Advance Linux Commands

• User management

    A user is an entity, in a Linux operating system, that can manipulate files and perform several other operations.
    Each user is assigned an ID that is unique for each user in the operating system.
    After installation of the operating system, the ID 0 is assigned to the root user and
    the IDs 1 to 999 (both inclusive) are assigned to the system users and hence the ids for local user begins from
    1000 onwards.
  
    In a single directory, we can create 60,000 users.
  

  commands

  • cat /etc/passwd -> this command shows list of all users in system

    The info about user is in specific format as:

    

      username : x : user id : user group id : : /home/username : /bin/bash
    

  • id <username> -> by using this command we can get id of user

    

  • sudo useradd -m <username> -> this will create user with mentioned username

    

  • passwd <username> -> this will set password of user

    

  • sudo userdel -r <username> -> this command deletes mentioned user. If the user is part of a group then it will not be deleted directly, hence we will have to first remove him from the group and then we can delete him.

    

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• Group management

    There are 2 categories of groups in the Linux operating system i.e. Primary and Secondary groups.
    The Primary Group is a group that is automatically generated while creating a user with a unique user
    ID simultaneously a group with an ID the same as the user ID is created and the user gets added to the
    group and becomes the first and only member of the group. This group is called the primary group.
    A secondary group is a group that can be created separately with the help of commands and we can then
    add users to it by changing the group ID of users.
  

  commands

  - sudo groupadd <groupname> -> this will creates group with mentioned name

  

  - sudo usermod -G <groupname> <username> -> this command adds user to existing group

  

  note: If we add a user to a group then it automatically gets removed from the previous groups, we can prevent this by the command given below

  - sudo usermod -aG <groupname> <username> -> this command adds user to existing group without removing previous groups

  

  - sudo gpasswd -M <user1>, <user2> ... <group> -> To add multiple users to a group simultaneously, you can utilize the gpasswd command with the -M option. This command allows you to specify a list of usernames separated by commas.

  note: this will overwrite existing users in group

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• grep, awk, find commands

  • grep

      The grep filter searches a file for a particular pattern of characters and displays all lines
      that contain that pattern. The pattern that is searched in the file is referred to as the regular
      expression (grep stands for global search for regular expression and printout).
    

    To know more about regular expression click here.

    commands

      grep <options> pattern <files>
        <options>: These are command-line flags that modify the behavior of grep.
        <pattern>: This is the regular expression you want to search for.
    

    • case insensetive search

      

    • Displaying the count of number of matches

      

    • Display the file names that matches the pattern

      

    • Checking for the whole words in a file

      

    • Show line number while displaying the output using grep -n

      

    • Matching the lines that start with a string

      

    • Matching the lines that end with a string

      

  • find

      The find command in Linux is a dynamic utility designed for comprehensive file and directory searches
      within a hierarchical structure. Its adaptability allows users to search by name, size, modification time,
      or content, providing a flexible and potent solution.
    

    commands

      find <path> <options> <expression>
    
      path: Starting directory for the search.
      Example: find /path/to/search
    
      options: Additional settings or conditions for the search.
      Example: find /path/to/search -type f -name "*.txt"
    
      expression: Criteria for filtering and locating files.
      Example: find /path/to/search -type d -name "docs"
    

    • Find A Specific File

      in below example, it seeks "test.txt" within current folder

      

    • Search Files with a Pattern Using

      In this case, it identifies files ending with ‘.sh’ within the current directory.

      

    • Empty Files and Directories

      

  • awk

      Awk is a scripting language used for manipulating data and generating reports. The awk command programming
      language requires no compiling and allows the user to use variables, numeric functions, string functions, and logical operators. 
    
      Awk is a utility that enables a programmer to write tiny but effective programs in the form of statements that
      define text patterns that are to be searched for in each line of a document and the action that is to be taken
      when a match is found within a line. Awk is mostly used for pattern scanning and processing.
      It searches one or more files to see if they contain lines that matches with the specified patterns and then
      perform the associated actions. 
    
      Awk is abbreviated from the names of the developers – Aho, Weinberger, and Kernighan. 
    
      What can we do with AWK?
      1. AWK Operations: 
      (a) Scans a file line by line 
      (b) Splits each input line into fields 
      (c) Compares input line/fields to pattern 
      (d) Performs action(s) on matched lines 
    
      2. Useful For: 
      (a) Transform data files 
      (b) Produce formatted reports 
    
      3. Programming Constructs: 
      (a) Format output lines 
      (b) Arithmetic and string operations 
      (c) Conditionals and loops
    
      Built in variables in awk
      Awk’s built-in variables include the field variables—$1, $2, $3, and so on ($0 is the entire line) —
      that break a line of text into individual words or pieces called fields. 
    
      NR: NR command keeps a current count of the number of input records. Remember that records are usually lines.
          Awk command performs the pattern/action statements once for each record in a file. 
      NF: NF command keeps a count of the number of fields within the current input record. 
      FS: FS command contains the field separator character which is used to divide fields
          on the input line. The default is “white space”, meaning space and tab characters.
          FS can be reassigned to another character (typically in BEGIN) to change the field separator. 
      RS: RS command stores the current record separator character. Since, by default, an input
          line is the input record, the default record separator character is a newline. 
      OFS: OFS command stores the output field separator, which separates the fields when Awk prints them.
          The default is a blank space. Whenever print has several parameters separated with commas,
          it will print the value of OFS in between each parameter. 
      ORS: ORS command stores the output record separator, which separates the output lines when Awk prints them.
      The default is a newline character. print automatically outputs the contents of ORS at the end of whatever it is given to print.
    

    commands

    Consider following content of file employee.txt

      ajay manager account 45000
      sunil clerk account 25000
      varun manager sales 50000
      amit manager account 47000
      tarun peon sales 15000
      deepak clerk sales 23000
      sunil peon sales 13000
      satvik director purchase 80000
    

    • Default behavior of Awk: By default Awk prints every line of data from the specified file.

      awk '{print}' employee.txt

      

    • Print the lines which match the given pattern.

      

    • Printing specfic columns

      

    • Display Line Number

      

    • Display Line From specific line number to specific line number

      

• file permission

  

  There’s a lot of information in those lines.

  1. The first character indicates it is file or directory.

  2. The next nine characters (rw-r–r–) shows the security.

     

  3. The next column shows the owner of the file. (Here it is root)

  4. The next column shows the group owner of the file. (Here it is root which has special access to these files)

  5. The next column shows the size of the file in bytes.

  6. The next column shows the date and time the file was last modified. Last Column = File_name or Directory_name. (For example, here 90DaysOfDevOps, B, C, test.txt etc...)

commands

chmod <permisions> <file/directory> to change permissions of file/directory we can use command

we can cahnge file permission's using to way one is using letters and other is using numbers (octal)

• using letters

  • to give either read or write or execute permisson to user/group/other, following are the exapmles

    • chmod u+w <file/directory> -> this will append write permission to user for mentioned file/dir

    • chmod o+x <file/dirr> -> this will append execute permission to others for mentioned file/dir

    • chmod u+w,g+r,o+x <file/dir> -> this will append write permission to user, read permision to group and execute permission to others for mentioned file/dir

    • chmod o-x <file/dir> ->this will remove execute permission from user for mentioned file/dir

• using numbers

  for numeric permission following table is required

in short, we can say value for 'r' is '4', value for 'w' is '2' and value for 'x' is '1', using summation we can assign permissions

• chmod 755 <file/dir> means user has all permissions i.e. rwx: 4 + 2 + 1 = 7 group has read and execute permissions not write i.e. r-x: 4 + 1 = 5 similar to others