How to delete the rest of each line after a certain pattern or a string in a file?

To explicitly delete everything that comes after ".com", just tweak your existing sed solution to replace ".com(anything)" with ".com":

sed 's/\.com.*/.com/' file.txt

I tweaked your regex to escape the first period; otherwise it would have matched something like "thisiscommon.com/something".

Note that you may want to further anchor the ".com" pattern with a trailing forward-slash so that you don't accidentally trim something like "sub.com.domain.com/foo":

sed 's/\.com\/.*/.com/' file.txt

You can use awk's field separator (-F) following way:

$ cat file
google.com/funny
unix.stackexchange.com/questions
isuckatunix.com/ireallydo

$ cat file | awk -F '\\.com' '{print $1".com"}'
google.com
unix.stackexchange.com
isuckatunix.com

Explanation:

NAME
       awk - pattern scanning and processing language

-F fs
       --field-separator fs
              Use fs for the input field separator (the value of the FS predefined variable).

As you want to delete every things after .com, -F '.com' separates line with .com and print $1 gives output only the part before .com. So, $1".com" adds .com and gives you expected output.

The best tool for non-interactive in-place file editing is ex.

ex -sc '%s/\(\.com\).*/\1/ | x' file.txt

If you've used vi and if you've ever typed a command that begins with a colon : you've used an ex command. Of course many of the more advanced or "fancy" commands you can execute this way are Vim extensions (e.g. :bufdo) and are not defined in the POSIX specifications for ex, but those specifications allow for a truly astonishing degree of power and flexibility in non-visual text editing (whether interactive or automated).

The command above has several parts.

-s enables silent mode to prepare ex for batch use. (Suppress output messages et. al.)

-c specifies the command to execute once the file (file.txt, in this case) is opened in a buffer.

% is an address specifier equivalent to 1,$—it means that the following command is applied to all lines of the buffer.

s is the substitute command that you are likely familiar with already. It is commonly used in vi and has essentially identical features to the s command of sed, though some of the advanced regex features may vary by implementation. In this case from ".com" to the end of the line is replaced with just ".com".

The vertical bar separates sequential commands to be executed. In many (most) ex implementations you can also use an additional -c option, like so:

ex -sc '%s/\(\.com\).*/\1/' -c x file.txt

However, this is not required by POSIX.

The x command exits, after writing any changes to the file. Unlike wq which means "write and quit", x only writes to the file if the buffer has been edited. Thus if your file is unaltered, the timestamp will be preserved.