Linux merge text files

Linux merge command

On Unix-like operating systems, the merge command performs a three-way file merge.

The merge process analyzes three files: a base version, and two conflicting modified versions. It attempts to automatically combine both sets of modifications, based on the shared base version, into a single merged file. If automatic merge is not possible, it facilitates manual merging.

This page describes the GNU/Linux version of merge.

Description

merge is part of the RCS version control package. It is used to perform a three-way file merge.

merge analyzes three files — an original file, and two modified versions of the original — and compares them, line-by-line, attempting to resolve the differences between the two sets of modifications to create a single, unified file which represents both sets of changes.

Depending on the differences between the two sets of changes, this may be an automatic process or may require user input.

If neither set of changes conflicts with the other, merge can usually figure out what to do on its own. But if the two sets of changes conflict — for example, if the same line of text is worded differently in both modified files — merge shows the conflict in the resulting merged file.

How merging works

merge incorporates all changes that lead from file2 to file3 into file1. The result ordinarily goes into file1.

Suppose file2 is the original, and both file1 and file3 are modifications of file2. Then merge combines both changes.

A conflict occurs if both file1 and file3 have changes in a common segment of lines. If a conflict is found, merge normally outputs a warning and brackets the conflict with » >>>>>>» lines. For instance, a typical conflict looks like this:

If there are conflicts, the user should edit the result and delete one of the alternatives.

Syntax

Options

-A	Output conflicts using the -A style of diff3(1) (if supported by diff3). This merges all changes leading from file2 to file3 into file1, and generates the most verbose output.
-E, -e	These options specify conflict styles that generate less information than -A. See diff3(1) for details. The default is -E. With -e, merge does not warn about conflicts.
-L label	This option may be given up to three times, and specifies labels to be used in place of the corresponding file names in conflict reports. That is, merge -L x -L y -L z a b c generates output that looks like it came from files x, y, and z instead of from files a, b, and c.
-p	Send results to standard output instead of overwriting file1.
-q	Quiet mode. Do not warn about conflicts.
-V	Print RCS’s version number.

Examples

Let’s say we have a file named orig.txt with the following contents.

. and a file named mod1.txt, which is a modified version of orig.txt:

. and a file named mod2.txt, which is also a modified version of orig.txt:

. and we run merge as follows:

It analyzes all three files, write to mod1.txt, and display the following warning:

This means the merge was successful, but we should be aware that there was a conflict. If we open mod1.txt — which by default is the file where the merge is written — we find that it now contains the following text:

It is up to us to decide which «Oranges are. » line to keep (or to combine them in our own way), and make the edit to the file manually.

Related commands

diff — Identify the differences between two files.

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how to merge multiple files into one single file in linux

Many a times you may have multiple files that needs to merged into one single file. It could be that you previously split a single file into multiple files, and want to just merge them back or you have several log files that you want merged into one. Whatever the reason, it is very easy to merge multiple text files into a single file in Linux.

The command in Linux to concatenate or merge multiple files into one file is called cat. The cat command by default will concatenate and print out multiple files to the standard output. You can redirect the standard output to a file using the ‘>‘ operator to save the output to disk or file system.

Another useful utility to merge files is called join that can join lines of two files based on common fields. It can however work only on two files at a time, and I have found it to be quite cumbersome to use. We will cover mostly the cat command in this post.

Merge Multiple files into One in Order

The cat command takes a list of file names as its argument. The order in which the file names are specified in the command line dictates the order in which the files are merged or combined. So, if you have several files named file1.txt, file2.txt, file3.txt etc…

bash$ cat file1.txt file2.txt file3.txt file4.txt > ./mergedfile.txt

The above command will append the contents of file2.txt to the end of file1.txt. The content of file3.txt is appended to the end of merged contents of file1.txt and file2.txt and so on…and the entire merged file is saved with the name mergedfile.txt in the current working directory.

Many a time, you might have an inordinately large number of files which makes it harder to type in all the file names. The cat command accepts regular expressions as input file names, which means you can use them to reduce the number of arguments.

bash$ cat file*.txt my*.txt > mergedfile.txt

This will merge all the files in the current directory that start with the name file and has a txt extension followed by the files that start with my and has a txt extension. You have to be careful about using regular expressions, if you want to preserve the order of files. If you get the regular expression wrong, it will affect the exact order in which the files are merged.

A quick and easy way to make sure the files get merged in the exact order you want, is to use the output of another file listing program such as ls or find and pipe it to the cat command. First execute the find command with the regular expression and verify the file order…

bash$ find . -name «file*.txt» -o -name «my*.txt»

This will print the files in order such that you can verify it to be correct or modify it to match what you want. You can then pipe that output into the cat command.

bash$ find . -name «file*.txt» -o -name «my*.txt» | xargs cat > ./mergedfile.txt

When you merge multiple files into one file using regular expressions to match them, especially when it is piped and where the output file is not very obvious, make sure that the regular expression does not match the filename of the merged file. In the case that it does match, usually the cat command is pretty good at error-ing out with the message “input file is output file”. But it helps to be careful to start with.

