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.
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
Understanding the Cat Command
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]
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 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
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
catcommand. In “cat < filename”, the filename is not passed as an argument, and the program may default to reading from stdin instead.
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.
cat command is primarily used to concatenate and display files. It reads files sequentially and writes their contents to standard output.
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.
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