Category: Linux

Mv is one of the Linux commands that must be learned. Mv stands for transferring files or directories from one place to another and is primarily used for moving them.

The syntax is similar to the cp command in Linux, but there is a fundamental distinction between these two commands.

The cp command can be called a copy-paste method. The mv instruction, while the cut-paste process can be equivalent.

This means the file or directory is transferred to a different location using the mv command on a file or directory. The source file/directory is no longer there.

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mv Command How can you use it?

The mv(transfer) command will move files and directories from place to place. It is also ideal for renaming files and folders.

mv [OPTIONS] source destination

• The source may be a single file or directory in the above command. The destination is always a single file or directory.
• When we have several files or folders, it is always a directory destination. Both source files and folders, in this case, are transferred to the directory of the destination. When we have a single source file and a destination directory, the file is transferred to the target folder.
• One crucial point is that when we transfer files and folders, we will obtain permission refused if we don’t have written permissions both for the source and destination.

mv mv image.png PNG

The current working directory transfers the image.png file to the PNG folder in the current work directory.

The original filename is renamed as the destination file if the destination directory isn’t present.

The image.png file is called PNG if it is not present in the existing working directory.

Transfer several folders and files

Specify the files you want to transfer as the source to move several files and folders. For instance, you would type to transfer file1 and file2 to the directory dir1:

mv File1 File2 dir1

You can also use pattern matching with the mv button. For, e.g., you would like to transfer all pdf files to the ~/Documents directory from the existing directory:

mv *.pdf ~/Documents

Drag a folder inside a separate folder with the mv command
We may use the following command to transfer a directory within another directory:

mv mv abcd abcd_New

It passes the abcd directory to another abcd New directory in our existing working directory.
The source directory is reset to the destination directory if the destination directory is not present.

How to transfer several files to another directory:
All source files and the path to the target directory are defined to transfer several files within a different directory.

mv <source(source)file path 3>

Our current working directory transfers the files 1.jpg, 2.jpg, and 2.png into a separate image directory in the current working directory.

Within a directory, we can transfer multiple files using regular expressions that match the filenames to be transferred.

Mv *jpg JPG

All files with mv backup:
We use the -b option to back up current files. It is intended to create a backup of the overwritten ~ character file with the attached backup file name.

mv -b a.jpg 1.jpg

ls

File rename

The mv command is essential for file renaming. The source file shall be renamed to the target file if you are using an mv command and specify a file name in your destination.
mv source_file target directory/target file

Suppose the target file does not exist in the target directory. In that case, the target file will be generated in the above case.

However, it overwrites without asking if the target file already exists. This means that with the source file’s content, the content of the current target file will be modified.

OverRight file when moving:

The existing file contents would be automatically overridden if a file is transferred and there is already a file with the same name.

In all cases, this might not be optimal. The overwriting scenario is available in a variety of ways.
You may use the -n option to avoid overwriting existing files. So mv will not overwrite the current file.

mv -n source_file target_directory

Forced movement of the file:

If you are shielded from writing the target file, you will be required to check until the target file is overwritten.

mv file1.txt target

Mv: substitute 'target/file1.txt' for 0444 overriding (r—r—r—) mode?

You may use the force option -f to bypass this prompt and overwrite the file immediately.

mv -f File1.txt target

tcpdump is a command-line utility that you can manage to capture and examine network traffic going to and from your system. It is the most regularly used tool amongst network administrators for troubleshooting network issues and security testing.

Notwithstanding its name, with tcpdump, you can also catch non-TCP traffic such as UDP, ARP, or ICMP. The captured packets can be written to a file or standard output. One of the most critical features of the tcpdump command is its capacity to use filters and charge only the data you wish to analyze.

In this article, you will learn the basics of how to use the tcpdump command in Linux.

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Installing tcpdump

tcpdump is installed by default on most Linux distributions and macOS. To check if the tcpdump command is available on your system type:

$ tcpdump --version

The output should look something like this:

Output:

tcpdump version 4.9.2

libpcap version 1.8.1

OpenSSL 1.1.1b 26 Feb 2019

If tcpdump is not present on your system, the command above will print “tcpdump: command not found.” You can easily install tcpdump using the package manager of your distro.

Installing tcpdump on Ubuntu and Debian

$ sudo apt update && sudo apt install tcpdump

Installing tcpdump on CentOS and Fedora

$ sudo yum install tcpdump

Installing tcpdump on Arch Linux

$ sudo pacman -S tcpdump

Capturing Packets with tcpdump

The general syntax for the tcpdump command is as follows:

tcpdump [options] [expression]

• The command options allow you to control the behavior of the command.
• The filter expression defines which packets will be captured.

Only root or user with sudo privileges can run tcpdump. If you try to run the command as an unprivileged user, you’ll get an error saying: “You don’t have permission to capture on that device.”

