Search within a search

Question

I've gotten used to using grep for my command line searches and wanted to know how to successfully do a search using the result of another search.

Here's my attempt, where I am looking for 'tool' within my result:

grep tool | grep -rl embed=

This returns some results and then the console hangs. Are there any simple/elegant solutions to this?

Answer 1

Pipelines run from left to right. More precisely, the processes run in parallel, but the data flows from left to right: the output of the command on the left becomes the input of the command on the right.

Here the command on the left is grep tool. Since you're passing a single argument to grep, it's searching in its standard input. Since you haven't redirected the standard input, grep is reading from the terminal: it's waiting for you to type.

To search in a file, use

grep tool path/to/file | …

To search in a directory recursively, use

grep -r tool path/to/directory | …

You can filter the results to list only the lines that contain embed=. Drop the -l and -r options, they make no sense when the input is coming from standard input.

grep -r tool path/to/directory | grep 'embed='

This lists lines containing both tool and embed= (in either order). An alternative method with simpler plumbing would be to do a single search with an or pattern; this is always possible, but if the patterns can overlap (not the case here), the pattern can get very complicated.

grep -E -r 'tool.*embed=|embed=.*tool' path/to/directory

If you wanted to list files containing both tool and embed=, you'd need a different command structure, with the first grep listing file names (-l) and the second one receiving those file names as arguments, not as input. Converting standard input into command line arguments what the xargs command is for.

grep -lZ -r tool path/to/directory | xargs -0 grep -l 'embed='

Answer 2

This command is going to search for foo in the file filename and in the results for bar:

grep foo filename | grep bar

An alternative would be with awk: awk '/foo/ && /bar/' filename

Answer 3

You can use grep in different ways, for instance:

cat myfile | grep keyword

This above example will dump the file to stdout. The pipe character | will take that output and feed it to grep keyword command as stdin. In the end, the lines in myfile containing keyword will be written to the stdout, which is generally terminal screen. This is a very ineffective use of resources, the same result can be accomplished with this command as well:

grep keyword myfile

if you want to use grep in a cascaded menner, here is how it goes:

grep keyword1 myfile | grep keyword2

This time, first grep command, finds the lines containing keyword1 in myfile. Second grep command takes the output of the first one, and searches for keyword2 in those. Effectively, you are looking for lines containing both keyword1 and keyword2 in myfile.

you can multiply these commands with different command line switches to find what exactly you are looking for, using more than one pipe. If you can elaborate what you are trying to do with your command in the original question in plain English, myself or someone else can help you further.

Answer 4

Essentially what you are trying to do is logical AND. grep is a line matching tool, so in terms of a line patter1 AND patter2 logic mean they are going to be on the same line ; when you search a file , where you want to find line with one term and another on the same line, you can use regular expression grep 'pattern1.*pattern2' input_file.txt

$ ps -ef | grep 'xieerqi.*firefox'
xieerqi   5069  2616  7 17:43 ?        00:07:25 /usr/lib/firefox/firefox
xieerqi  24317  6901  0 19:22 pts/19   00:00:00 grep xieerqi.*firefox

In this example, instead of searching all processes with my username and then piping to another grep that filters firefox processes.

For more, refer to this tutorial