Linux-and-Terminal

Bash commands

• Basic Operations:
  • Locating Applications:
    • which [command] - Locates and displays source of file
    • whereis [command] - Locates and displays source of file. It also looks for packages in a broader range of system directories.
  • Accessing Directories:
    • pwd - Displays the present working directory.
    • cd - Go to home directory
      • cd .. - Go back </br> . means current directory and .. means previous directory
      • cd ../folder-name –> In the previous directory there is another directory.
      • cd - - Change to previous working directory; - (minus)
  • Exploring the File System:
    • cd / - Changes your current directory to the root (/) directory (or path you supply).
    • ls - List the contents of the present working directory.
      • ls -a - List all files, including hidden files and directories (those whose name start with . )
      • ls -l - Shows users, permissions, group owners, etc.
      • ls -li /path/to/directory - Used to list files and directories in a directory. The -l option provides a long listing format, and the -i option displays the inode number of each file.
      • ls -l *pattern* - Used to list all files and directories in the current directory that match the specified pattern.
      • ls -al - Combination of ls -a and ls -l
      • ls -R - Also shows files in sub directories. </br>
    • tree - Displays a tree view of the filesystem.
      • tree -d - Displays directories only.
  • Hard and Soft Links:
    • ln <source> <destination> - Used to create hard links.
    • ln -s <source> <destination> - Used to create soft(symbolic) links.
  • Navigating the directory history:
    • pushd <directory> - Used to change the current directory and also save the current directory onto the directory stack.
    • popd <directory> - Used to remove the top directory from the directory stack and change the current directory to the one that was removed from the stack.
    • dirs - Displays the directories in the directory stack in reverse order, with the most recent directory listed first.
• File Operations:
  • cat - Used for viewing files that are not very long; it does not provide any scroll-back.
    • cat -n - Also displays the line numbers.
  • wc <file> - Prints the number of lines, words and characters in file.
  • tac- Used to look at a file backwards, starting with the last line.
  • less - Used to view larger files because it is a paging program. It pauses at each screen full of text, provides scroll-back capabilities, and lets you search and navigate within the file.

  NOTE: Use / to search for a pattern in the forward direction and ? for a pattern in the backward direction. An older program named more is still used, but has fewer capabilities: “less is more”.

  • head <file-name> - Displays the first 10 lines of file.
    • head -4 <file-name> - Displays the first 4 lines of file.
  • tail <file-name> - Displays the last 10 lines of file by default.
    • tail -2 <file-name> - Displays last 2 lines of file by default.
  • touch <file-name> - Creates a new file.
  • touch <folder-name>/<file-name> - Creates a file in a folder.
  • echo > <file-name> - Creates a new file
  • mv <file1-name> <file2-name> - Renames file1-name to file2-name.
  • mv <file-name> <dir-name> - Moves file into directory.
  • mv <file-name ../<newFile-name> - Move and create a new file in previous directory.
  • rm <file-name> - Removes a file permanently.
    • rm -f <file-name> - Forces deletion of file in certain scenarios when the file is unable to delete. For example, if the file is open, etc.
    • rm -i <file-name> - Used to remove (delete) files and directories from the file system. The system will display a prompt for each item, asking you to confirm whether you want to delete it. You can then respond with “y” (yes) or “n” (no).
    • rm -rf <file-name> - Forcefully remove a directory recursively.
    • locate <file-name> - locates the file.
    • locate ".txt" - displays all the files that end with an extension of .txt
    • find . -type d - displays files in current directory of type directory.
    • find . -type f - displays files in current directory of type file.
    • find . -type f -name <file-name>
      • -type f -name f* - shows files starting with f.
      • -type f -name "*.txt" - shows files of type .txt
      • -type f -iname "*.txt" - shows files of type .txt that are not case sensitive.
      • -type f -mmin -20 - displays files that were modified less than 20 min ago.
      • -type f -mmin +20 - displays files that were modified more than 20 min ago.
      • -type f -maxdepth 2 -
    • find . -size +1k - shows files/folders with size greater than 1kb.