The most simple use case is to invoke tcpdump without any options and filters:

$ sudo tcpdump

Output:

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode

listening on ens3, link-type EN10MB (Ethernet), capture size 262144 bytes

15:47:24.248737 IP linuxize-host.ssh > desktop-machine.39196: Flags [P.], seq 201747193:201747301, ack 1226568763, win 402, options [nop,nop,TS val 1051794587 ecr 2679218230], length 108

15:47:24.248785 IP linuxize-host.ssh > desktop-machine.39196: Flags [P.], seq 108:144, ack 1, win 402, options [nop,nop,TS val 1051794587 ecr 2679218230], length 36

15:47:24.248828 IP linuxize-host.ssh > desktop-machine.39196: Flags [P.], seq 144:252, ack 1, win 402, options [nop,nop,TS val 1051794587 ecr 2679218230], length 108

... Long output suppressed

23116 packets captured

23300 packets received by filter

184 packets dropped by kernel

tcpdump will continue to capture packets and write to the standard output until it receives an interrupt signal. Use the Ctrl+C key combination to send an interrupt signal and stop the command.

For more verbose output, pass the -v option, or -vv for even more verbose output:

$ sudo tcpdump -vv

You can specify the number of packets to be captured using the -c option. For example, to capture only ten packets, you would type:

$ sudo tcpdump -c 10

After capturing the packets, tcpdump will stop.

When no interface is specified, tcpdump uses the first interface it finds and dumps all packets going through that interface.

Use the -D option to print a list of all available network interfaces that tcpdump can collect packets from:

$ sudo tcpdump -D

For each interface, the command prints the interface name, a short description, and an associated index (number):

 Output:

1.ens3 [Up, Running]

2.any (Pseudo-device that captures on all interfaces) [Up, Running]

3.lo [Up, Running, Loopback]

The output above shows that ens3 is the first interface found by tcpdump and used when no interface is provided to the command. The second interface any is a special device that allows you to capture all active interfaces.

To specify the interface you want to capture traffic, invoke the command with the -i option followed by the interface name or the associated index. For example, to capture all packets from all interfaces, you would specify any interface:

$ sudo tcpdump -i any

By default, tcpdump performs reverse DNS resolution on IP addresses and translates port numbers into names. Use the -n option to disable the translation:

$ sudo tcpdump -n

Skipping the DNS lookup avoids generating DNS traffic and makes the output more readable. It is recommended to use this option whenever you invoke tcpdump.

Instead of displaying the output on the screen, you can redirect it to a file using the redirection operators > and >>:

 $ sudo tcpdump -n -i any > file.out

You can also watch the data while saving it to a file using the tee command:

$ sudo tcpdump -n -l | tee file.out

The -l option in the command above tells tcpdump to make the output line buffered. When this option is not used, the output will not be written on the screen when a new line is generated.

Understanding the tcpdump Output

tcpdump outputs information for each captured packet on a new line. Each line includes a timestamp and information about that packet, depending on the protocol.

The typical format of a TCP protocol line is as follows:

[Timestamp] [Protocol] [Src IP].[Src Port] > [Dst IP].[Dst Port]: [Flags], [Seq], [Ack], [Win Size], [Options], [Data Length]

Let’s go field by field and explain the following line:

15:47:24.248737 IP 192.168.1.185.22 > 192.168.1.150.37445: Flags [P.], seq 201747193:201747301, ack 1226568763, win 402, options [nop,nop,TS val 1051794587 ecr 2679218230], length 108

• 15:47:24.248737 – The timestamp of the captured packet is local and uses the following format: hours:minutes: seconds. Frac, where frac is fractions of a second since midnight.
• IP – The packet protocol. In this case, IP means the Internet protocol version 4 (IPv4).
• 192.168.1.185.22 – The source IP address and port, separated by a dot (.).
• 192.168.1.150.37445 – The destination IP address and port, separated by a dot (.).
• Flags [P.] – TCP Flags field. In this example, [P.] means Push Acknowledgment packet, which acknowledges the previous packet and sends data. Other typical flag field values are as follows:

• • [.] – ACK (Acknowledgment)
  • [S] – SYN (Start Connection)
  • [P] – PSH (Push Data)
  • [F] – FIN (Finish Connection)
  • [R] – RST (Reset Connection)
  • [S.] – SYN-ACK (SynAcK Packet)
• seq 201747193:201747301 – The sequence number is in the first: last notation. It shows the number of data contained in the packet. Except for the first packet in the data stream where these numbers are absolute, all subsequent packets use as relative byte positions. In this example, the number is 201747193:201747301, meaning that this packet contains bytes 201747193 to 201747301 of the data stream. Use the -S option to print absolute sequence numbers.
• Ack 1226568763 The acknowledgment number is the sequence number of the next data expected by the other end of this connection.
• Win 402 – The window number is the number of available bytes in the receiving buffer.
• options [nop,nop,TS val 1051794587 ecr 2679218230] – TCP options. or “no operation,” is padding used to make the TCP header multiple of 4 bytes. TS val is a TCP timestamp, and ecr stands for an echo reply. Visit the IANA documentation for more information about TCP options.
• length 108 – The length of payload data

tcpdump Filters

When tcpdump is invoked with no filters, it captures all traffic and produces a tremendous output, making it very difficult to find and analyze the packets of interest.

