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+# Grep
+
+Search a file for lines matching a regular expression pattern. Return the line
+number and contents of each matching line.
+
+The Unix [`grep`](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html) command can be used to search for lines in one or more files
+that match a user-provided search query (known as the *pattern*).
+
+The `grep` command takes three arguments:
+
+1. The pattern used to match lines in a file.
+2. Zero or more flags to customize the matching behavior.
+3. One or more files in which to search for matching lines.
+
+Your task is to implement the `grep` function, which should read the contents
+of the specified files, find the lines that match the specified pattern
+and then output those lines as a single string. Note that the lines should
+be output in the order in which they were found, with the first matching line
+in the first file being output first.
+
+As an example, suppose there is a file named "input.txt" with the following contents:
+
+```text
+hello
+world
+hello again
+```
+
+If we were to call `grep "hello" input.txt`, the returned string should be:
+
+```text
+hello
+hello again
+```
+
+### Flags
+
+As said earlier, the `grep` command should also support the following flags:
+
+- `-n` Print the line numbers of each matching line.
+- `-l` Print only the names of files that contain at least one matching line.
+- `-i` Match line using a case-insensitive comparison.
+- `-v` Invert the program -- collect all lines that fail to match the pattern.
+- `-x` Only match entire lines, instead of lines that contain a match.
+
+If we run `grep -n "hello" input.txt`, the `-n` flag will require the matching
+lines to be prefixed with its line number:
+
+```text
+1:hello
+3:hello again
+```
+
+And if we run `grep -i "HELLO" input.txt`, we'll do a case-insensitive match,
+and the output will be:
+
+```text
+hello
+hello again
+```
+
+The `grep` command should support multiple flags at once.
+
+For example, running `grep -l -v "hello" file1.txt file2.txt` should
+print the names of files that do not contain the string "hello".
+
+
+Run the tests with:
+
+```bash
+bats grep_test.sh
+```
+
+After the first test(s) pass, continue by commenting out or removing the
+`[[ $BATS_RUN_SKIPPED == true ]] || skip` 
+annotations prepending other tests.
+
+To run all tests, including the ones with `skip` annotations, run:
+
+```bash
+BATS_RUN_SKIPPED=true bats grep_test.sh
+```
+
+## Source
+
+Conversation with Nate Foster. [http://www.cs.cornell.edu/Courses/cs3110/2014sp/hw/0/ps0.pdf](http://www.cs.cornell.edu/Courses/cs3110/2014sp/hw/0/ps0.pdf)
+
+
+## External utilities
+`Bash` is a language to write "scripts" -- programs that can call
+external tools, such as
+[`sed`](https://www.gnu.org/software/sed/),
+[`awk`](https://www.gnu.org/software/gawk/),
+[`date`](https://www.gnu.org/software/coreutils/manual/html_node/date-invocation.html)
+and even programs written in other programming languages, 
+like [`Python`](https://www.python.org/).
+This track does not restrict the usage of these utilities, and as long
+as your solution is portable between systems and does not require
+installation of third party applications, feel free to use them to solve
+the exercise.
+
+For an extra challenge, if you would like to have a better understanding
+of the language, try to re-implement the solution in pure `Bash`,
+without using any external tools. Note that there are some types of
+problems that bash cannot solve, such as performing floating point
+arithmetic and manipulating dates: for those, you must call out to an
+external tool.
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others
+have completed the exercise.