there are lots of compression methods that work recursively command line and its good to know who the end audience is.

i.e. if it is to be sent to someone running windows then zip would probably be best:

zip -r file.zip folder_to_zip

unzip filenname.zip

for other linux users or your self tar is great

tar -cvzf filename.tar.gz folder

tar -cvjf filename.tar.bz2 folder  # even more compression

#change the -c to -x to above to extract

One must be careful with tar and how things are tarred up/extracted, for example if I run

cd ~
tar -cvzf passwd.tar.gz /etc/passwd
tar: Removing leading `/' from member names
/etc/passwd


pwd

/home/myusername

tar -xvzf passwd.tar.gz

this will create /home/myusername/etc/passwd

unsure if all versions of tar do this:

 Removing leading `/' from member names

This will output a gz archive for each matching file, files are replaced by the archive:

gzip fileprefix*

Have a look to the '-r' flag too.

The gzip command will not recursively compress a directory into a single zip file, when using the -r switch. Rather it will walk that directory structure and zip each file that it finds into a separate file.

Example

before

$ tree dir1/
dir1/
|-- dir11
|   |-- file11
|   |-- file12
|   `-- file13
|-- file1
|-- file2
`-- file3

now run the gzip command

$ gzip -r dir1

after

$ tree dir1/
dir1/
|-- dir11
|   |-- file11.gz
|   |-- file12.gz
|   `-- file13.gz
|-- file1.gz
|-- file2.gz
`-- file3.gz

If you'd prefer to zip up the directory structure then you'll likely want to use the tar command, and then compress the resulting .tar file.

$ tar zcvf dir1.tar.gz dir1/

Example

$ tar zcvf dir1.tar.gz dir1/
dir1/
dir1/file1
dir1/file2
dir1/dir11/
dir1/dir11/file11.gz
dir1/dir11/file12.gz
dir1/dir11/file13.gz
dir1/file3

Which results in the following single file:

$ ls -l | grep tar
-rw-rw-r-- 1 saml saml  271 Oct  1 08:07 dir1.tar.gz

You can confirm its contents:

$ tar ztvf dir1.tar.gz 
drwxrwxr-x saml/saml         0 2013-10-01 08:05 dir1/
-rw-rw-r-- saml/saml         0 2013-10-01 07:45 dir1/file1
-rw-rw-r-- saml/saml         0 2013-10-01 07:45 dir1/file2
drwxrwxr-x saml/saml         0 2013-10-01 08:04 dir1/dir11/
-rw-rw-r-- saml/saml        27 2013-10-01 07:45 dir1/dir11/file11.gz
-rw-rw-r-- saml/saml        27 2013-10-01 07:45 dir1/dir11/file12.gz
-rw-rw-r-- saml/saml        27 2013-10-01 07:45 dir1/dir11/file13.gz
-rw-rw-r-- saml/saml         0 2013-10-01 07:45 dir1/file3

No need for loops or anything more than find and gzip:

find . -type f ! -name '*.gz' -exec gzip "{}" \;

This finds all regular files in and below the current directory whose names don't end with the .gz extension (that is, all files that are not already compressed). It invokes gzip on each file individually.

Edit, based on comment from user unknown:

The curly braces ({}) are replaced with the filename, which is passed directly, as a single word, to the command following -exec as you can see here:

$ touch foo
$ touch "bar baz"
$ touch xyzzy
$ find . -exec echo {} \;

./foo
./bar baz
./xyzzy

Easy and very fast answer that will use all your CPU cores in parallel:

parallel gzip ::: *

GNU Parallel is a fantastic tool that should be used far more in this world where CPUs are only getting more cores rather than more speed. There are loads of examples that we would all do well to take 10 minutes to read... here

you can do that with a for loop to find every file then compress it:

for i in `find | grep -E "\.css$|\.html$"`; do gzip "$i" ; done

gzip will not combine files.. It is not like zip. You need to use tar with gzip

tar cf file.tar dir
gzip file.tar

or

tar zcf file.tar.gz dir

To build on @SvenW's answer (which will only work on the current directory), if you have a HUGE number of files or want to do it on a recursive directory structure you can also use

find . -type f -exec gzip \{\} \;

and if you need to put the output into a different directory (in this example, ../target) and don't want to remove the originals, you can do something like:

find . -type f -print | while read fname ; do
    mkdir -p "../target/`dirname \"$fname\"`"
    gzip -c "$fname" > "../target/$fname.gz"
done