Lecture 6: Linux Files

1. Core Concepts

The foundational principle of the Linux file system is that in the Linux system, everything is a file. If it is not a file, it is a process.

The term "file" in Linux has a broad scope. It encompasses not only standard data like text files, images, and compiled programs, but also system components such as disk partitions, hardware device drivers, and even directories.

Filenames in Linux are always case-sensitive. Consequently, demo.txt and Demo.txt are recognized as two distinct and separate files.

2. Linux File Types

Linux classifies files into several distinct types, each identified by a specific character in detailed directory listings.

ls -l prints the file type as the first character of the permission string.

Example using stat command (simplified):

Identifier	Description
-	Regular: Contains programs, executable files, and text files.
d	Directory: Contains a list of other files and is shown in blue color.
b	Block file
c	Character device file
p	Named pipe file (FIFO)
l	Symbolic link file
s	Socket file

NOTE

The file types identified by b, c, p, l, and s are classified as Special files.
The type of any given file can be determined using the file command.

3. File Management Commands

The following commands are essential for basic file management operations.

Command	Description
file	Determines file type.
touch	Used to create a file.
rm	To remove a file.
cp	To copy a file.
mv	To rename or to move a file.
rename	To rename multiple files.

4. Copy & Move Commands

cp Command

Copies files and directories.

• cp <existing> <new> (Same directory).
• cp <file> <destination_path/>
• cp -r : Copy a directory recursively.
• cp *.txt <dest_dir> : Copy multiple files using wildcard.
• cp --backup : Backup existing destination file.
• cp -i : Confirm before overwriting (interactive).
• cp -p : Preserve file properties.
• cp -u -v : Copy only when source is newer or dest is missing (verbose).

5. File Creation Methods

Linux offers multiple methods for creating new files, ranging from command-line utilities to full-featured text editors.

• Using the cat command
• Using the touch command
• Using the redirect > symbol
• Using the echo or printf commands
• Using text editors such as vim, nano, or vi

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5.1 touch Command

The touch command serves two primary functions: creating a new, empty file or updating the access and modification timestamps of an existing file. It is particularly useful when you need to create a file placeholder before the data is ready to be stored.

# Create an empty file
touch new.txt

The key difference between touch and cat for file creation is that touch creates a blank, zero-byte file, whereas cat allows you to specify the file's content directly from the command prompt.

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5.2 cat Command for Creation

The cat command can be combined with the > redirection operator to create a new file and immediately add content to it.

cat > test.txt

After executing the command, you can type the desired content directly into the terminal. To save the content and exit, press CTRL + D.

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5.3 Redirect > Symbol

The redirection operator > can be used without a preceding command to create an empty file.

> test5.txt

It can be also used with a preceding command to redirect the output into the file.

# Redirect output of ls command into output.txt file overwriting its content.
ls -l > output.txt

# Redirect the output of the commands into new.txt appending into the end of the file.
pwd >> new.txt
which bash >> new.txt

It is important to distinguish between the two redirection operators:

• > (overwrite): Redirects output to a file. If the file exists, its contents are overwritten. If it does not exist, it is created.
• >> (append): Appends output to the end of a file. If the file does not exist, it is created.

5.4 echo and printf Commands

The echo and printf commands are used to create a file with content specified directly on the command line.

echo "File content" > test6.txt

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5.5 Text Editors

Standard Linux text editors provide a more interactive way to create and edit files. Common editors include vim, nano, and vi.

To save and exit the nano editor, follow these steps:

1. Press CTRL + X to exit.
2. Press y to confirm that you want to save the changes.
3. Press Enter to save the file with the current name.

6. File Content Viewing and Manipulation

6.1 cat Command

Beyond file creation, the cat command is a versatile tool for viewing and manipulating file content.

