Basic Linux Command Line Interface Guide

This basic Linux Command-Line Interface (CLI) Guide provides a general explanation of commonly used Bash shell commands for the Barracuda CloudGen Firewall. You can access the command-line interface by connecting to the hostname with a terminal utility (such as PuTTY). It is strongly recommended that the administrator reads the manual (man) page for bash (# man bash) after connecting to the system, and any man pages for commands listed in this article where a further explanation is needed. It is also important to remember that more options are available for commands than what are outlined in the following list.

To verify the directory path that you are currently working in, use the pwd command. It stands for "Print name of current Working Directory." From your shell, enter:

# pwd

To display the contents of the current or specified directory, use the ls command. It stands for "list directory contents" and can be thought of as "List to Screen." To display the contents of the current directory, enter:

# ls

To print the contents of the directory in long list format, add the -l option: 

# ls -l

Notice the different outputs between running ls and ls -l.

To attach a file system to a device or several devices, use the mount command. You can also run the command by itself to view the mounted partitions:

# mount 

( /dev/sda#  shows the mounted devices.) 

To view the total and available disk space for the file system, use the df command. It stands for “report file system disk space usage” but is commonly referred to as “disk free.” To print the output in a “human readable” format, add the -h option:

# df -h

To “change directories” or switch from one directory to another, use the cd command.

For example:

1. Change to the /home/remote directory.
   # cd /home/product

2. Print the current working directory to verify that you are now in the /home/product directory.
   # pwd 
   You should see the following output:
   /home/product

The touch command is “officially” used to change a file’s time stamp, but you can use it to create files.

For example, you can create a file named myfile in the /mail/tmp directory.

1. Change to the /mail/tmp directory.
   # cd /mail/tmp

2. Create a file named myfile.
   # touch myfile

3. List the directory contents to verify that myfile has been created.
   # ls
   In the output, myfile should be listed.

To change a file’s permissions, use the chmod command. When changing the file permissions with chmod, the permissions are binary counted. Before using the chmod command, identify what permissions should be given to which group. For example, you may choose to grant the following permissions:

• Owner — Read, Write, and Execute

• Group — Read and Execute

• Other — Read and Execute

For example, to change the permissions of a file named myfile:

1. Allow read, write, and execute permissions for the file.
   # chmod 777 myfile

2. List the directory contents in long list format to view the file permissions.
   # ls -l
   You should see output that is similar to the following:
   -rwxrwxrwx 1 root root     0 2013-03-06 12:17 myfile

To create or edit plain text documents or programs, use the vi or vim (Vi Improved) command. The vi editor has two modes:

• Insert mode — Lets you use input text into the document or program.

• Command mode — Lets you use commands. It is recommended that you read the man page for a comprehensive list of all commands.

You can also learn more about using vi by reading the Vimtutor and following the steps outlined in the files. To access the Vimtutor, run:

# vimtutor

The following table lists common commands that you can use with vi:

For example, to edit a file named myfile and save your changes:

1. Open the file in the vi editor.
   # vi myfile

2. Press the I key to enter the insert mode. When you are in insert mode, --INSERT -- is displayed at the lower left hand corner of the shell.

3. Type the following:

   #!/bin/bash echo "hello, $USER. I will list the files in the current directory" echo "The current directory is, $PWD" ls # list files

4. Press the Esc key to return to command mode.

5. Save and exit the file. Enter:
   :wq

6. To run this file, enter:
   # ./myfile.

To later remove a line from myfile without saving your changes:

1. Open the file in the vi editor: 
   # vi myfile

2. Put the cursor in the line that you want to delete and then press d+d.

3. Exit the file without saving. Enter:
   :q!

4. Run the file.
   # ./myfile.
   The output should not have changed.

You can use the wc command with various options to view information about a file:

Use the head command to print the first 10 lines of a file. For example, to print the first ten lines of a file named myfile:

# head myfile

To print lines based on other criteria, you can add options to the command.

Use the tail command to print the last 10 lines of a file. Usually, the command is used with the –f option to view a log file as it is being written. For example, enter:

# tail –f /mail/log/debug

To separate the last 10 lines that have been written from the appended output, press Enter.

To exit out of tail, press Ctrl+C.

To “concatenate” a file and print the standard output, use the cat command. It is one of many commands that you can use to view the contents of a file. For example, to view myfile:

# cat myfile

Using cat to create a file:

You can also use the cat command to create a file and write text to that file.

For example, to create a file named usernamecatfile:

1. Create the usernamecatfile file.
   # cat > usernamecatfile
   The > will redirect the output to a file. You can now write to the file. 

2. Type the following information: 
   This is my example of an output redirect

3. Press the Enter key to create a new line, and then press Ctrl+D to quit the cat.

4. Use cat for the file you created to verify the file was written to.

Using cat to output to a file:

You can also use the cat command to redirect the output to a new file. This is essentially a method of copying a file.

For example:

1. Redirect the output of usernamecatfile to usernamecatredir.
   # cat usernamecatfile > usernamecatredir

2. Use cat for the new file and review the output.

To view a file incrementally, use the less command. This command is especially useful on larger files and is more useful than the more command because it allows for forward and backward movement. When viewing the file, using /pattern will search forward within the file for the specified regex pattern. When viewing the file, using ?pattern will search backwards within the file for the specified regex pattern.

For example:

1. View myfile.
   # less myfile

2. Review the output. Notice how different it is from the output from using the cat command.

3. To exit, press the Q key.

To search a file for a given pattern, use the grep command. If a line has the requested pattern, the entire line is printed.