Merge Two Files at Arbitrary Location

Sometimes you might want to merge two files, but at a particular location within the content of a file. This is more like the process of inserting contents of one file into an another at a particular position in the file.

If the file sizes are small and manageable, then vi is a great editor tool to do this. Otherwise the option is to split the file first and then merge the resulting files in order. The easiest way is to split the file is based on the line numbers, exactly at where you want to insert the other file.

bash$ split -l 1234 file1.txt

You can split the file into any number of output files depending on your requirement. The above example will split the file file1.txt to chunks of 1234 lines. It is quite possible that you might end up with more than two files, named xaa, xab, xac etc..You can merge all of it back using the same cat command as mentioned earlier.

bash$ cat xaa file2.txt xa

The above command will merge the files in order with the contents of file2.txt in between the contents of xaa and xab.

Another use case is when you need to merge only specific parts of certain files depending on some condition. This is especially useful for me when I have to analyze several large log files, but am only interested in certain messages or lines. So, I will need to extract the important log messages based on some criteria from several log files and save them in a different file while also maintaining or preserving the order of the messages.

Though you can do this using cat and grep commands, you can do it with just the grep command as well.

bash$ grep -h «[Error]» logfile*.log > onlyerrors.log

The above will extract all the lines that match the pattern [Error] and save it to another file. You will have to make sure that the log files are in order when using the regular expression to match them, as mentioned earlier in the post.

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Use CAT Command to Combine Text Files in Ubuntu 18.04

The CAT command in Linux is not only helpful in creating text files, displaying their contents, but also in merging text from two or more text files. The merged text can then be saved to another text file. In this tutorial, you will learn the use of the CAT command to combine text from two or more files into a single one. This will help you in achieving a power user status on Ubuntu from an average beginner. We have run the commands mentioned in this tutorial on an Ubuntu 18.04 LTS system.

Let us present a few examples in this article which will help you in understanding the proper usage of the CAT command in the following four scenarios:

• Merging text from multiple text files to a single text file.
• Merging text from multiple files, and saving the output to another file in alphabetical order.
• Appending text from one text file to another.
• Appending text from the Ubuntu Terminal directly to a text file.

Note: It is a good practice to backup important files before altering their contents.

Example 1: Merging text from three files to another text file

We have created three sample text files on our system by the name of textfile1.txt, textfile2.txt, and textfile3.txt. All of these files contain a line of text. The following use of the CAT command will display the text from all of these files in a single output.

Open the Ubuntu Terminal by either pressing CTRl+Alt+T or through the Dash, and then enter the following command:

In the following image you can see how the output from my three text files is printed as a single merged output:

Linux allows you to print the output of a command to a file by using the following syntax:

Let us make use of this command and the cat command to save the text from three different text files to a new text file:

In the following image, I am saving the merged text from my three files to a new file textfile4.txt; I am then printing the contents of the new file to the screen for you to view:

Please remember that if the destination text file already exists in your system, its contents will be overwritten.

Example 2: Merging text from three files, and saving the output to another file in alphabetical order

Suppose you have three text files; each containing some text. You want to merge the text from all three and save the output to a fourth file, but in alphabetical order. This is how you will do it: Advertisement

In the following image, you can view the text from each of my text files. If I simply combine the text to a new file textfile4.txt, the output will be as follows:

However, I want an alphabetically sorted output to be printed to my text file, so I will use the following command:

You can see how my newly created textfile5.txt contains merged and sorted text from my three source files.

Example 3: Appending text from one text file to another

The cat command can also be used to append text from a source file to a destination file without messing up with the contents of the later.

Here is a sample destination file:

Here is a sample source file:

The syntax for appending text:

Here is how my destination file looks after I append the text from my source file to it:

Example 4: Appending text from the Terminal directly to a file

If you want to append some text, from the command line, at the end of an already existing text file, you can use the following syntax:

After entering this command, a cursor will appear for you to enter the text you want to add to the specified file. Enter the text and press Ctrl+D. Your entered text will be appended at the end of the file without disturbing its already existing contents.

You can see this text added to the file in the following image:

We hope that the detailed examples described in this article along with the syntax of the cat command in each case will help in merging the contents of multiple files into a single one. Moreover, you can excel sorting and appending of text not only from one file to another but also directly from the Ubuntu Terminal.

Karim Buzdar

About the Author: Karim Buzdar holds a degree in telecommunication engineering and holds several sysadmin certifications. As an IT engineer and technical author, he writes for various web sites. You can reach Karim on LinkedIn

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