我可以在bash shell脚本中使用正则表达式吗

Question

I am a first year Computer technician student, and before now have never actually used linux. I grasped the basics of scripting fairly quickly and am trying to write a script that will create a directory and a soft link for each assignment. seeing as we average two assignments a week i thought this would be helpful.

I am having trouble making my script accept only numbers as variables. I have it mostly working (mostly) with use of 3 case statements, but would rather use basic regex with an if statement, if I can.

if [ $# != 1 ]; then

    red='\033[0;31m'
    NC='\033[0m'
    blue='\033[1;34m'
    NC='\033[0m'
    echo 1>&2 "${blue}ERROR:${NC} $0 :${red}expecting only one variable, you gave $#($*)${NC}"
    echo 1>&2 "${blue}Usage:${NC} $0 :${red}number of assignment.${NC}"
    exit 2

fi


case $1 in

    [[:punct:]]*) echo 1>&2 "Numeric Values Only"
    exit 2
    ;;
    [[:alpha:]]*) echo 1>&2 "Numeric Values Only"
    exit 2
    ;;
    [[:space:]]*) echo 1>&2 "Numeric Values Only"
    exit 2
    ;;
esac

the script then makes the directory and creates a soft link for the marking script (if its posted), and ends. can anyone help me shorten/eliminate the case statements

Answer 1

You cannot use regular expressions in portable shell scripts (ones that will run on any POSIX compliant shell). In general, patterns in the shell are globs , not regular expressions.

That said, there are a few other options. If you are using Bash specifically, you have two choices; you can use extended globs, which give you some regex like functionality:

shopt -s extglob

case $1 in
    +([[:digit:]]) ) echo "digits" ;;
    *) echo "not digits" ;;
esac

Another option is that Bash has the =~ operator in the [[ ]] conditional construct to match strings against a regex; this doesn't work in a case statement, but works in an if :

if [[ $1 =~ [0-9]+ ]]
then
    echo "digits"
fi

Finally, if you want to do regular expression matching portably (so it will run in other POSIX shells like ash , dash , ksh , zsh , etc), you can call out to grep ; if you pass -q , it will be silent about the matches but return success or failure depending on whether the input matched:

if echo "$1" | grep -qE "[0-9]+"
then
    echo "digits"
fi