Basic Shell Skills

Introduction to Shells

A shell is a command-line interpreter that provides a user interface for accessing the services of the operating system. It allows users to execute commands, run programs, and manage files. There are several types of shells available in Unix-like systems, each with its own features and capabilities.

Common Shells

• Bourne Shell (sh): The original Unix shell, known for its simplicity and scripting capabilities.
• Bourne Again Shell (bash): An improved version of the Bourne shell, offering more features and better scripting capabilities. It is the default shell in many Linux distributions, including RHEL.
• C Shell (csh): A shell with C-like syntax, useful for scripting and interactive use.
• Korn Shell (ksh): A powerful shell that incorporates features from both the Bourne and C shells.
• Z Shell (zsh): An extended version of the Bourne shell, offering many enhancements and interactive features.

Basic Shell Commands

Understanding basic shell commands is crucial for navigating and managing a Unix-like system.

• ls: List directory contents.
• cd: Change the current directory.
• pwd: Print the current working directory.
• cp: Copy files and directories.
• mv: Move or rename files and directories.
• rm: Remove files and directories.
• mkdir: Create directories.
• rmdir: Remove empty directories.
• touch: Create empty files or update the timestamp of existing files.
• echo: Display a line of text.
• cat: Concatenate and display file content.
• more/less: View file content one page at a time.
• head/tail: Display the beginning or end of a file.

Editing Files with vim

vim (Vi IMproved) is a powerful and highly configurable text editor, an improved version of the classic Unix text editor vi. It’s widely used for editing files on Unix-like systems due to its efficiency and robustness.

Starting vim

To start vim, type vim followed by the name of the file you want to edit.

vim filename

If the file does not exist, vim will create it.

Basic vim Modes

vim operates in several modes, each serving a different purpose:

1. Normal Mode: This is the default mode. In this mode, you can navigate through the text and perform various commands.
2. Insert Mode: This mode is used for editing text. You can enter Insert Mode from Normal Mode by pressing i.
3. Visual Mode: This mode is used for selecting text. Enter Visual Mode from Normal Mode by pressing v.
4. Command-Line Mode: This mode is used for executing commands. Enter Command-Line Mode from Normal Mode by pressing :.

Basic Navigation

• Move the cursor: Use h, j, k, and l to move the cursor left, down, up, and right, respectively.
• Move to the beginning of the line: Press 0.
• Move to the end of the line: Press $.
• Move to the beginning of the file: Press gg.
• Move to the end of the file: Press G.

Editing Text

• Enter Insert Mode: Press i.
• Exit Insert Mode: Press Esc.
• Delete a character: Press x in Normal Mode.
• Undo the last change: Press u.
• Redo the last undone change: Press Ctrl-r.

Saving and Exiting

• Save changes: Type :w in Command-Line Mode.
• Save changes and exit: Type :wq in Command-Line Mode.
• Exit without saving: Type :q! in Command-Line Mode.

Searching and Replacing

• Search for a pattern: Type /pattern and press Enter in Normal Mode.
• Move to the next match: Press n.
• Move to the previous match: Press N.
• Search and replace: Type :s/old/new/g in Command-Line Mode to replace all occurrences of old with new in the current line. Use :%s/old/new/g to replace in the entire file.

Understanding the Shell Environment

The shell environment in Unix-like systems is a powerful interface for interacting with the operating system. Understanding the shell environment is crucial for efficient system administration.

Environment Variables

Environment variables are dynamic values that affect the behavior of processes on a system.

• Common Environment Variables:
  • PATH: Specifies the directories where executable files are located.
  • HOME: Specifies the current user’s home directory.
  • USER: Specifies the current user’s username.
  • SHELL: Specifies the current user’s shell.
• Viewing Environment Variables: Use the echo command to view the value of an environment variable.

echo $PATH

• Setting Environment Variables: Use the export command to set a new environment variable or change the value of an existing one.

export VARIABLE_NAME=value

Shell Configuration Files

Shell configuration files customize the shell environment and can be used to set environment variables, aliases, and functions.

• Common Shell Configuration Files:
  • ~/.bashrc: Executed for interactive non-login shells.
  • ~/.bash_profile: Executed for login shells.
  • ~/.profile: Another configuration file executed for login shells.

Aliases

Aliases are shortcuts for longer commands.

• Creating an Alias: Use the alias command to create a new alias.

alias ll=’ls -la’

• Viewing Aliases: Use the alias command without arguments to list all current aliases.

alias

• Removing an Alias: Use the unalias command to remove an alias.

unalias ll

Finding Help

Finding help and documentation is crucial for learning and troubleshooting in Unix-like systems.

Using the man Command

The man (manual) command displays the user manual for commands and other aspects of the system.

• View a Manual Page: Type man followed by the command name.

man ls

• Navigate the Manual Page: Use the arrow keys or Page Up and Page Down to scroll. Press q to quit.

Using the info Command

The info command provides more detailed documentation than man pages.

• View an Info Page: Type info followed by the command name.

info ls

• Navigate the Info Page: Use the arrow keys to scroll. Press q to quit.

