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# Grep
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Search a file for lines matching a regular expression pattern. Return the line
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number and contents of each matching line.
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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
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that match a user-provided search query (known as the *pattern*).
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The `grep` command takes three arguments:
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1. The pattern used to match lines in a file.
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2. Zero or more flags to customize the matching behavior.
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3. One or more files in which to search for matching lines.
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Your task is to implement the `grep` function, which should read the contents
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of the specified files, find the lines that match the specified pattern
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and then output those lines as a single string. Note that the lines should
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be output in the order in which they were found, with the first matching line
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in the first file being output first.
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As an example, suppose there is a file named "input.txt" with the following contents:
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```text
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hello
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world
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hello again
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```
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If we were to call `grep "hello" input.txt`, the returned string should be:
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```text
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hello
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hello again
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```
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### Flags
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As said earlier, the `grep` command should also support the following flags:
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- `-n` Print the line numbers of each matching line.
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- `-l` Print only the names of files that contain at least one matching line.
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- `-i` Match line using a case-insensitive comparison.
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- `-v` Invert the program -- collect all lines that fail to match the pattern.
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- `-x` Only match entire lines, instead of lines that contain a match.
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If we run `grep -n "hello" input.txt`, the `-n` flag will require the matching
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lines to be prefixed with its line number:
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```text
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1:hello
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3:hello again
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```
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And if we run `grep -i "HELLO" input.txt`, we'll do a case-insensitive match,
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and the output will be:
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```text
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hello
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hello again
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```
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The `grep` command should support multiple flags at once.
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For example, running `grep -l -v "hello" file1.txt file2.txt` should
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print the names of files that do not contain the string "hello".
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Run the tests with:
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```bash
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bats grep_test.sh
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```
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After the first test(s) pass, continue by commenting out or removing the
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`[[ $BATS_RUN_SKIPPED == true ]] || skip`
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annotations prepending other tests.
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To run all tests, including the ones with `skip` annotations, run:
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```bash
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BATS_RUN_SKIPPED=true bats grep_test.sh
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```
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## Source
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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)
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## External utilities
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`Bash` is a language to write "scripts" -- programs that can call
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external tools, such as
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[`sed`](https://www.gnu.org/software/sed/),
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[`awk`](https://www.gnu.org/software/gawk/),
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[`date`](https://www.gnu.org/software/coreutils/manual/html_node/date-invocation.html)
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and even programs written in other programming languages,
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like [`Python`](https://www.python.org/).
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This track does not restrict the usage of these utilities, and as long
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as your solution is portable between systems and does not require
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installation of third party applications, feel free to use them to solve
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the exercise.
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For an extra challenge, if you would like to have a better understanding
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of the language, try to re-implement the solution in pure `Bash`,
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without using any external tools. Note that there are some types of
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problems that bash cannot solve, such as performing floating point
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arithmetic and manipulating dates: for those, you must call out to an
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external tool.
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## Submitting Incomplete Solutions
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It's possible to submit an incomplete solution so you can see how others
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have completed the exercise.
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88
bash/grep/grep.sh
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bash/grep/grep.sh
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#!/usr/bin/env bash
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usage () {
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echo "usage: grep.sh [-nlivx] [-e] pattern file [...]"
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exit 1
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}
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# no arguments
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LN= # be sure variables are reset
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FN=
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RV=
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FL=
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MULTI=
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# parse args:
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while getopts ":nlivxe" arg; do
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case $arg in
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n) LN=t ;; # will print line number
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l) FN=t ;; # only file names
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i) shopt -s nocasematch ;; # case insensitive
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v) RV=t ;; # reverse search
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x) FL=t ;; # full line search
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e) break ;; # not in exercise: end args
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*) usage ;;
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esac
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done
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shift $((OPTIND-1))
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# need at least pattern and 1 file.
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# TODO: consider stdin if no file
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[[ $# < 2 ]] && usage
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[[ $# > 2 ]] && MULTI=t
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# full line match
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if [[ $FL = t ]]; then
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pattern="^$1\$"
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else
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pattern=".*$1.*"
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fi
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shift
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# will prevent leading/trailing whitespaces to be trimmed
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IFS=""
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while [[ $# > 0 ]]; do
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file="$1"
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if [[ ! -r "$file" ]]; then
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echo "$file: no such file, or not readable."
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exit 1
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fi
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lnum=0 # line number
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match=n
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while read -r line ; do # will consider '\' as normal char
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(( lnum ++ ))
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if [[ $line =~ $pattern ]] ; then # line match
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match=y
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# print only filename, go to next file
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[[ $FN = t ]] && echo "$file" && break
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# not reverse matching only
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if [[ $RV != t ]]; then
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# multiple files: print filename
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[[ $MULTI == t ]] && echo -n "$file:"
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[[ $LN = t ]] && echo -n "$lnum:"
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echo "$line"
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fi
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else
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# reverse match
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if [[ $RV == t ]]; then
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# print only filename, go to next file
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[[ $FN = t ]] && echo "$file" && break
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# multiple files: print filename
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[[ $MULTI == t ]] && echo -n "$file:"
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[[ $LN = t ]] && echo -n "$lnum:"
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echo "$line"
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fi
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fi
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done < "$1"
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shift # next file
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done
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exit 0
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