Filters are one of the most powerful features of the tcpdump command. They since they allow you to capture only those packets matching the expression. For example, when troubleshooting issues related to a web server, you can use filters to obtain only the HTTP traffic.

tcpdump uses the Berkeley Packet Filter (BPF) syntax to filter the captured packets using various machining parameters such as protocols, source and destination IP addresses and ports, etc.

In this article, we’ll take a look at some of the most common filters. For a list of all available filters, check the pcap-filter manpage.

Filtering by Protocol

To restrict the capture to a particular protocol, specify the protocol as a filter. For example, to capture only the UDP traffic, you would run:

sudo tcpdump -n udp

Another way to define the protocol is to use the proto qualifier, followed by the protocol number. The following command will filter the protocol number 17 and produce the same result as the one above:

sudo tcpdump -n proto 17

For more information about the numbers, check the IP protocol numbers list.

Filtering by Host

To capture only packets related to a specific host, use the host qualifier:

$ sudo tcpdump -n host 192.168.1.185

The host can be either an IP address or a name.

You can also filter the output to a given IP range using the net qualifier. For example, to dump only packets related to 10.10.0.0/16, you would use:

$ sudo tcpdump -n net 10.10

Filtering by Port

To limit capture only to packets from or to a specific port, use the port qualifier. The command below captures packets related to the SSH (port 22) service by using this command:

$ sudo tcpdump -n port 23

The port range qualifier allows you to capture traffic in a range of ports:

sudo tcpdump -n port range 110-150

Filtering by Source and Destination

You can also filter packets based on the origin or target port or host using src, dst, src and dst, and src or dst qualifiers.

The following command captures coming packets from a host with IP 192.168.1.185:

sudo tcpdump -n src host 192.168.1.185

To find the traffic coming from any source to port 80, you would use:

sudo tcpdump -n dst port 80

Complex Filters

Filters can be mixed using the and (&&), or (||), and not (!) operators.

For example, to catch all HTTP traffic coming from a source IP address 192.168.1.185, you would use this command:

sudo tcpdump -n src 192.168.1.185 and tcp port 80

You can also use parentheses to group and create more complex filters:

$ sudo tcpdump -n 'host 192.168.1.185 and (tcp port 80 or tcp port 443)'

To avoid parsing errors when using special characters, enclose the filters inside single quotes.

Here is another example command to capture all traffic except SSH from a source IP address 192.168.1.185:

$ sudo tcpdump -n src 192.168.1.185 and not dst port 22

Packet Inspection

By default tcpdump, catches only the packet headers. However, sometimes you may need to examine the content of the packets.

tcpdump enables you to print the content of the packets in ASCII and HEX.

The -A option tells tcpdump to print each packet in ASCII and -x in HEX:

$ sudo tcpdump -n -A

To show the packet’s contents in both HEX and ASCII, use the -X option:

$ sudo tcpdump -n -X

Reading and Writing Captures to a File

Another useful feature of tcpdump is to write the packets to a file.

This is handy when you are taking a large number of packages or carrying packets for later analysis.

To start writing to a file, use the -w option followed by the output capture file:

$ sudo tcpdump -n -w data.pcap

This command up will save the capture to a file named data. pcap. You can name the file as you want, but it is a standard protocol to use the .pcap extension (packet capture).

When the -w option is used, the output is not represented on the screen. tcpdump writes raw packets and generates a binary file that cannot be read with a regular text editor.

To inspect the contents of the file, request tcpdump with the -r option:

$ sudo tcpdump -r data.pcap

If you need to run tcpdump in the background, add the ampersand symbol (&) at the command end.

The capture file can also be examined with other packet analyzer tools such as Wireshark.

When obtaining packets over a long period, you can allow file rotation. tcpdump enables you to generate new files and rotate the dump file on a defined time interval or fixed size. The following command will create up to ten 200MB files, named file.pcap0, file.pcap1, and so on: before overwriting older files.

$ sudo tcpdump -n -W 10 -C 200 -w /tmp/file.pcap

Once ten files are created, the older files will be overwritten.

Please take care that you should only run tcpdump only during troubleshooting issues.

