FC Command: Comparing Files from the Windows Command Prompt

File Comparison with the FC Command Line Tool

For PC users, a powerful command line tool exists for identifying differences between files. This tool, known as File Compare or FC, facilitates the detection of content discrepancies or variations in binary code.

Functionality and Capabilities

FC is a straightforward program designed to analyze the contents of both text and binary files. It supports comparisons of files encoded in both ASCII and Unicode text formats.

The primary function of this utility is to pinpoint and display lines that differ between two files, or between two sets of files.

How it Works

By utilizing the FC command, users can quickly identify any mismatches in file content. This is particularly useful for version control, debugging, and ensuring data integrity.

The tool presents a clear output, highlighting the lines where differences are found, allowing for easy review and correction.

Essentially, FC provides a robust method for determining if files are identical or if modifications have been made.

File Compare’s Switches and Parameters

The File Compare utility, often referred to as FC, utilizes several switches to modify its behavior during file comparisons. These options allow for tailored analysis based on specific needs.

Switches for Comparison Control

Here's a breakdown of the available switches and their functions:

/B – Initiates a binary comparison, treating files as sequences of bytes.
/C – Performs a case-insensitive comparison, disregarding differences in letter casing.
/A – Displays only the initial and final lines of each differing section. This provides a summarized view of changes.
/U – Interprets files as Unicode text, enabling accurate comparison of Unicode characters.
/L – Treats files as ASCII text, suitable for comparisons involving standard ASCII characters.
/N – When used with ASCII files, this switch displays corresponding line numbers alongside the differences.
/LBn – Limits the number of consecutive differing lines FC will process before halting. Replace "n" with the desired limit; the default is 100.
/nnnn – Specifies the number of consecutive matching lines required after a mismatch for FC to continue. This prevents significant desynchronization between files.
/T – Prevents the expansion of tabs into spaces during the comparison process.
/W – Compresses whitespace, including tabs and spaces, to streamline the comparison and ignore minor formatting differences.

The Pathname Parameter

A single parameter is required for operation: the Pathname. This parameter must be specified twice, indicating the locations of the two files you wish to compare.

Providing the correct pathnames is essential for FC to accurately locate and analyze the files.

Understanding FC Command Syntax

As with any command-line utility, correct syntax is crucial when using the File Compare (FC) tool. Two primary approaches exist for utilizing FC, depending on your comparison needs.

You can compare either individual files or multiple files simultaneously. The latter is achieved through the use of wildcards, specifically the question mark (?) and asterisk (*).

FC Command Structure

The basic structure of the FC command is as follows:

FC [pathname1] [pathname2] – This form compares two specific files.
FC [switches] [pathname1] [pathname2] – This allows for the inclusion of switches to modify the comparison process.

Interpreting Error Levels

The FC command provides feedback through %errorlevel% values, indicating the outcome of the comparison. These values are:

-1 – Indicates an error in the command's syntax.
0 – Signifies that the files being compared are identical.
1 – Confirms that differences were detected between the files.
2 – Suggests that one or both of the specified files could not be located.

Understanding these error levels is essential for troubleshooting and verifying the results of your file comparisons.

Getting Started with File Comparison

Prior to beginning, it’s recommended to download the three example text files provided for testing purposes. Each file contains a paragraph of text featuring similar word arrangements. Once downloaded, these files can be copied into any directory on your computer. For this demonstration, we will place them on the desktop for ease of access.

FCsample
FCexercise
FCexercise2

Next, an elevated command prompt window needs to be opened. In Windows 7 and 10, access the start menu, or utilize the search function in Windows 8, and type CMD. Then, right-click on the result and select “Run as administrator.” Although not strictly necessary, using an elevated command prompt can prevent unwanted confirmation prompts.

Today’s tutorial will focus on several straightforward scenarios, which will be detailed below.

Comparing two text files located in the same folder using the File Compare utility.
Utilizing the “/lbn” switch with File Compare to compare files within the same folder.
Comparing two files that are identical in content.
Conducting a binary comparison of both differing and identical files.

Scenario 1 – Comparing Two Text Files

With your command prompt window open and the text files on your desktop, a basic file comparison can now be performed. This section will demonstrate a fundamental comparison, followed by the addition of various options. Start by entering the following command to compare “FCsample” and “FCexercise.” Ensure the file path is adjusted to match your system, and note that the command prompt is not case-sensitive.

fc C:\Users\Martin\Desktop\FCsample.txt C:\Users\Martin\Desktop\FCexercise.txt

The output displays the complete text from both documents because they contain discrepancies.

Scenario 2 - Comparing Files with the “/lbn” Switch

Let’s proceed with another comparison, instructing FC to halt after identifying two lines of differing data. This is achieved by incorporating the “/lbn” switch.

fc /lb2 C:\Users\Martin\Desktop\FCsample.txt C:\Users\Martin\Desktop\FCexercise.txt

The resulting message, “Resync Failed. Files are too different,” indicates that more than two consecutive lines of mismatched data were found. Experiment with different numerical values or modify the files directly to observe the resulting changes in the file comparison tool.

Scenario 3 - Comparing Identical Files

The downloaded files include two named “FCexercise” and “FCexercise2.” These files possess identical content, so a comparison will be executed to examine the outcome.

fc C:\Users\Martin\Desktop\ FCexercise.txt C:\Users\Martin\Desktop\FCexercise2.txt

As illustrated, FC reports that no differences were detected. If one file were edited, even by adding a single character, and the command rerun, the results would reflect the change.

Note that only the addition of the letter “a” was made.

Scenario 4 – Performing a Binary Comparison

For this instance, a binary comparison will be conducted between the “FCexercise” and “FCsample” files.

fc /b C:\Users\Martin\Desktop\ FCexercise.txt C:\Users\Martin\Desktop\sample.txt

The process begins by informing you that the files are being compared. Subsequently, a series of binary digits are displayed, representing the side-by-side comparison of the files. Finally, a report indicates that “FCexercise” is longer than “FCsample.”

For the subsequent example, a binary comparison will be performed on the “FCexercise” and “FCexercise2” files.

fc /b C:\Users\Martin\Desktop\ FCexercise.txt C:\Users\Martin\Desktop\FCexercise2.txt

In this comparison of two identical files, FC confirms that no differences exist. Having learned the fundamentals of the FC tool, feel free to experiment with the switches and explore new possibilities. Remember to utilize sample files, such as those provided, to prevent accidental data loss during testing.

Image Credit: Nikki on Flickr

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