Grep
Internal
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:
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.
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: