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Cat Command: File Display with and without Input Redirection

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The cat command is among the most commonly used commands in Unix-like operating systems. It is primarily used to concatenate and display files. In this article, we will delve into the differences between using the cat command with and without input redirection.

Quick Answer

The cat command can be used both with and without input redirection to display the contents of files. When using cat without input redirection, you directly specify the file name as an argument. With input redirection, the shell opens the file and connects its contents to the standard input of the cat command. The main difference lies in how the file is opened and passed to the cat command.

Understanding the Cat Command

The cat command, short for ‘concatenate’, is a standard Unix utility that reads files sequentially, writing them to standard output. The name is derived from its function to concatenate files.

The basic syntax is as follows:

cat [options] [filenames]

The filenames parameter specifies the files to read. If no files are given, cat reads from standard input.

Cat Command without Input Redirection

When using the cat command without input redirection, you directly specify the file name as an argument. For instance:

cat filename

Here, cat opens the file named ‘filename’ and writes its content to standard output. The permissions of the file determine whether the command can read its contents.

Cat Command with Input Redirection

Input redirection is a feature of the shell that allows you to redirect the input of a command from a file, rather than from the keyboard. When using the cat command with input redirection, the shell opens the file and connects its contents to the standard input (stdin) of the cat command.

The syntax for cat with input redirection is:

cat < filename

In this case, the shell, not the cat command, opens the file. The permissions of the shell executing the command are checked, as the shell is the one opening the file.

Differences between Cat with and without Input Redirection

While both commands produce the same output, there are key differences in their operation:

  • Permissions: As mentioned earlier, the permissions of the file or the shell determine whether the command can read the file’s contents.
  • File Expansion: If the filename argument contains wildcards or other characters that the shell expands into multiple filenames, “cat filename” will treat each expanded filename as a separate file to concatenate, while “cat < filename” will treat the expanded filenames as separate input streams.
  • Positional Parameters: Some programs may expect a filename as a positional parameter. In “cat filename”, the filename is passed as an argument to the cat command. In “cat < filename”, the filename is not passed as an argument, and the program may default to reading from stdin instead.

Conclusion

In conclusion, while “cat filename” and “cat < filename” may seem similar, the difference lies in how the file is opened and passed to the cat command. Understanding these differences can help you use the cat command more effectively and troubleshoot issues related to file permissions and input streams.

For more information on the cat command and input redirection, you can check the GNU Coreutils manual and the Bash manual.

What is the purpose of the `cat` command?

The cat command is primarily used to concatenate and display files. It reads files sequentially and writes their contents to standard output.

How do I use the `cat` command without input redirection?

To use the cat command without input redirection, you simply specify the file name as an argument. For example, cat filename will open the file named ‘filename’ and display its contents.

How do I use the `cat` command with input redirection?

To use the cat command with input redirection, you use the < symbol followed by the file name. For example, cat < filename will open the file and connect its contents to the standard input of the cat command.

Where can I find more information on the `cat` command and input redirection?

For more information, you can refer to the GNU Coreutils manual and the Bash manual. These resources provide detailed documentation on the cat command and various features of the shell, including input redirection.

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