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Chapter 3

  1. ls List directory in depth

    • ls list all dir and files in current dir.

    • ls /Desktop same for a sub dir Desktop.

    • ls HP/ Desktop/ Downloads/ specifying multiple dir.

    • ls -l for long detailed view.

    • Commands are often followed by one or more options that modify their behavior and, further, by one or more arguments, the items upon which the command acts:
      command -options arguments

    • ls -lt to get long listin along with result sorted on modification time in ascending order of time.

    • ls -lt --reverse same as above just reverse the result of time sorting.

    • Other options

      • -a List all files, even those with names that begin with a period

      • -F append an indicator character (/) to the end of each listed name while file name remains same.

      • -r display the result in reverse order

      • -S sort result by size

      • -t sort by modification time

        himanshu in ~: ls -l
total 68
drwxrwxr-x 2 himanshu himanshu 4096 Mar 29 10:50  bin
drwxr-xr-x 2 himanshu himanshu 4096 Mar 29 16:21  Desktop
drwxr-xr-x 2 himanshu himanshu 4096 Mar 31 11:43  Documents
drwxr-xr-x 2 himanshu himanshu 4096 Apr  4 20:56  Downloads
-rw-rw-r-- 1 himanshu himanshu  851 Mar 23 00:50  dump.rdb
drwxrwxr-x 4 himanshu himanshu 4096 Mar 28 16:04  HP
drwxr-xr-x 2 himanshu himanshu 4096 Feb 25 23:38  Music
drwxr-xr-x 3 himanshu himanshu 4096 Mar 14 11:31  Pictures
        1. -rw-rw-r-- first - show its a file while d shows its a directory.
        2. Next three characters are the access rights for the file’s owner, the next three are for members of the file’s group, and the final three are for everyone else.
        3. 1 show file's number of hard links
        4. himanshu The user name of the file’s owner
        5. himanshu The name of the group that owns the file.
        6. 851 Size of the file in bytes.
        7. Mar 28 16:04 Date and time of the file’s last modification.
        8. dump.rdb Name of the file.

  2. file Determine file type

    • file filename is the format

    • Identify directory and files

      $: file HP/
HP/: directory
      $: file MyTextFile.txt 
MyTextFile.txt: ASCII text

    • hello

  3. less View text file content

    • less filename is the format. To exit press Q. less falls into the class of programs called pagers, programs that allow the easy viewing of long text documents in a page-by-page manner

    • less /etc/passwd file that defines all the system’s user accounts.

    • Shortcuts

      • PAGE UP or b : Scroll back one page

      • PAGE DOWN or Spacebar: Scroll forward one page.

      • G Move to the end of the text file.

      • g Move to the beginning of the text file.

      • /characters then hit enter Search forward to the next occurrence of characters.

  4. Directories Found on Linux Systems

    • / The root directory, where everything begins.

    • /bin Contains binaries (programs) that must be present for the system to boot and run.

    • /boot Contains the Linux kernel, initial RAM disk image, and the boot loader.
      Interesting files:

      • /boot/grub/grub.conf or menu.lst, which are used to configure the boot loader
      • /boot/vmlinuz the Linux kernel

    • /dev Contains device nodes. “Everything is a file” also applies to devices. Here is where the kernel maintains a list of all the devices.

    • /etc The /etc directory contains all of the system-wide configuration files. It also contains a collection of shell scripts that start each of the system services at boot time

    • /home Ordinary users can write files only in their home directories. This limitation protects the system from errant user activity.

    • /lib Contains shared library files used by the core system programs. These are similar to DLLs in Windows.

    • /lost+found It is used in the case of a partial recovery from a filesystem corruption event. this directory will remain empty.

    • /media will contain the mount points for removable media such as USB drives, CD-ROMs, etc.

    • /opt used to install “optional” software. This is mainly used to hold commercial software products that may be installed on your system.

    • /proc It’s not a real filesystem in the sense of files stored on your hard drive. Rather, it is a virtual filesystem maintained by the Linux kernel. The “files” it contains are peepholes into the kernel itself.

    • /root This is the home directory for the root account.

    • /sbin This directory contains “system” binaries. These are programs that perform vital system tasks that are generally reserved for the superuser.

    • /tmp for storage of temporary, transient files created by various programs. Some configurations cause this directory to be emptied each time the system is rebooted.

    • /usr It contains all the programs and support files used by regular users.

    • /usr/bin contains the executable programs installed by your Linux distribution.

    • /usr/lib The shared libraries for the programs in /usr/bin.

    • /usr/local programs that are not included with your distribution but are intended for system-wide use are installed. Programs compiled from source code are normally installed in /usr/local/bin.

    • /usr/sbin Contains more system administration programs

    • /usr/share contains all the shared data used by programs in /usr/bin. This includes things like default configuration files, icons, screen backgrounds, sound files, etc.

    • /usr/share/doc Most packages installed on the system will include some kind of documentation. In /usr/share/doc, we will find documentation files organized by package.

    • /var The /var directory tree is where data that is likely to change is stored. Various databases, spool files, user mail, etc. are located here

    • /var/log contains log files, records of various system activity. These are very important and should be monitored from time to time.

  5. Symbolic Link

    lrwxrwxrwx 1 himanshu himanshu   52 Mar 28 16:04 Tutorials -> '/media/himanshu/Local Disk/dev/Courses and Tutorials'

    This is a special kind of a file called a symbolic link (also known as a soft link or symlink).

    Notice the first character l in lrwxrwxrwx it's neither - not d. Also looks the dir have two name but in actual first name Tutorials is actual name of symlink while another name /media/himanshu/Local Disk/dev/Courses and Tutorials is the location of actual file/directory.

    Usefull in updateing the lib so when version update from 1.0 to 1.1 or something then previous program which refereed to this lib using symlink foo can now point to version 1.1 using new symlink foo just we need to update foo to point to new version.