[Day 4] 1. Few more Commands

Types of Commands in Linux

1. Internal Commands

• Definition: Commands implemented within the shell program itself.
• Executable File: Not available on the disk.

2. External Commands

• Definition: Commands implemented separately from the shell.
• Executable Locations: Found on the hard disk in directories such as
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  /bin

  ,
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  /sbin

  ,
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  /usr/bin

  , and
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  /usr/sbin

  .

Command Types and Tools

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  type

  Command: Checks if a command is internal or external.
  • Examples:
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      type echo

      →
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      echo is a shell builtin

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      type mkdir

      →
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      mkdir is /usr/bin/mkdir

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      type cd

      →
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      cd is a shell builtin

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  which

  Command: Provides the path of a given command (location on the hard disk).
  • Examples:
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      which ls

      →
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      /usr/bin/ls

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      which cd

      →
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      /usr/bin/cd

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      which echo

      →
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      /usr/bin/echo

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      which pwd

      →
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      /usr/bin/pwd

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      which mkdir

      →
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      /usr/bin/mkdir

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  whereis

  Command: Gives the path of the command and its manual page.
  • Examples:
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      whereis ls

      →
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      ls: /usr/bin/ls /usr/share/man/man1/ls.1.gz

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      whereis cd

      →
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      cd:

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      whereis alias

      →
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      alias:

File Handling in Shell

• Internal commands are executed inside the shell only.

• The shell deals with text files and not special files like PDFs.

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  open

  Command:
  • Use to open files based on their paths. It checks the file's magic number and opens the appropriate application.
  • Example: Opening a PDF file will launch a PDF viewer.

Tools for Executable Files

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  readelf

  : Displays detailed information from ELF (Executable and Linkable Format) files, such as headers, sections, and symbols.
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  objdump

  : Offers broader functionality, including disassembling code and showing headers, sections, and symbol tables for various binary formats.

Manual Pages

• Use
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  man

  to access manual entries for commands.
  • Example:
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    man alias

    returns "No manual entry for alias" because it's built-in.
  • Instead, use
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    man bash

    for bash-related commands.

Command Execution

• Synchronous Execution: Commands execute one after the other, with the shell waiting for the current command to finish before proceeding.

• Asynchronous Execution: Commands execute in the background, allowing immediate execution of other commands. This is done using

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  &

  at the end of the command.
  • Example:
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    cat &

  • Check status with
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    ps

    command.
  • Use
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    kill

    command to stop background processes.
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      kill -l

      : Lists signals (64 in total, each with a predefined meaning).
    • Examples of killing a process:
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        kill -9 5480

        (using PID)
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        kill -SIGKILL 5480

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        pkill -9 cat

        (can kill multiple processes)

Logical Operators

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  &&

  (AND Operator):
  • Nest commands; the second command executes only if the first is successful.
  • Example:
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    ls && pwd

  • If the first command fails (e.g.,
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    ls -W

    ), the second won't execute.

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  ;

  (Separator):
  • Executes the second command regardless of the first command's success or failure.

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  ||

  (OR Operator):
  • The second command executes only if the first fails.
  • Example:
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    ls || pwd

  • If
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    ls -W

    generates an error, the error is displayed, and the second command executes.