Grep

Internal

• Linux
• sed

Overview

grep accepts regular expressions. Both " " and ' ' can be used to enclose the expression to search for.

If the expression is found, the enclosing lines are sent to stdout the command returns 0 (success).

If the expression is NOT found, the command returns 1 (failure).

    if grep -q "some regex" ./file.txt; then
        # do something if expression is found
    else
        # do something else if the expression is not found
    fi

Options

-q

Quiet.

-v, --invert-match

Invert the sense of matching, to select non-matching lines.

-e

GNU grep uses -e to introduce a regular expression to match. Also see:

grep Regular Expressions

Regular Expressions

grep Regular Expressions

Recipes

Including TWO expressions at the same time

cat /proc/cpuinfo | grep 'processor\|siblings'

processor	: 0
siblings	: 2
processor	: 1
siblings	: 2

The method works even if there are spaces and other regular expression characters within the alternative regexes:

... | grep ' logger: [1-9]\+\.\|(concurrent mode failure)'

This will select either "logger: [1-9]\+\." or "(concurrent mode failure)"

Excluding TWO expressions at the same time

     echo "something" | grep -v "blah" | grep -v "blah2"

Do not Send Anything to stout

... and avoid > /dev/null:

    grep -q

--color

grep --color 'context.layer='

Prefix the Line with the "Line Number" (-n)

-n prefixes the line with the line number.

sed: Insert a Line/Append in a Specific Position (line number) in a file

Return Only the First Match

-m means "stop reading the file about num matches. So to return the first match:

-m 1

Print More Lines of Trailing Context after Each Match

Use -A to print a number of lines of trailing context after each match:

grep -A 10 "something" ./file.txt

Iterate over Lines Selected by grep

IFS="$(printf '\n\r')";
for i in $(grep "something ${some_file}); do
  ...
done
IFS="$(printf ' \t\n')"

See also:

IFS