    • find . -empty - shows files that are empty.

    • • cp <file1-name> <file2-name> - copy file1 contents in file2.

  • diff <file1-name> <file2-name> - compares the contents of two file line by line and outputs the lines that do not match.

  • sort <file-name> - sorts alphabetically.
    • -r - sorts in reverse order
    • -f - case insensitive sorting
    • -n - returns result in numerical order
  • sort <file-name> | uniq - removes duplicate entries.
  • paste <file1-name> <file2-name> - combines fields from file1 and file2.
  • join <file1-name> <file2-name> - combines lines from two files based on a common field or key.
  • Zipping files: zip <zip-file-name> <file1-name> <file2-name>
  • Unzipping files: unzip <file-name>
  • cut -c 1-2 <file-name> - shows first 2 columns of file.
  • which [command] - locates and displays source of file
  • whereis [command] - locates and displays source of file. It also looks for packages in a broader range of system directories.

  • file <file/directory-name> - To find the type of files/directories.

  • cat <file-name> - displays contents of file.
  • cat > <file-name> - creates a new file. > basically means where do you want to output.
  • cat <file1-name> <file2-name> - prints the contents of two files.
  • cat <file1-name> <file2-name> > <file3-name> - merges file1 and file2 contents in file3.
  • cat <file1-name> | tr a-z A-Z > <file2-name> - | is known as pipe which helps in executing multiple commands simultaneously. The tr command is used to translate the contents of a file and > is used to redirect it into another file.
  • echo "message" - prints message
  • echo "message" > file-name - adds message in a file.
  • echo "message" >> existing-file-name - appends message in an already existing file.
  • echo $variable - contents of specific environment variable are displayed.
  • less <file-name> - displays the contents of a file one page at a time. We can switch to the next page by click the space bar.
  • sed 's/pattern/replacement/' filename - searches for pattern in file and replaces them with the given replacement text.
  • wc <file> - prints the number of lines, words and characters in file.
    • -l - prints just the number of lines.
• Directory Operations:
  • open . - opens current directory in files
  • pwd - displays the present working directory
  • mkdir <dir-name> - creates a new directory.
  • mkdir <dir1-name> <dir2-name> - creates a new directory in existing directory.
  • mkdir -p <dir1-name> <dir2-name> <dir3-name> - creates a new directory in between existing directories.
  • cd folder-name
  • cd - go to home directory
  • cd .. - go back </br> . means current directory and .. means previous directory
  • cd ../folder-name –> in the previous directory there is another directory.
  • rm -r folder-name - removes a folder permanently.
  • rmdir <dir-name> - to remove an empty directory.
    • rmdir -rf <dir-name> - removes a non-empty directory.
  • find . - find in current directory.
  • find .. - find in previous directory.
  • find <folder-name> - find in folder.
• Environmental Variables:
  • env - displays environmental variables
• Processes:
  • ps - Displays information about processes currently running on the system.
    • ps aux - Shows a detailed list of all running processes.
    • ps -e - Lists all processes.
    • ps -ef - Provides a full listing with additional details.
    • ps -eLf - Displays one line of information for every thread.
  • pstree - Displays the processes running on the system in the form of a tree diagram showing the relationship between a process and its parent process and any other processes that it created.
  • top - Provides an interactive real-time view of system processes.
  • htop - Enhanced version with a more user-friendly interface and additional features.
  • pidstat - Reports statistics for processes, including CPU, memory, and I/O usage.
  • vmstat - Provides information about system-wide virtual memory statistics and can offer insights into process behavior.
  • iotop - Monitors I/O usage by processes and helps identify processes causing high I/O loads.
  • w - Displays information about currently logged-in users, their activities, and system load.
  • top - Provides a dynamic real-time view of system processes and resource usage. provides a dynamic real-time view of system processes and resource usage.
  • uptime - Provides a summary of system uptime and load averages.
• Text Editors:
  • nano <file> - start the editor and edit file.
  • gedit <file> - start the text editor and edit file.
  • vi <file> - start the editor and edit file.
  • emacs <file> - Start emacs and edit file.
• System Commands:
  • sudo - used for administrative permissions.
  • df - used to check the system disk space.
    • df -m - in megabytes.
    • df -hg - in gigabytes.
  • du - shows disk usage statistics.
  • top - shows process running and CPU usage.
  • kill <process-id> - to kill the process.
  • uname - shows kernel name.
    • -o - shows type of kernel.
    • -a - shows architechture.
    • -r - shows kernel version
  • cat /etc/os-release - shows all the info about the OS.
  • lscpu - shows CPU details.
  • free - shows free memory.
    • -h - shows used and free memory.
  • vmstat - shows virtual memory.
  • id - shows all id’s.
    • id <user-name> - shows id of a particular user.
  • useradd <new-user-name> - creates a new user
  • passwd <user-name> - creates a password for user.
  • userdel <user-name> - deletes a user.
  • getent group <user-name> - to check user exists or not
  • lsof - shows all open files.
    • lsof -u <user-name> - shows open files for particular user.
• Permissions:
  There are three types of permissions: read, write and execute and there are three types of owners: user, group and others.
  • find . -perm 777 - shows all files that have read, write and execute permissions.
    chmod command is use to change file modes or permissions.
  • chmod u=rwx, g=rx, o=r <file-name> - changes the user, group and others permission of a file.
    
