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Linux.md

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File System

  • /
    • Contains the entire filesystem hierarchy.
  • /bin
    • Contains essential binary executables and command-line tools, such as:
      • Shell interpreters (e.g., bash, sh)
      • Basic system utilities (e.g., ls, cp, mv, rm)
      • Text processing tools (e.g., grep, sed, awk)
  • /etc
    • Stores system-wide configuration files, including:
      • Network configuration (e.g., /etc/network/interfaces)
      • User and group information (e.g., /etc/passwd, /etc/group)
      • System startup scripts (e.g., /etc/init.d)
      • Application configurations (e.g., /etc/apache2/apache2.conf)
  • /home
    • Contains user home directories, which typically include:
      • Personal files and documents
      • User-specific configuration files (e.g., .bashrc, .profile)
      • Application data (e.g., browser bookmarks, email client data)
  • /opt
    • Houses optional or third-party software packages, such as:
      • Proprietary applications (e.g., Google Earth)
      • Custom-compiled software
      • Large software suites (e.g., IBM WebSphere)
  • /tmp
    • Provides temporary storage for various processes and users:
      • Temporary files created by applications
      • Session data
      • Cached information
  • /usr
    • Contains user-related programs, libraries, and data:
      • Additional binaries and executables (/usr/bin)
      • System administration commands (/usr/sbin)
      • Libraries (/usr/lib)
      • Header files (/usr/include)
      • Documentation (/usr/share/doc)
  • /var
    • Stores variable data that changes during system operation:
      • Log files (/var/log)
      • Spool directories for print jobs (/var/spool)
      • Temporary files that persist between reboots (/var/tmp)
      • Website content for web servers (/var/www)

Commands

File System

  • rm - remove files or directories

    • rm -rf dir1 dir2 dir3

  • touch filename - create a file

  • ls - list directory contents

    • ls -a - list all contents
    • ls -l - list all contents with detailed view

  • cat - create, view and concatenate files

    • cat > file - create new file
    • cat srcfile > destfile - copy srcfile into destfile

  • less - views the content of a file one screen at a time, allowing for scrolling.

  • mv src [src2 src3 ...] dir - move source files to directory

    • mv srcdir existing_dir - move folder (existing_dir/srcdir)
    • mv old_dir new_dir - rename folder
    • mv srcfile destfile - rename source to destination

  • cp - copy files or directories

    • By default, existing files are overwritten without confirmation.
    • cp file1 file2
    • cp -r dir1 dir2 - copy all contents of dir1 to dir2 (-> dir2/dir1)
    • cp -i file1 file2 - prompt for confirmation before overwriting

  • pwd - present working directory

  • diff file1.txt file2.txt - compare file content differences

  • grep [OPTION...] PATTERNS [FILE...] - search and print lines that match patterns

    • -r allows recursive searching through directories.

  • find DIR -name PATTERN - search for file in a directory hierarchy

  • chmod - change/modify file permissions

  • sort file - sort lines of text files

  • Piping (|) - take command and feed it to another

  • Redirection (>) - write output to a file

    • e.g. scoop list > scoop-installs.txt

Processes

  • ps - displays information about currently running processes.
    • ps aux - detailed view.
  • kill - terminates processes by their PID (Process ID).
    • kill -9 - forceful termination.
  • top - provides a dynamic, real-time view of running processes and system resource usage.
  • htop - interactive process viewer, with more user-friendly interface than top.

Networking

  • ping - tests connectivity to another host.
  • curl - transfers data from or to a server, supporting various protocols. Useful for testing APIs.
  • scp - securely copies files between hosts on a network.

System Info

  • df -h - displays disk space usage in a human-readable format.
  • du -sh - summarizes disk usage of a directory.
  • free -m - shows memory usage.
  • uname -a - displays system information including kernel version and architecture.

Miscellaneous

  • man - user manual for any command
  • history - shows a list of previously executed commands, which can be useful for recalling complex commands.
  • sudo - runs commands as the root user
    • -u <user> <command> - specifies that a command should be run as the specified user.
      • e.g. sudo -u postgres psql, sudo -u nginx nginx -t
    • systemctl [start|stop|status] <service> - newer and preferred method of managing services on systems that use systemd.
      • e.g. sudo systemctl start postgresql
    • service <service> [start|stop|status] - An older, more traditional command available on most Linux distributions for backward compatibility.
      • A high-level wrapper that can work across different init systems, including systemd.
      • On systems using systemd, the service command often redirects to systemctl.
      • e.g. sudo service postgresql start
    • sudo su - allows a user with sudo privileges to switch to the root user account without needing to know the root password.
      • Effectively gives the user a root shell with full system access.
      • Generally discouraged in favor of sudo -i or sudo -s for security reasons, as these methods provide better logging and control over elevated privileges.
    • apt - CLI for managing packages
      • install - install a package
      • update - checks to see which packages can be updated.
      • upgrade - performs the update for packages that can be updated

Bash Scripting

  • Number of arguments: $#
  • All positional arguments (as a single word): $*
  • All positional arguments (as separate strings): $@
  • List array elements: ${array[*]}

Vim

  • vim filename - edit a file via Vim
    • i - used to enter input mode.
    • l - moves the cursor to the right.
    • h - moves the cursor to the left.
    • k - moves the cursor up.
    • j - moves the cursor down.
    • w - place cursor after a word
    • b - jump to beginning of a word
    • dd - delete a line
    • u - undo a command
    • dw - delete word where cursor is located
    • esc - enter command mode
    • :wq - save and quit vim
    • . (period) - repeat a previous command