Using the --help Option

Most commands have a --help option that displays a brief summary of how to use the command.

• View Help for a Command: Type the command followed by --help.

ls –help

Online Resources

There are numerous online resources for Unix-like systems documentation and community support.

• Official Documentation: Refer to the official documentation for your specific Unix-like system.
• Community Forums: Participate in community forums and mailing lists for help and discussions.
• Online Tutorials: Many websites offer tutorials and guides for Unix-like systems.

Conclusion

Mastering basic shell skills, including editing files with vim, understanding the shell environment, and finding help, is crucial for effective system administration in Unix-like systems. This guide provides a comprehensive overview of these essential skills, offering the necessary knowledge and practical examples to enhance your proficiency in managing RHEL systems.

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Exercise

Authenticating on the Server

• Action: Authenticate on the server1 server that you created.
  • Description: Login to the server as the user created during installation.
  • Purpose: Gain access to the server to perform the exercises.

Opening the Terminal

• Action: Click Activities, type term, and click the terminal icon.
  • Description: Open a terminal in the graphical user interface.
  • Purpose: Access the terminal for command-line exercises.

Executing the time ls Command

• Command: time ls
  • Description: This command executes the ls command, listing the contents of the current directory. The Bash built-in time command shows information about the time it took to complete this command.
  • Purpose: To measure and display the execution time of the ls command.

Finding the Path of time

• Command: which time
  • Description: This command shows the filename /usr/bin/time that was found in the $PATH variable.
  • Purpose: To locate the executable file for the time command.

Checking time as a Shell Keyword

• Command: time
  • Description: This command shows that time is a shell keyword.
  • Purpose: To differentiate between the internal shell keyword and the external command.

Displaying the $PATH Variable

• Command: echo $PATH
  • Description: This command shows the contents of the $PATH variable.
  • Purpose: To verify the directories included in the search path for executable files.

Executing /usr/bin/time ls Command

• Command: /usr/bin/time ls
  • Description: This command runs the ls command using the external /usr/bin/time command.
  • Purpose: To show the difference between the internal shell keyword time and the external time command.

Changing to Home Directory

• Command: cd
  • Description: This command changes the current directory to the user’s home directory.
  • Purpose: To ensure the user is in their home directory before starting the exercise.

Printing the Working Directory

• Command: pwd
  • Description: This command prints the current working directory.
  • Purpose: To verify the current directory.

Listing Directory Contents

• Command: ls
  • Description: This command lists the contents of the current directory.
  • Purpose: To display files and directories.

Redirecting STDOUT to Null Device

• Command: ls > /dev/null
  • Description: This command redirects standard output to the null device, effectively discarding it.
  • Purpose: To suppress the command output.

Executing a Non-Existent Command

• Command: ls ilwehgi > /dev/null
  • Description: This command attempts to list a non-existent file, showing an error message on the screen.
  • Purpose: To demonstrate error handling.

Redirecting STDERR to Null Device

• Command: ls ilwehgi 2> /dev/null
  • Description: This command redirects standard error to the null device, suppressing error messages.
  • Purpose: To suppress error messages.

Listing /etc Contents while Hiding Errors

• Command: ls ilwehgi /etc 2> /dev/null
  • Description: This command lists the contents of the /etc directory while hiding error messages.
  • Purpose: To show valid and invalid output redirection.

Redirecting STDOUT and STDERR Separately

• Command: ls ilwehgi /etc 2> /dev/null > output
  • Description: This command redirects standard output to a file named output while suppressing error messages.
  • Purpose: To save valid output to a file.

Displaying File Contents

• Command: cat output
  • Description: This command displays the contents of the file output.
  • Purpose: To verify the contents of the output file.

Overwriting File Contents

• Command: echo hello > output
  • Description: This command overwrites the output file with the text “hello”.
  • Purpose: To demonstrate file content manipulation.

Appending to a File

• Command: ls >> output
  • Description: This command appends the directory listing to the output file.
  • Purpose: To add content to an existing file without overwriting it.

Recursively Listing All Files and Directories

• Command: ls -R /
  • Description: This command recursively lists all files and directories starting from the root directory.
  • Purpose: To show all files and directories, which may produce a lengthy output.

Paginating Output with less

• Command: ls -R / | less
  • Description: This command pipes the recursive directory listing into less, allowing for paginated viewing.
  • Purpose: To improve the readability of lengthy output.

Quitting less

• Command: q
  • Description: This command quits the less pager.
  • Purpose: To exit the pager and end the command.

Attempting to Write to a Terminal Device File

• Command: ls > /dev/tty1
  • Description: This command attempts to write the directory listing to the terminal device file /dev/tty1.
  • Purpose: To demonstrate permission restrictions for device files.

Displaying Command History

• Command: history
  • Description: This command displays the history of commands used in the current session.
  • Purpose: To review previously executed commands.

Reverse Searching Command History

• Command: Ctrl-r
  • Description: This command initiates a reverse search in the command history.
  • Purpose: To quickly find and execute a previously used command.