If you need to start tcpdump at a particular time, you can use a cronjob. tcpdump does not have an alternative to exit after a given time. You can use the timeout command to stop tcpdump after any time. For example, to exit after 5 minutes, you would use:

$ sudo timeout 300 tcpdump -n -w data.pcap

Conclusion: 

To analyze and troubleshoot network related issues, the tcpdump command-line tool is used.

This article presented you with the basics of tcpdump usage and syntax. If you have any queries related to tcpdump, feel free to contact us.

Among those who work with Linux, the command’ pwd’ is very helpful that tells the directory you are in, starting from the root directory (/). For Linux newbies, who may get lost amid the wide variety of directories found on the command line, ‘pwd’ (Print Working Directory) comes to the rescue. ‘pwd ‘stands for ‘print working directory’ As you can tell, the command ‘pwd ‘prints where the user is currently at. It prints the current directory name, combined with the complete path, with the root folder listed first. This manual command is built into the shell and is available on most of the shells.

If both ‘-L ‘and ‘-P’ options are used, option ‘L ‘is taken into priority. If a choice isn’t specified at the prompt, pwd will only traverse symbolic links, i.e., take option -P into consideration. Using the pwd command, we will demonstrate how to identify your current working directory.

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What is the working directory?

The working directory is that in which the user is currently working. When you are working in the command prompt each time, you are in a directory. The default directory in which a Linux system opens when it is first booted is a user’s home directory. Change directories by using the cd command to delete any file from the current working directory (root directory), you would type:

$ cd /tmp

If you have a customized shell prompt, the path to your current working directory may be displayed.

user@host:/tmp#

Copy

pwd Command

The pwd command is “print working directory.” It is one of the essential and most commonly used Linux commands. When this command is invoked, the complete path to the current working directory will be displayed. The /pwd command is a command introduced in most modern shells such as bash and zsh. The standalone/bin/pwd is not the same as the /bin/pwd executable. The type command lets you display all files containing the “pwd” string.

$ type -a pwd

pwd refers to the shell builtin.

pwd is /bin/pwd

From the output, you can see the built-in Bash function ‘pwd’ has priority over the Bash standalone program and is used whenever you enter ‘pwd.’ If you wish to use the /bin/pwd standalone executable, enter the full path you saved the binary file how to change your current directory.

To find out the current directory, type pwd in your terminal and press return.

$ pwd

The resulting outputs will look similar to this.

/home/linuxcent

The pwd command determines the path of the PWD environment variable. The final output will be the same if you write:

$ echo $PWD

/home/linuxcent

The pwd command accepts only two arguments:

• -L (—logical) – Do not resolve symlinks.
• -P (—physical) – Display the physical directory without any symbolic links.

If no passphrase is specified, pwd behaves as if the -L option is specified.

To illustrate the operation of the -P option, I will create a directory and symlink.

$ mkdir /tmp/directoryln

$ -s /tmp/directory /tmp/symlink

Now, if you want navigate to the /tmp/symlink directory and you type pwd in your terminal:

$ pwd

The output shows your current working directory: /tmp/symlink

If you run the same command using -P option: $ pwd -P

The command will print the directory to which the symlink points to: /tmp/directory

Conclusion

The working directory is the current directory that your terminal is in. The pwd command lets you know where you are right now. If you have any kind of issues or comments, we would be delighted to hear them.

The tee command records from the regular input and writes both standard output and one or more files simultaneously. Tee is frequently used in sequence with other commands through piping.

In this article, we will cover the basics of working the tee command.

tee Command Syntax

The syntax for the tee command is as below:

tee [OPTIONS] [FILE]

Where OPTIONS can be:

• • -a (–append) – Do not overwrite the files; instead, affix to the given files.
  • -i (–ignore-interrupts) – Ignore interrupt signals.
  • Use tee –help to view all available options.
• FILE_NAMES – One or more files. Each of which the output data is written to

 How to Use the tee Command

The tee command’s most basic method represents the standard output (stdout) of a program and writing it in a file.

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In the below example, we use the df command to get information about the available disk space on the file system. The output is piped to the tee command, expressing the result to the terminal, and writes the same information to the file disk_usage.txt.