  • We can also use octal numbers to change permissions. chmod 777 <file-name> - sets read, write and execute permissions to all owners. Here the number 777 is divided into 3 categories - user, group, others. 4 stands for read, 2 stands for write, 1 stands for execute and 0 stands for no permission. So, if we want to set read and write permissions to all the owners i.e. 4 + 2 = 5 –> chmod 555 <file-name>
  • chown root <file-name> - changes the file owner to root.
    root is a super user account used for administrative purposes and has the highest number of access rights in the system.
  • find . -perm 777 - displays files that have permission to read, write and execute.
• Search operations:
  • find . -type f -name "*.txt" -exec rm -rf {} + - Here {} are parameters.
  • grep - global regular expression print. It allows us to search for text within our files and is case sensitive.
  • grep -V - version of grep
  • grep <keyword> <file-name> - searching for particular keyword in a file.
  • `grep -w ' - displays complete word.
  • grep -i <keyword> <file-name> - searches for keyword ignoring case.
  • grep -iw <keyword> <file-name> - shows complete word ignoring case.
  • grep -n <keyword> <file-name> - displays line number.
  • grep -win <keyword> <file-name> - combination of all three tags.
  • grep -B 3 <keyword> <file-name> - shows previous three lines that comes before particular keyword.
  • grep -win <keyword> ./ .txt - searching a keyword in all text files in current directory.
  • grep -rwin <keyword> . - searching recursively in current directory.
  • grep -wirl <keyword> . - displays all the files that have particular keyword.
  • grep -wirc <keyword> . - displays count of files that contain particular keyword.
  • strings <file> | grep <my_string> - searches for my_string in file.
  • history - shows history of all the commands we are using.
  • Piping output of one command into other commands using grep:
    history | grep "<command>" - shows history of a particular command.

• What are jobs?
  It is a process started by the shell. We can use jobs command to list the running jobs.