Filtering Command History

• Command: history | grep cat
  • Description: This command pipes the history output through grep to search for commands containing the text “cat”.
  • Purpose: To filter history by specific commands.

Repeating a Command from History

• Command: !nn
  • Description: This command executes the command with the history number nn.
  • Purpose: To repeat a specific command from history.

Clearing Command History

• Command: history -c
  • Description: This command clears the in-memory command history.
  • Purpose: To erase the current session’s history.

Attempting Tab Completion

• Command: gd then Tab and Tab again
  • Description: This command attempts to use tab completion to find commands starting with “gd”.
  • Purpose: To demonstrate Bash’s tab completion features.

Completing a Command with More Context

• Command: gdi then Tab
  • Description: This command completes the command using more specific input to “gdi”.
  • Purpose: To show accurate tab completion with more context.

Changing to /etc Directory

• Command: cd /etc
  • Description: This command changes the current directory to /etc.
  • Purpose: To navigate to a specific directory.

Completing a File Name

• Command: cat ps then Tab
  • Description: This command completes the filename to passwd when there is only one match.
  • Purpose: To demonstrate file name auto-completion.

Editing a File with Vim

• Command: vim ~/testfile
  • Description: This command opens Vim and creates/edits a file named testfile in the home directory.
  • Purpose: To practice using the Vim editor.

Entering Input Mode in Vim

• Command: i
  • Description: This command switches Vim to input mode for typing text.
  • Purpose: To start entering text in Vim.

Exiting Input Mode in Vim

• Command: Esc
  • Description: This command exits input mode and returns Vim to command mode.
  • Purpose: To switch from text entry to command execution.

Writing Changes in Vim

• Command:
  • Description: This command writes the changes to the file.
  • Purpose: To save the file in Vim.

Navigating to a Specific Line in Vim

• Command: :3
  • Description: This command moves the cursor to line 3 in the file.
  • Purpose: To navigate within the file.

Deleting a Line in Vim

• Command: dd
  • Description: This command deletes the current line in Vim.
  • Purpose: To remove text.

Undoing the Last Change in Vim

• Command: u
  • Description: This command undoes the last change made in Vim.
  • Purpose: To reverse a previous action.

Opening a New Line in Vim

• Command: o
  • Description: This command opens a new line below the current line.
  • Purpose: To insert new lines for additional text.

Substituting Text in Vim

• Command: :%s/ox/OX/g
  • Description: This command substitutes all instances of “ox” with “OX” in the file.
  • Purpose: To perform text substitution.

Writing and Quitting Vim

• Command:
  • Description: This command writes changes and quits Vim.
  • Purpose: To save and exit Vim.

Displaying the LANG Variable

• Command: echo $LANG
  • Description: This command shows the current language setting.
  • Purpose: To verify language and locale settings.

Viewing Command Help

• Command: ls –help
  • Description: This command displays help information for the ls command.
  • Purpose: To learn about command options.

Temporarily Changing the Language

• Command: LANG=es_ES.UTF-8
  • Description: This command temporarily changes the language setting to Spanish.
  • Purpose: To demonstrate language setting changes.

Editing .bashrc

• Command: vim .bashrc
  • Description: This command opens the .bashrc file in Vim for editing.
  • Purpose: To modify user-specific shell settings.

Adding an Environment Variable to .bashrc

• Command: COLOR=red
  • Description: This command adds a custom environment variable COLOR set to “red” in .bashrc.
  • Purpose: To demonstrate adding a variable to configuration.

Verifying an Environment Variable

• Command: echo $COLOR
  • Description: This command displays the value of the COLOR variable.
  • Purpose: To check that the variable is set correctly.

Opening the man Manual Page

• Command: man man
  • Description: This command opens the manual page for the man command.
  • Purpose: To read documentation on using man.

Searching Within man Pages

• Command: /-k
  • Description: This command searches within the man page for the -k option.
  • Purpose: To find specific options in the manual.

Continuing the Search in man Pages

• Command: n
  • Description: This command continues searching in the man page.
  • Purpose: To navigate through search results.

Quitting man Pages

• Command: q
  • Description: This command quits the man page.
  • Purpose: To exit the manual viewer.

Reading the apropos Manual Page

• Command: man apropos
  • Description: This command opens the manual page for the apropos command.
  • Purpose: To understand the apropos command.

Reading the mandb Manual Page

• Command: man mandb
  • Description: This command opens the manual page for the mandb command.
  • Purpose: To learn about updating the man database.

Updating the Manual Page Database

• Command: sudo mandb
  • Description: This command updates the manual page database.
  • Purpose: To refresh the man page index as a superuser.

Reading the ls Manual Page

• Command: man ls
  • Description: This command opens the manual page for the ls command.
  • Purpose: To get detailed information about ls.

Navigating to the End of the man Page

• Command: G
  • Description: This command moves to the end of the man page.
  • Purpose: To quickly navigate to the end of the document.

Opening ls Info Page with pinfo

• Command: pinfo ‘(coreutils) ls invocation’
  • Description: This command opens the ls info page with pinfo.
  • Purpose: To read detailed documentation with better formatting.