$ df -h | tee disk_usage.txt

Output:

Filesystem      Size  Used Avail Use% Mounted on

dev             7.8G     0  7.8G   0% /dev

run             7.9G  1.8M  7.9G   1% /run

/dev/nvme0n1p3  212G  159G   43G  79% /

tmpfs           7.9G  357M  7.5G   5% /dev/shm

tmpfs           7.9G     0  7.9G   0% /sys/fs/cgroup

tmpfs           7.9G   15M  7.9G   1% /tmp

/dev/nvme0n1p1  511M  107M  405M  21% /boot

/dev/sda1       459G  165G  271G  38% /data

tmpfs           1.6G   16K  1.6G   1% /run/user/120

Write to Multiple File

By using the tee command, you can write to multiple files also. To do so, define a list of files separated by space as arguments:

$ command | tee file1.out file2.out file3.out

Append to File

By default, the tee command will overwrite the specified file. Use the -a (–append) option to append the output to the file :

$ command | tee -a file.out

Ignore Interrupt

To ignore interrupts use the -i (–ignore-interrupts) option. This is useful when stopping the command during execution with CTRL+C and want the tee to exit gracefully.

$ command | tee -i file.out

Hide the Output

If you don’t want the tee to write to the standard output, you can redirect it to /dev/null:

$ command | tee file.out >/dev/null

Using tee in Conjunction with sudo

Let us say you need to write to a file owned by root as a sudo user. The following command will fail because the redirection of the output is not operated by sudo. The redirection is executed as the unprivileged user.

$ sudo echo "newline" > /etc/file.conf

The output will look something like this:

Output:

bash: /etc/file.conf: Permission denied

Prepend sudo before the tee command as shown below:

$ echo "newline" | sudo tee -a /etc/file.conf

the tee will receive the echo command output, upgrade to sudo permissions and then write to the file.

Using tee in combination with sudo enables you to write to files owned by other users.

Conclusion:

If you want to read from standard input and writes it to standard output and one or more files, then the tee command is used.

The source command on Linux is a pretty popular function run by system administrators daily.

But what is the function of the source command?

Used to refresh the current shell environment, the source command can also be used in order to import functions into other bash scripts or to run scripts into the current shell environment.

In today’s tutorial, we are going to see how the source command should be used on Linux.

The commands will be executed in a Debian 10 environment with GNOME, but they will work for every distribution.

Source command internals

Before starting, it is important to have a complete understanding of what environment and shell variables are.

By default, on Linux, your system already owns a couple of environment variables to store various information such as the shell to use, your hostname or your current username.

Environment variables are initialized by the system and are inherited by all processes on the system. As a consequence, when you are running a shell instance, you are to get the value of environment variables on your system.

$ echo $USER
devconnected

On the other hand, shell variables are variables declared within the context of a shell instance and they are not shared with other shells or with child processes.

$ VARIABLE=devconnected
$ echo $VARIABLE
devconnected

On Linux, when you execute a script, it is most likely executed in a subshell.

As a consequence, you won’t be able to have the variables defined in your first shell running in the script, they simply don’t share the same environment.

This is what the source command solves.

The source command is used in order to evaluate the file passed as an argument in the context of the current execution. Shortened as ‘.’, the source command is mostly used in the context of shells run in terminal windows.

$ source filename [arguments]

Note that when arguments are provided, they are set as positional parameters for the script specified.

Note : the source documentation is located inside the bash documentation. To read it, type “man bash” and search for the source command paragraph.

Source to update your current shell environment (.bashrc)

One of the main reasons to use source is to refresh the current shell environment by running the bashrc file.

As a reminder, .bashrc is a script file executed whenever you launch an interactive shell instance.

It is defined on a per-user basis and it is located in your home directory.

Let’s say for example that you want to add a new alias to your shell environment.

Open your .bashrc file and a new entry to it.

alias ll='ls -l'

Now try to run your new alias command directly in the terminal.

As you can see, the changes were not directly applied in your current environment.

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For the changes to be applied, run the source command with the .bashrc file as an argument.

$ source ~/.bashrc

An alternative way to do it is to run it with the dot syntax

$ . ~/.bashrc

If you try to execute your alias again, you should be able to run it

Source to execute a script in the current context

As explained before, the source command can be used to execute a script in the context of the current environment.

To illustrate this point, let’s define a local shell variable in your current shell environment.

$ VARIABLE=devconnected

Now, create a new script and paste the following content in it

#!/bin/bash

echo $VARIABLE
echo $LOGNAME

The VARIABLE variable represents the local shell variable we created before and the LOGNAME variable is an environment variable on your host.

Save your script and give execute permissions to your newly created script (you will need sudo privileges to execute this command)

$ sudo chmod ugo+x <script>

If you try to execute the script, you should see the following result

As you can see, the $VARIABLE local variable was not printed to the standard output.

This is because the script was executed in a subshell having its own set of local variables. The environment variable was printed however.

In order to execute this script in the context of our current shell, we are going to use the source command.

As you can see, in this case, the $VARIABLE variable was correctly printed to the standard output.

Source to import a script function

One of the great features of the source command is to update a script function, allowing a greater reusability of your existing shell functions.

To illustrate this point, let’s take the example of a function that prints the current user to the standard output.

Create a new script file and paste the following content inside.

