev/null.

Redirect STDIN from a File

The lower-than operator (<) redirects the contents of a file to the standard input of a command:

cat < stdout.txt

In this example, cat receives the contents of stdout.txt through STDIN.

Redirect an STDIN Stream to a File

The double lower-than operator (<<) can be used to create a here-document.

A here-document allows multiple lines of input to be supplied to a command:

cat << EOF > stream.txt
Hack The Box
EOF

In this example:

• << EOF starts the input stream.
• Hack The Box is the content supplied through STDIN.
• The second EOF ends the input stream.
• > stream.txt redirects the output to stream.txt.

The word EOF is a delimiter and can be replaced with another identifier.

Pipes

A pipe (|) sends the standard output of one command to the standard input of another command.

The following command searches for files ending in .conf and filters the results to include only lines containing systemd:

find /etc/ -name "*.conf" 2>/dev/null | grep systemd

The command works as follows:

1. find searches for .conf files.
2. 2>/dev/null discards error messages.
3. | sends the remaining output to grep.
4. grep systemd displays only lines containing systemd.

Chain Multiple Pipes

Multiple pipes can be chained together.

The following command searches for .conf files, filters results containing systemd, and counts the number of matching lines:

find /etc/ -name "*.conf" 2>/dev/null | grep systemd | wc -l

The output from each command becomes the input for the next command:

find → grep → wc

Exercises

How many .log files exist on the system?

find / -iname "*.log" 2>/dev/null | wc -l

Command breakdown:

How many .bak files exist on the system?

find / -iname "*.bak" 2>/dev/null | wc -l

Output:

4

Submit the Full Path of the xxd Binary

whereis -b xxd

Output:

xxd: /usr/bin/xxd

The full path is:

/usr/bin/xxd