For example:

1. Run the command:
   # grep TIMING /mail/log/info

2. Review the output.

The grep command is an excellent tool with tailing for a specific pattern, sending an output to the grep with a pipe, or just greping a file for the pattern. Examples of this include:

• tail –f /mail/log/info | grep “domain.com”

• less /mail/log/info | grep “domain.com”

• grep “domain.com” /mail/log/info

To send the output of one command to the input of another, use the pipe ( | ) operator.

For example:

1. Use the less command to view the info file.
   # less /mail/log/info

2. Press the Q key to exit.

3. Press the up arrow key to display the previous command (# less /mail/log/info) and then add: 
   …/info| grep TIMING

4. Review the output. Notice the difference between the output for the less command and the grep command.

To move a file to another directory, use the mv command.

For example:

1. Move myfile from the /mail/tmp directory to the /home/remote directory.
   # mv /mail/tmp/myfile /home/remote/myfile.

2. List the contents of the /mail/tmp directory to verify that myfile is no longer in it.
   # ls /mail/tmp

3. Change to the /home/remote directory and then list its contents.
   # cd /home/remote/
# ls

To make a copy of a file or create a copy of a file in another location, use the cp command. It is recommended that you create a copy of any file before modifying it in any way.

For example:

1. Create a copy of myfile that is named myfile2.
   # cp myfile myfile2

2. List the files in the directory to verify that myfile2 was created.

For example, to create a copy of a file in another location:

1. Create a copy of myfile2 in another directory.
   # cp /home/remote/myfile2 /tmp/myfile2

2. List the files in the /home/remote and /tmp/ directories. Both directories should list myfile2.

To “remove” a file or directory, use the rm command. It is good practice to add the file or directory to be deleted before putting the rm command into the string.

For example:

1. Type the following command without pressing Enter:
   # /tmp/myfile2

2. Use the left arrow key to go to the beginning of the line and add rm.
   # rm /tmp/myfile2

3. Press Enter.

4. List the files in the tmp directory to verify that the file has been removed.

To “make links between files,” use the ln command. Many files and file system locations are already symbolically linked on Barracuda Networks products.

For example: 

1. Symbolically link /home/remote/myfile to /mail/tmp/seemyfile.
   # ln –s /home/remote/myfile /mail/tmp/seemyfile

2. Run a “long list” for /mail/tmp to see the symbolic link between seemyfile and /home/remote/seemyfile.

3. Cat both files to see how the content is similar.

To search for files in a directory hierarchy, use the find command. This command is especially useful when you do not know the name or location of a file.

For example, to search for files in the /home directory that have yfile in their name:

# find /home –iname ‘*yfile*’

The output prints all files in the /home directory that have the simple pattern of yfile in their name.

To display the full path of shell commands (executables or scripts), use the which command.

For example:

# which qm.pl

To create shortcuts by defining aliases, use the alias command.

For example:

1. Create an alias named homepers for listing the contents of the /home directory in long list format.
   # alias homepers=”ls –l /home”

2. Enter the alias to verify that it works.
   # homepers
   The contents of the /home directory should be listed in long list format.

The alias is removed when you log out of the shell. To permanently keep the alias, add it to the ~remote/.bashrc file.

To view how long the system has been running, use the uptime command.

To report a snapshot of the current running processes, use the ps command. It is useful for getting the process ID to send kill signals. If you are looking for a specific process, add a pipe and the requested process.

For example:

# ps fax

If you are looking for a specific process, you can pipe the ps command to a grep for a pattern.

For example:

# ps fax | grep mysql

Compare the outputs from both ps commands.

To display the top processes and memory used, use the top command. To sort by memory (%MEM), press Shift+M.

To send a kill signal to a process ID, use the kill command. The -9 is a common option and is used to “kill all”. For example: 

# kill -9 <process ID>

To trace system calls and signals, use the strace command. This command essentially intercepts and records the system calls of a process and the signals received by a process. Before using the strace command, use the top command to get the required process ID. After getting the process ID, use the following syntax to trace the process:

# strace –p <process ID>

ping

To send an ICMP ECHO_REQUEST to a host and listen for a response, use the ping command. This is a good tool for verifying that the host is responding to requests. However, make sure that ICMP has not been disabled on a network or the host; otherwise, no response is provided and the request will time out.

For example:

1. Ping your company website.
   # ping <company website address>  
   You should receive responses.

2. Press Ctrl+C to kill the ping to the website.

3. Ping www.aol.com.
   # ping www.aol.com 
   This request should not return any responses.

4. Press Ctrl+C to kill the ping to AOL.

5. Review the output.

To query a DNS server for a record, use the dig command. It is a DNS lookup utility. However, the dig command relies on DNS and will not reference the /etc/hosts file to resolve a name to an IP address.

Example:

• # dig barracuda.com (general DNS query for the barracuda.com a record)

• # dig @4.2.2.2 barracuda.com (DNS query for the barracuda.com a record but at a specific server (4.2.2.2))

• # dig mx barracuda.com (DNS query for a Mail eXchange (MX) record)

• # dig –x 64.235.145.81 (DNS query for a reverse lookup of an IP address) 

To query Internet domain name servers, use the nslookup command. It has two modes

• Interactive mode — Allows the user to query a name server for information about various hosts and domains.

• Non-interactive mode — Prints just the name and request information for a host or domain.

For example, to query the MX record for barracuda.com in interactive mode:

1. Start the nslookup.
   # nslookup

2. Specify that you are looking up the MX record.
   > set type=mx