uniq Command

uniq removes duplicate consecutive lines in a text file and is useful for simplifying the text display.

Because uniq requires that the duplicate entries must be consecutive, one often runs sort first and then pipes the output into uniq; if sort is used with the -u option, it can do all this in one step.

To remove duplicate entries from multiple files at once, use the following command:

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sort file1 file2 | uniq > file3

or

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sort -u file1 file2 > file3

To count the number of duplicate entries, use the following command:

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uniq -c filename

zcat, zless, zdiff and zgrep.

When working with compressed files, many standard commands cannot be used directly. For many commonly-used file and text manipulation programs, there is also a version especially designed to work directly with compressed files. These associated utilities have the letter "z" prefixed to their name. For example, we have utility programs such as zcat, zless, zdiff and zgrep.

Here is a table listing some z family commands:

Command	Description
$ zcat compressed-file.txt.gz	To view a compressed file
$ zless somefile.gz or $ zmore somefile.gz	To page through a compressed file
$ zgrep -i less somefile.gz	To search inside a compressed file
$ zdiff file1.txt.gz file2.txt.gz	To compare two compressed files

Note that if you run zless on an uncompressed file, it will still work and ignore the decompression stage. There are also equivalent utility programs for other compression methods besides gzip.

Let's Learn "cat" the most frequently used Linux command line utilities.

cat is short for concatenate and is one of the most frequently used Linux command line utilities. It is often used to read and print files, as well as for simply viewing file contents. To view a file, use the following command:

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$ cat <filename>

For example, cat readme.txt will display the contents of readme.txt on the terminal. However, the main purpose of cat is often to combine (concatenate) multiple files together. You can perform the actions listed in the table using cat.

The tac command (cat spelled backwards) prints the lines of a file in reverse order. Each line remains the same, but the order of lines is inverted. The syntax of tac is exactly the same as for cat, as in:

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$ tac file
$ tac file1 file2 > newfile

Command	Usage
cat file1 file2	Concatenate multiple files and display the output; i.e. the entire content of the first file is followed by that of the second file
cat file1 file2 > newfile	Combine multiple files and save the output into a new file
cat file >> existingfile	Append a file to the end of an existing file
cat > file	Any subsequent lines typed will go into the file, until Ctrl-D is typed
cat >> file	Any subsequent lines are appended to the file, until Ctrl-D is typed