Check if a string matches a regex in Bash script

You can use the test construct, [[ ]], along with the regular expression match operator, =~, to check if a string matches a regex pattern.

For your specific case, you can write:

[[ $date =~ ^[0-9]{8}$ ]] && echo "yes"

Or more a accurate test:

[[ $date =~ ^[0-9]{4}(0[1-9]|1[0-2])(0[1-9]|[1-2][0-9]|3[0-1])$ ]] && echo "yes"#           |\______/\______*______/\______*__________*______/|#           |   |           |                  |              |#           |   |           |                  |              |#           | --year--   --month--           --day--          |#           |          either 01...09      either 01..09      |# start of line            or 10,11,12         or 10..29      |#                                              or 30, 31      |#                                                        end of line

That is, you can define a regex in Bash matching the format you want. This way you can do:

[[ $date =~ ^regex$ ]] && echo "matched" || echo "did not match"

where commands after && are executed if the test is successful, and commands after || are executed if the test is unsuccessful.

Note this is based on the solution by Aleks-Daniel Jakimenko in User input date format verification in bash.

In other shells you can use grep. If your shell is POSIX compliant, do

(echo "$date" | grep -Eq  ^regex$) && echo "matched" || echo "did not match"

In fish, which is not POSIX-compliant, you can do

echo "$date" | grep -Eq "^regex\$"; and echo "matched"; or echo "did not match"

In bash version 3 you can use the '=~' operator:

if [[ "$date" =~ ^[0-9]{8}$ ]]; then    echo "Valid date"else    echo "Invalid date"fi

Reference: http://tldp.org/LDP/abs/html/bashver3.html#REGEXMATCHREF

NOTE: The quoting in the matching operator within the double brackets, [[ ]], is no longer necessary as of Bash version 3.2

A good way to test if a string is a correct date is to use the command date:

if date -d "${DATE}" >/dev/null 2>&1then  # do what you need to do with your dateelse  echo "${DATE} incorrect date" >&2  exit 1fi

from comment: one can use formatting

if [ "2017-01-14" == $(date -d "2017-01-14" '+%Y-%m-%d') ]