Linux SysAdmin [RHEL] - Lesson #2

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Linux SysAdmin [RHEL] - Lesson #2

This thread is continue of Linux System Administrator topic based on RedHat.


General Information and Terms:
Environment Variables - Some values have been set in Linux OS directly. It includes $PATH variables, system and user information($HOME, $USER, $SHELL, $LANG), and temp configuration($TMPDIR)


General Usage:

1. env - Shows all Environment Variables

1.1. export variable_name="variable_value" - Sets environment variable temporary for user's current session.

1.2. unset variable_name - Removes environment variable temporary for user's current session.

1.3. Adding export variable_name="variable_value" at the end of the line in /home/user_name/.bashrc - Sets environment variable permanent for current user.
After adding line, using source /home/user_name/.bashrc applies settings. If you do not use this command, custom Environment Variables will not set for current session. All of user's new terminal sessions will load with custom Environment Variables.

1.4. Adding export variable_name="variable_value" at the end of the line in /etc/profile - Sets environment variable system wide, for all users permanently.

2. which command_name - Shows command's path, where is command's executable file located.

3. type command_name - If command is internal, says only command_name is a shell bultin, if command is external, shows command's executable file path as which command, but with its words.

4. Redirection of Command Outputs

4.1. Overriding: command_name > file.txt - Replacing (Overriding) content of file.txt with command's Standard Output (stdout). If this file already contains any content, all of them will be deleted and command's output will be written inside.

4.2. Adding: command_name >> file.txt - Adds command's Standard Output (stdout) to the end of content, with a new line in file.txt.

4.3. Redirecting: command_name 0< file.txt - Redirects file's content to command as argument.
P.S> It is NOT Best Practise. Most commands do not support to get arguments from Standard Input (stdin).
Best Practise Alternative: cat file.txt | xargs command_name

As shown in below picture, Standard Input (stdin) is not supported for setting argument by most commands.

4.4. Standard Outputs(stdout), Standard Inputs(stdin) and Standard Errors(stderr)

4.4.1. Standard Outputs: Command's normal output with Error Code 0 (Error Code 0 means program finished successfully, without error)
It redirects using 1> or 1>>, 1 means Standard Output (stdout).
P.S> When using > or >>, it defaults to using 1> or 1>> in fact.
Example: pwd >> file.txt is pwd 1>> file.txt

4.4.2. Standard Inputs: Command's input redirected from a file or command output.
P.S> Input is entering data while program is in progress, Argument is data which setting via parameters. They are different things.
It redirects using < or 0<, 0 means Standard Input (stdin).

4.4.3. Standard Errors: Command's output with Error Code x(>0) (Error Code x(>0) means program finished with error, code is higher than 0)
It redirects using 2> or 2>>, 2 means Standard Error (stderr).
P.S> If you want to get command output only normal output without errors, you can redirect errors to not existing path/file.

Usage: command_name >> file.txt 2> /dev/null
It will redirect errors to /dev/null file which it called as "Black Hole".

5. vim file_name.ext - Text editor, new and improved version of vi

5.1. Pressing i button, entering insert mode. After this, you can add and modify content.

5.2. Pressing [ESC] button, entering command mode (exiting insert mode).
5.2.1. Unsaved Quit: :q - quiting without changing content. If any changes, it asks for saving.
P.S> Using ! mark, force quit without save, without interaction.
:q! - quiting without changing content, without interaction.

5.2.2. Saved Quit: :wq - quiting with changing content.
Fast Alternative: In command mode, [SHIFT]+[Z][Z]

5.2.3. Search Content: /search_term - searchs term and marks it in content.

6. nano file_name.ext - Text editor as vim, but more comfortable.

6.2. Nano is easier to use than vim. All shortcuts is on terminal screen.
6.2.1. [CTRL]+[O] - Saves changes
6.2.2. [CTRL]+[X] - Exits nano, if unsaved changes exists, it asks for saving.
6.2.3. [CTRL]+[W] - Searchs terms inside of content.

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