$ nano script

#!/bin/bash
# Print the current user to the standard output
printUser() {
   echo $USER
}

Create another script and use the source function in order to import your printUser function.

$ nano import-script

#!/bin/bash
source ./script
printUser

Assign the correct permissions to the file you want to execute. You don’t need to have the execute permissions for the file containing the function you want to import.

$ sudo chmod u+x import-script
$ ./import-script

This is what you should see on your screen.

As you can see, the function was correctly imported and executed, printing the name of the current user of my host.

Source to read variables from a file

Another great usage of the source command on Linux is to read variables from a file.

Let’s say for example that you have a file with the following content in a file named “variables“

NAME=devconnected
WEBSITE=devconnected.com
DESCRIPTION="best educational website online"

In another script file, you would be able to use those variables by using the source command to read variables.

Create a script and paste the following content in it.

$ nano variable-script

#!/bin/bash
source ./variables
echo $NAME is a website located at $WEBSITE and it is probably the $DESCRIPTION

Troubleshooting

In some cases, you may run into errors when trying to source your files using the source command.

Here is a list of the common errors when trying to source files.

Source command not found on Linux

In some cases, you may run into this error

$ source: command not found

Or using sudo, you might also have a source command not found error.

$ sudo: source: command not found

The easy fix here is to execute the command as the root user by using “su”.

$ sudo -s
$ source .bashrc

Note that you can also use the “dot syntax” which is an equivalent to the source command.

$ . .bashrc

Conclusion

In today’s tutorial, you learnt what is the source command on Linux and how you can use to execute scripts in the context of the current environment.

You also learnt that it can be used to import functions or variables from a simple file to a script file.

If you want more tutorials related to Linux system administration, we have a complete category dedicated to it on devconnected. Click on the image below to read them.

On Debian 10, users are able to change their password pretty easily.

It is also possible, if you have sudo rights, to change user passwords as well as to define rules for password change on the host.

In this tutorial, we are going to see how you can change the user password on Debian 10 through the command-line and the user interface if you are using a GNOME desktop.

Change User Password using passwd

The first way to change the user password is to use the passwd command.

$ passwd

Changing password for devconnected.
Current password:
New password:
Retype new password:
passwd: password updated successfully

If you type the same password, you are going to have a warning message saying

Password unchanged

Change Another User’s Password with passwd

Before running the passwd command, make sure that you have sudo rights on your Debian 10 host.

To check sudo rights quickly, run the sudo command and make sure that you have error messages.

$ sudo -v

If you have sudo rights, you can run the passwd command.

Note: when updating another’s user account, you are not forced to know the current user password. It is very handy if you want to restrict the access to a user.

$ sudo passwd <user>

New password:
Retype new password:
passwd: password updated successfully

Delete Another User’s Password with passwd

Sometimes you want to reset the user password, maybe because it has lost it or because the password has been compromised.

You can set the password for the user, or you can delete the existing password to make the account passwordless.

To delete another user’s password, run the following command

$ sudo passwd -d <user>
passwd: password expiry information changed

Now when logging via the GNOME user interface, you won’t be prompted with a password. The account will automatically be logged in.

Note: deleting a user password must be done under rare circumstances and the account should be updated quickly to set a secure and long password.

User data might be compromised if no passwords are set for the account.

• How To Change Root Password on Debian 10
• How To Manage Root Account on Ubuntu 20.04
• How To Add A User to Sudoers On CentOS 8 | Centos 8 Add User to Group

Expire Another User’s Password with passwd

When setting a passwd on Debian, the password will never expire by default.

But sometimes, because you want to apply correct password policies, you may want to set an expiration time or to expire some accounts after a given time.

To expire another user’s password on Debian, run the following command

$ sudo passwd --expire <user>
passwd: password expiry information changed

Now when logging on the other user account, it should be prompted to change its password.

Change your password on the GNOME desktop

If you are using Debian 10 with a GNOME desktop, you can modify your password via the user interface.

System administrators tend to use the command line to perform administrative operations, but nothing forces you to do it this way.

1. In the Activities search box, type “Settings” and open it.

2. In the Settings window, choose the “Details” option.

3. Choose the “Users” option, and find the user you want to modify.

4. Click on the password field. Specify your old password and change your password to a secure one.

Click on “Change” and your password should be changed. Make sure to log again to test your new password.

Troubleshooting

In some cases, you may run into some errors while changing your password on Debian 10.

Here is the list of the most common errors and their solutions.

Default root password on Debian 10

By default, there is no default password for the root account on Debian 10.

This is because the root account is locked by default and setting a root password will unlock the account.

If you forgot your root password, you will have to reset it by rebooting and starting a bash shell into the GRUB.

Forgotten password on Debian 10

If you forgot your password on Debian, you will have to reset your password using the passwd command.

