Zcat, Zless, Zgrep, Zdiff

From - http://www.thegeekstuff.com/2009/05/zcat-zless-zgrep-zdiff-zcmp-zmore-gzip-file-operations-on-the-compressed-files/

In this article let us review how to perform normal file operation on a compressed files using the powerful Linux Z commands.

Some of these z commands uncompresses the file temporarily in the /tmp directory to perform the specified operation. Some of the z commands uncompresses it on the fly to perfom the specified operation. But, under any case, z commands gives the peace of mind, as you don’t want to worry about the overhead of uncompressing the compressed file to perform an operation.

You can do the following normal file operations on the compressed file

1. Viewing the compressed file with zcat.
2. Paging the compressed file with zless / zmore.
3. Searching inside the compressed file with zgrep / zegrep.
4. Comparison of file using zdiff / zcmp

Example 1: View Compressed File and Uncompress with zcat

Compressing a file using gzip creates a compressed file with *.gz extension. You can view a compressed file with zcat with the following way. Which would be as same as the uncompressed file operation ‘cat filename’. zcat uncompresses the file and shows it in the stdout.

$ zcat filename.gz | more

$ ls -l big-file.*
-rw-r--r-- 1 ramesh ramesh 24853275 May  9 15:14 big-file.txt

$ gzip big-file.txt 
[Note: Compress the file]

$ ls -l big-file.*
-rw-r--r-- 1 ramesh ramesh 9275204 May  9 15:14 big-file.txt.gz

$ zcat big-file.txt.gz 
[Note: View the file without uncompressing it]

zcat big-file.txt.gz > big-file.txt
[Note: Uncompress the file]

Example 2: View a gzipped file which don’t have the gz suffix.

You can uncompress a gzipped file which don’t have the gz suffix. If you try to uncompress a gzipped file which don’t have the gz suffix with “gunzip” or “gzip -d” command you will face the following error.

gunzip: auth.log: unknown suffix -- ignored

But this zcat will uncompress the file and shows the content as shown below.

$ cat > test-file.txt
This is a test file used for gunzip and zcat testing

zcat is awesome command.

$ gzip test-file.txt

$ mv test-file.txt.gz test-file-no-ext

$ gzip -d test-file-no-ext
gzip: test-file-no-ext: unknown suffix -- ignored

$ zcat test-file-no-ext
This is a test file used for gunzip and zcat testing

zcat is awesome command.

Example 3: Display the file content without worrying about whether it is compressed or not

When you are not sure whether a file is compressed or not, you can still view the file without worrying about it’s compression status as shown below.

In this example, If the input-file is compressed zcat will display the content by uncompressing it. If the input-file is not compressed zcat will display the content as it is.

$ zcat -f input-file

Example 4: Paging the compressed file with zless / zmore.

You can paginate a compressed file with zless command or zmore command as shown below.

$ zcat filename.gz | more

$ zcat filename.gz | less

(or)

$ zless filename.gz
$ zmore filename.gz

Note: To open any kind of file type, refer to our previous article Open & View 10 Different File Types with Linux Less Command – The Ultimate Power of Less.

Example 5: Searching inside the compressed file with zgrep / zegrep.

You can search inside a compressed file with zgrep / zegrep as shown below. This would be as same as the uncompressed file operation ‘grep -i filename’. All the options to the zgrep command will be passed to grep, and the file will be fed to grep command. It may uncompress and feed the file to grep command if needed.

$ cat > test-file.txt
gzip, gunzip, zcat - compress or expand files
zless - file perusal filter for crt viewing of compressed text
zcmp, zdiff - compare compressed files

$ grep -i less test-file.txt
zless - file perusal filter for crt viewing of compressed text

$ gzip test-file.txt

$ zgrep -i less test-file.txt.gz
zless - file perusal filter for crt viewing of compressed text

Note: Become familiar with the grep command by reading our earlier article Get a Grip on the Grep! – 15 Practical Grep Command Examples.

Example 6: Comparison of file using zdiff / zcmp

You can compare two compressed files with zdiff / zcmp as shown below. This would be same as the uncompressed file operation ‘diff file1 file2′.

$ cat > file1.txt
This is line one
This is line two

$ cat > file2.txt
This is line 1
This is line two

$ diff file1.txt file2.txt
1c1
<> This is line 1

$ gzip file1.txt file2.txt

$ zdiff file1.txt.gz file2.txt.gz
1c1
<> This is line 1