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Netstat is a powerful tool that is used by experienced network administrators, but can also be used as a basic tool to see active connections to a system.

The use of any Linux command, such as netstat, requires the use of the proper syntax. Know the basic structure of the command and the switches.

Common Shell Commands

4.10 Use common shell commands and expressions.

Basic tools are built into the shells, which are started as the environment when a user or administrator logs in to a Linux system. The most common shell for Linux is Bash or Bourne Again SHell. Other major shell programs used for Linux are Public Domain Korn Shell (PDKSH), the TCSH shell, and the Z-shell.

See Chapter 8 for more information on shell programs.

Bash is an sh-compatible command language interpreter, which is intended to conform with the IEEE POSIX Shell and Tools specification. Bash also incorporates useful features from the Korn and C shells (ksh and csh). The following are the configuration files for Bash:

/bin/bash: The bash executable

/etc/profile: The system wide initialization file for login shells

~/.bash_profile: The personal initialization file for login shells

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~/.bashrc: The individual per-interactive-shell startup file

~/.bash_logout: The login shell cleanup file that executed when the shell exits

~/.inputrc: Individual read line initialization file

Instructions for the Bash shell are very large and complex — in fact, the MAN page contains over 5000 lines of text, and thus covering this topic would require another book. Some of the most common and useful tools included in the shell are shown in Table 10-2.

Table 10-2

Bash Shell Commands and Tools Used

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This is by no means the definitive list of tools for Bash. With experience, you will learn more of the included shell commands and the necessary commands for each environment. These common commands can help make Linux tools — especially the history tool — easier to use for everyone.

Basic shell scripts

4.18 Program basic shell scripts using common shell commands (e.g., grep, find, cut, if)

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You can use the common shell commands to create some very simple scripts to perform routine tasks. You can chain together a number of basic commands to perform a specific task. Here is an example of a shell script:

#!/bin/sh

# This script changes to your home directory and does a directory list

cd /home/user1 ls -al

In this simple script, all we are doing is changing to a user’s home directory, and listing the contents of that directory.

The first line identifies this as a shell script that is passed to the BASH shell program at /bin/sh. This is important to ensure that the script is run under the BASH shell and not another one such as C shell.

The second line is a comment, telling the user what the script does. All comments are preceded with a “#” mark.

The next two commands are simply BASH shell commands to change to the user home directory, and list the contents.

Here is a list of common commands that can be very useful when programming shell scripts:

grep — This command searches for a specific pattern of text in a file. This is useful for searching large amounts of text for a certain name or error message.

find — This command is similar to grep, as it searches a directory for a pattern or file that you are looking for.

cut — This command removes sections of text from lines of a file.

if — If you need to perform some type of logical decision making within a script, you would use the if command. For example:

if [ “yes” = “yes” ]; then echo “The answer is yes”

These simple examples barely scratch the surface of the many large tasks you can perform using shell scripts.