• Network Operations:
  • cat \etc\hosts - displays the contents of /etc/hosts that contains the mappings of IP addresses to hostnames.
  • host [options] <hostname or IP address> - allows to query DNS records and retrieve information about domain names or IP addresses.
  • ping <hostname> - checks the status of the remote host.
  • route [options] [command] - used to view and manipulate routing table.
    • route -n - displays the current routing table.
  • traceroute <address> - prints the route taken by the packet to reach the network host.
  • wget <url> - use to download files from the internet.
    • wget -o <custom-file-name> <url> - to set custom name for the downloaded file.
  • curl <url> - reads a URL.
  • hostname - use to obtain the dns name.
    • -i - shows IP address.
  • nslookup <url/IP address> - to check IP address for particular domain.
  • netstat - shows all active ports
  • telnet
• Finding Documentation:
  • man command_name - To view the manual page of a specific command.
    • man -f <command_name> - To list all pages on the topic. Generates the same result as typing whatis.
    • man -k <command_name> - To list all pages that discuss a specific topic (even if the specified subject is not present in the name). Generates the same result as typing apropos.
    • man -a <command_name> - Used to display the manual pages for a specified command from all available manual page sections.
    • info <topic name> - To view help for a particular topic. The system then searches for the topic in all available info files.
    • <command_name> --help - Help provides basic information about shell commands.
• Basic Packaging Commands(deb):
  • dpkg --install foo.deb - Install package.
  • apt install foo - Install package, dependencies.
  • dpkg --remove foo.deb - Remove package.
  • apt autoremove foo - Remove package, dependencies.
  • dpkg --install foo.deb - Update package.
  • apt install foo - Update package, dependencies.
  • apt dist-upgrade - Update entire system.
  • dpkg --list - Show all installed packages.
  • dpkg --listfiles foo - Get information on package.
  • dnf list "foo" apt-cache search foo - Show packages named foo.
  • apt-cache dumpavail foo - Show all available packages.
  • dpkg --search file - What package is file part of?
• Basic shortcuts:
  • CTRL-L - Clears the screen
  • CTRL-D - Exits the current shell
  • CTRL-Z - Puts the current process into suspended background
  • CTRL-C - Kills the current process
  • CTRL-H - Works the same as backspace
  • CTRL-A - Goes to the beginning of the line
  • CTRL-W - Deletes the word before the cursor
  • CTRL-U - Deletes from beginning of line to cursor position
  • CTRL-E - Goes to the end of the line
  • Tab - Auto-completes files, directories, and binaries
  • CTRL+K - removes everything after the cursor.
  • CTRL+R - backward search / reverse intelligent search.
  • Running previous commands using history number/name: !<history-number> or !<command-name>
  • CTRL+L or clear - used to clear terminal.
  • CTRL+L (In Files) - used to access path of specific directory.
  • We can run multiple commands using ;
    Example: git init;git add .;git commit -m "message";git push
  • !!(bang bang) - executes previous command.
• Operators
  • I/O Redirection:
    • > - Redirects standard output to a file, overwriting the file if it already exists.
    • >> - Redirects standard output to a file, appending the output to the file if it already exists.
    • < - Redirects standard input from a file.
    • 2> - Redirects standard error to a file.
    • 2>> - Redirects standard error to a file, appending to the file if it already exists.
    • &> - Redirects both standard output and standard error to a file.
    • | - Redirects the output of one command as input to another command (pipeline).
  • & - it will create a process in the background so that other commands can be running. We can check the background process using ps command and kill it if needed. Example: ping google.com & facebook.com
  • && - The command that is succeeding this operator will only execute when the previous one is finish executing. Example: echo "first" && "second"
  • || - The command that is after the OR operator will only execute if the execution of previous command fails. Example: echo "first" || echo "second"
  • ! - can be use to delete all the files except one particular file. Example: rm -rf !(<file-name>)
  • \ - used to split long commands and execute them as single command.
  • {} - combination operator use to group commands. </br>
• Wildcards
  • ? - Matches any single character.
  • * - Matches any string of characters.
  • [set] - Matches any character in the set of characters, for example [adf] will match any occurrence of a, d, or f.
  • [!set] - Matches any character not in the set of characters.

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