# View single file: Displays the entire content of a single file to the terminal.
cat file.txt

# View multiple files: Displays the content of multiple files sequentially, one after the other.
cat file-1.txt file-2.txt file-3.txt

# Show line numbers
cat -n file.txt

# Copy contents: Redirects the content of one file into another, overwriting the destination file.
cat old.txt > new.txt

# Concatenate/Merge files: The `>>` operator appends the content of one or more files to a destination file.
cat file-1.txt file-2.txt >> file-3.txt

# Using wildcards -> merge all .txt files into all.txt
cat *.txt >> all.txt

# Highlight end of lines: displays a $ character at the end of each line, making trailing whitespace visible.
cat -e file.txt

• Suppress Empty Lines: The -s option suppresses multiple adjacent blank lines, replacing them with a single blank line.

NOTE

You can use a wildcard (*) to select multiple files of the same type.
For example, cat *.txt >> filename.txt will merge all .txt files in the current directory.

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6.2 tac Command

The tac command (named as the reverse of cat) displays the contents of a file in reverse order, printing the last line first.

tac <file name>

7. File Deletion and Type Determination

7.1 rm Command Options

The rm (remove) command is used to delete files and directories.

Option	Description
rm *extension	Deletes all files matching the given extension in the current directory.
rm -r / rm -R	Recursively deletes a directory and all of its contents.
rm -i	Prompts for confirmation before deleting each file.
rm -rf	Forcefully deletes a directory and all of its contents without prompting, including write-protected files.
-f	Force mode: ignores non-existent files and suppresses confirmation prompts. Does not recursively delete directories on its own.
-v	Verbose mode: prints the names of files as they are deleted.
-d	Deletes an empty directory.
--one-file-system	Deletes only files on the same file system as the specified directory, ignoring mounted filesystems.

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7.2 file Command Options

The file command determines a file's type.

• -s: Used for special files (e.g., block or character device files).
• *: Lists the types of all files in the current directory.
• /directory_name/*: Lists the types of all files within a specified directory.
• [range]*: Lists the types for all files whose names begin with a character within the specified range (e.g., [a-c]*).

8. Search Commands

8.1 find Command

Searches for files in the filesystem based on conditions.

• find <location> <criteria> <term>
• . : Current directory. / : Root directory.
• find . -name "*.txt" (Find by name).
• find . -type f -name "*.bak" (Find files by name and type f).
• find . -type d -name ".git" (Find directories by name and type d).
• find . -newer msg.txt (Find files newer than msg.txt).
• find . -name Demo.txt -delete (Find and delete permanently).

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8.2 locate Command

Searches for files using a pre-built database (faster than find).

• locate <file name>
• Update database: updatedb
• locate -n 5 "*.txt" (Limit results).
• locate -c Demo* (Display The Number of Matching Entries).
• locate -i (Ignore case).
• locate -e *demo.txt* (Display only existing files, skips deleted files).
• locate -S (Show database stats).

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8.3 which Command

Locates the path of an executable or command.

• which <program_name>
• which -a <program_name>: Print all matches in PATH, not just the first
• which node cat ls

9. Viewing Commands

9.1 head Command

Displays the first part (default: 10 lines) of a file.

• head <file name>
• head file1.txt file2.txt (Multiple files).
• head -n <number> <file> (Show specific number of lines).
• head -c <number> <file> (Show specific number of bytes).

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9.2 tail Command

Displays the last part (default: 10 lines) of a file. Used to monitor logs. Can display multiple files.

• tail <file name>
• ls -t /etc | tail -n 6 (Piped with other commands).

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9.3 more Command

Displays file content one screen at a time (for large files). Cannot display binary files.

• more <file name> or cmd | more

• Navigation:

  • Enter (line down)
  • Space (next page)
  • b (back page)
  • / (search).

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9.4 less Command

An advanced version of more with additional features (adjusts to window size, supports search).

• less <file name> or cmd | less

The less command have navigation keys similar to the vim editor.

• Search:
  • /string (forward searching)
  • ?string (backward searching)
  • n (next match)
  • N (previous match).