If you are not the system administrator, you have to ask the admin to run the passwd command in order to reset your password and make it expire immediately.

If you are the system administrator, you can run the passwd yourself.

$ sudo passwd <user>

If you remember the root password, connect as root and change the user password over there.

$ su -

$ passwd <user>

Conclusion

With this tutorial, you learnt how to change user password on Debian 10 Buster.

Another method to authenticate on a server is to use SSH keys. Make sure to check this article if you are interested in logging with SSH keys on Debian 10.

I hope that you learnt something new today.

Until then, have fun, as always.

This tutorial focuses on how to create swap space on Debian 10 via a swap file or a swap partition on your hard drive.

On a Linux system, it is very common to run out of memory, because you run too many programs, or because you run programs that are consuming too much memory.

As a consequence, if your RAM is full, you won’t be able to launch new programs on your computer.

You will have to manually shut down programs or tweak them to consume less memory.

There is however another way to virtually increase your memory : by using swap space.

In this tutorial, we are going to see how you can add swap space on Debian 10, either by creating a swap file or by creating a disk partition dedicated to swap.

Looking to add swap space on CentOS 8?

What is Swap Space on Debian?

Swap space is a space allocated to handle additional programs running when your RAM memory is full.

Let’s say that you have 4 GBs of RAM on your computer, and that 3 programs are already taking 3.5 GBs out of the 4 available.

If you are trying to run a program that is taking 1 GB on normal usage, you won’t be able to do it as you don’t have the space necessary for the program.

You could buy some RAM (which is expensive), or you could choose to create some swap space on your host.

When running your 1 GB program, your operating system (Linux here) will “move” or “swap” one of the programs to a dedicated part of your hard drive (your swap partition) and run your 1 GB program on the newly allocated space.

As you can imagine, the OS can switch programs from swap to RAM and vice versa.

The threshold to determine when programs should be switched from RAM to Swap is called the swappiness, but configuring ths swappiness will be reserved for another tutorial.

Now that you have some basics on what the swap space is and how the swap space works on Linux, let’s see on you can create some swap space on Debian 10.

Prerequisites

Sudo privileges

In order to add swap space on Debian 10 Buster, you need to have sudo privileges on your host.

Make sure this is the case by running the following command

$ sudo -v

If you are not getting any errors messages, you are good to go.

• How To Add Swap Space on Ubuntu 20.04
• How To Install and Configure Debian 10 Buster with GNOME
• How To Encrypt Root Filesystem on Linux

Checking existing swap partitions

In order to see existing swap partitions available on your host, run the following command

$ sudo swapon --show

If a partition is already existing, you should get at least one line as a result.

As you can see, I already own a swap partition on my sda drive of size 8 GB.

As the current memory on my computer is sufficient, my host is not using swap at the moment.

If no swap spaces are configured on your system, this is the output that you should expect.

Add Swap Space with a swap file

The first method to add swap space on Debian is to use a dedicated swap file.

Many tutorials are not specifying this detail, but swap files cannot contain any holes at all.

It means that you should not use the cp command to create your swap file.

It is also not recommended to use the fallocate commands on file systems that support preallocated files such as XFS and ext4.

As a consequence, you are going to use the dd command in order to add swap space on Debian.

In this case, we are going to create a 2 GB swap file.

Note : there are no performance improvements in using a swap file rather than creating a file partition. Swap files are just easier to manage because the file size can be easily adjusted. Changing the partition size for swap can be trickier than changing the file size.

a – Create a swapfile using dd

To add swap space, run the following command

$ sudo dd if=/dev/zero of=swapfile bs=1MiB count=$((2*2014))

Make sure that your swap file was created by issuing the following command.

$ ls -l swapfile

b – Secure your swapfile with permissions

Swap files are only used by the operating system for memory optimization purposes.

As a consequence, it should not be modified by any users except for the root user.

Change the file permissions of your swapfile.

$ sudo chmod 600 /swapfile

c – Enable your swapfile

Now that your swapfile is secure, it is time to activate your swap space.

To enable swap space on Debian 10, run the following command.

$ sudo mkswap /swapfile

This is going to set the file as a swap file, setting the correct headers for the swapon binary.

Now that the swapspace is correctly set, you can enable it.

$ sudo swapon /swapfile

To verify that your swap space is active, you can run the initial command with the –show flag.

$ sudo swapon --show

d – Make your swap space permanent

Similarly to the creation of filesystems, changes won’t be made permanent if you don’t append some changes to the fstab file.

If you leave it this way, your swap space will be erased at the next host reboot.

To make your swap space permanent, edit the /etc/fstab file and paste the following changes.

