将字符串作为命令行参数传递时，转义如何工作？

Question

I am passing date in this format 02/03/2013 to awk script, I need to get all of the records matching this date, If I do it directly without using the script I have to escape the forwards slash

awk '$4 ~ /02\/03\/2013/ {print $1}' 

However I dont have to escape it in case of it being sent as a command line argument,I ran the script using 02/03/2013 and awk command changed to accomodate it.

Could anyone explain to me How does the escaping of character work when we sending the string as command line argument? What are precaution to be taken for harmful input?

Any online resources related to this would be really appreciated.

Answer 1

The forward slash is not a special character in the regular expression. The reason it needs to be escaped is because the regex constant uses forward slashes itself as demarcation. With the use of a string that is interpreted as a regex, these are not used so that slashes do not need to be escaped..

awk '$4~dd{print $1}' dd=02/03/2013

Instead of regex matching you could perhaps use exact matching

awk '$4==dd{print $1}' dd=02/03/2013

Answer 2

You can use parameter expansion in bash:

date=02/03/2013
date=${date//\//\\/}            # Backslash all slashes.
awk "\$4 ~ /$date/ {print \$1}" # Prevent bash from expanding $1.