[Day 3] 4. Some more commands

1. Head and Tail Commands:

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  head

  : Displays the first part of files.
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  head -5 filename.txt

  • This command prints the first 5 lines of
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    filename.txt

    .
  • You can also use negative numbers with
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    head

    , but this feature might vary based on the implementation.

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  tail

  : Displays the last part of files.

  • Example:

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    tail +5 filename.txt

  • This command starts printing from the 5th line onward to the end of

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    filename.txt

    .

  • Unlike

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    head

    ,
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    tail

    does not accept negative numbers.

2. Echo Command:

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  echo

  : Used to display messages or output text to the console.
  • Example:
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    echo "Hello Ojas"

    • This command prints
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      Hello Ojas

      on the console.

3. Translate Command (tr):

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  tr

  : Used to translate or delete characters.
  • Examples:

    • Translate space to

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      #

      :
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      echo "text with spaces" | tr ' ' '#'

      • This replaces spaces with
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        #

        .

    • Squeeze multiple spaces into one:

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      echo "text    with   multiple    spaces" | tr -s ' '

      • This command condenses consecutive spaces into a single space.

    • Change case:

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      echo "Hello World" | tr 'A-Z' 'a-z'

      • This converts uppercase letters to lowercase.

4. Cut Command:

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  cut

  : Used to remove sections from each line of files.

  • Example:

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    cut -d ',' -f 1 filename.csv

    • This command cuts and displays the first field (column) from a CSV file, using a comma as the delimiter.

    • You can also specify ranges:

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      cut -d ',' -f 1-3 filename.csv

    • This would cut and display fields 1 through 3.

Meta-characters/Wildcards in Shell:

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   *

   : Represents all characters.
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   ?

   : Represents a single occurrence of any character.
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   ;

   : Used to separate multiple commands on a single line.

Changing Permissions with chmod:

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  chmod

  : Changes the file permissions (also called access or mode).
  • Examples:

    • Add write permission:

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      chmod +w filename.txt

    • Add execute permission for all:

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      chmod +x filename.txt

• Setting permissions using user types:

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    u

    : User
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    g

    : Group
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    o

    : Others
  • Example:
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    chmod u+x,g-w filename.txt

• Octal Notation: Permissions can also be changed using octal numbers.

  • For example, if the original permissions are
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    rw- rw- r--

    , the octal number would be
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    664

    .
  • You can set this using:
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    chmod 664 filename.txt

Why Are Permissions Required?

Permissions ensure that only authorized users can read, write, or execute files, providing security and access control.

Changing File Ownership with chown:

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  chown

  : Changes the ownership of a file.
  • Example:
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    chown username filename.txt

• Change group ownership:

  • Example:
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    chown :groupname filename.txt

• Change both owner and group:

  • Example:
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    chown username:groupname filename.txt

• To change ownership of a directory and all its sub-directories, use

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  -R

  (recursive):
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  chown -R username directoryname/

Find Command:

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  find

  : Searches for files in a directory hierarchy.

  • Search by name:

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    find -name test.txt

  • Search by exact path:

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    find /home/sunbeam -name test.txt

  • Search by file type:

    • Simple files:

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      find -type f

    • Directories:

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      find -type d

  • Search by size:

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    find . -size +1M

    • This searches for files larger than 1 MB in the current directory.