$ cd /etc/
$ sudo nano fstab

/swapfile none swap defaults 0 0

This configuration specifies that :

• /swapfile: the name of the “swap filesystem” we are creating;
• none: there is mount point for this filesystem
• swap: the filesystem type used
• defaults: the filesystem options, set as default for this example
• 0: the dump option for the filesystem, as well as the pass option.

Save your file, and restart your changes to make sure that the changes are still effective.

$ sudo reboot
$ sudo swapon --show

Congratulations!

You successfully created swap space on Debian 10 using a swap file.

Add Swap Space with a swap partition

Another way to add swap space on Debian is to create a dedicated swap partition.

If you run the initial Debian 10 installation, there is a high chance that some swap partition is already created on your system.

However for this tutorial, we are going to start from scratch and create our own swap partition.

a – Create a swap space partition with fdisk

To have a look at the existing partitions on your host, run the following command

$ sudo fdisk -l

As you can see, I already own a ext4 primary partition and a second partition that is not currently used.

We are going to add a swap partition as an extended or logical partition on sda.

Run the fdisk utility, and create a new partition on the hard drive that you want (sda in my case)

$ sudo fdisk /dev/sda

Welcome to fdisk (util-linux 2.30.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Command (m for help): n

Run the “n” command to add a new partition to your disk.

Note : if your host is running out of primary partitions, you can add swap space within an extended partition.

All space for primary partitions is in use
Adding logical partition 5
First sector (48291898-65062911, default 48291840):

You can leave the first sector option as default by just pressing Enter.

On the next prompt, specify the size of your swap partition. In this case, I am going to use 2 GiB again.

Last sector, +/-sectors or +/-size{K,M,G,T,P} : +2G

Created a new partition 5 of type 'Linux' and of size 2 GiB.

As you can see, partitions are created with the Linux partition type by default.

This is not what we want, since we want to have a swap partition on our drive.

To change the partition type, run the “t” command in fdisk.

Command (m for help): t
Partition number (1,2,5, default 5): 5
Hex code (type L to list all codes): 82

On Linux, swap partitions have the partition type ID 82 in fdisk.

Hit Enter, and make sure that your partition type was correctly changed.

Changed type of partition 'Linux' to 'Linux swap / Solaris'

Don’t forget to write your changes to disk as fdisk does not directly write to disk unless you ask it do it.

To write on disk, run the “w” command in fdisk.

The partition table has been altered
Syncing disks.

Make sure that your swap partition was correctly added by running the fdisk command again.

Now that your swap partition is created, it is time to enable it on our Debian 10 host.

b – Enabling your swap partition

First, make sure to run the mkswap for the swap headers to be correctly set on your partition.

$ sudo mkswap /dev/sda5

Now that your headers are set, run the swapon command.

$ sudo swapon /dev/sda5

Similarly to the other method, make sure that your swap space was correctly created.

$ sudo swapon --show

c – Make your swap space permanent

In order to make your swap space permanent, it needs to be added to the fstab file.

First of all, get the UUID for your newly created partition.

$ sudo blkid

Copy the UUID value, and edit your fstab to append the following changes.

$ sudo nano /etc/fstab

UUID=4c46c5af-3530-486b-aabe-abca2543edca   none   swap  defaults   0   0

Save your file, and restart your system to make sure that your changes are permanent.

$ sudo reboot
$ sudo swapon --show

Congratulations, you correctly created a swap partition using fdisk on Debian 10 Buster.

Remove swap partition on Debian

Removing swap partitions on Debian is pretty straightforward : first you need to use the command “swapoff” on the swap partition you are trying to remove.

If you are not sure about your current existing partitions, run a simple “blkid” command.

$ blkid 

$ sudo swapon /dev/sda5

Finally, edit your fstab and remove the entry associated with the swap partition.

$ sudo nano fstab

UUID=4c46c5af-3530-486b-aabe-abca2543edca   none   swap  defaults   0   0     <--- To be removed.

Troubleshooting

When adding swap space on Debian 10 Buster, you may run into the following error.

swapon: /swapfile: read swap header failed.

This error is happening when you don’t run the mkswap command before running the swapon command.

As a reminder, mkswap sets the header for the file or the partition to be used as swap space.

If you forget to run the mkswap command, Linux won’t be able to assign it as swap space on your host.

Location of swap file on Linux

By default, swap files are located into the “/proc/swaps” directory of your system.

~$ cat /proc/swaps
Filename                Type        Size    Used    Priority
/swapfile               file        1025101 0       -1

From there, you know that your swap file is located at your root directory.

Another way to get the location of your swap file is to inspect the fstab file.

$ cat /etc/fstab

/swapfile    none     swap    sw     0       0

Conclusion

Today, you learnt that there are two ways to add swap space on a Debian 10 host, by creating a swap file or by creating a swap partition with fdisk.