grep 和 regex 存储在字符串中

Question

my question is quite short:

a="'[0-9]*'"
grep -E '[0-9]*' #for example, line containing 000 will be recognized and printed

but

grep -E $a #line containing 000 WILL NOT be printed, why is that?

Does substitution for grep regex change the command's behaviour or have I missed something from a syntactic point of view? In other words, how do I make it so that grep accepts regex from a string stored in a variable.

Thank you in advance.

Answer 1

Quotes go around data, not in data. That means, when you store data (in this case, a regex expression) in a variable, don't embed quotes in the variable; instead, put double-quotes around the variable when you use it:

a="[0-9]*"
grep -E "$a"

You can sometimes get away with leaving the double-quotes off when using variables (as in Avinash Raj's comment), but it's not generally safe. In this case, it'll work fine provided there are no files or subdirectories in the current working directories with names that happen to start with a digit. You see, without double-quotes around $a , the shell will take its value, try to split it into multiple words (not a problem here), try to expand each word that contains shell wildcards into a list of matching files (potential problem here), and pass that to the command ( grep ) as its list of arguments. That means that if you happen to have files that start with digits in the current directory, grep thinks you ran a command like this:

grep -E 1file.txt 2file.jpg 3file.etc

... and it treats the first filename as the pattern to search for, and any other filenames as files to be searched. And you'll be scratching your head wondering why your script works or fails depending on which directory you happen to be in.

Note: the pattern [0-9]* is a valid regular expression, and a valid shell glob (wildcard) pattern, but it means very different things in the two contexts. As a regex, it means 0 or more digits in a row. As a shell glob, it means something that starts with a digit. Speaking of which, grep -E '[0-9]*' is not actually going to be very useful, since everything contains strings of 0 or more digits, so it'll match every line of every file you feed it.