Quotes when using grep?

Question

Grep acts differently depending on what kind of quotes I surround the regex with. I can't seem to get a clear understanding of why this is. Here is an example of the problem:

hamiltont$ grep -e show\(  test.txt 
  variable.show();
  variable.show(a);
  variable.show(abc, 132);
  variableshow();
hamiltont$ grep -e "show\("  test.txt 
grep: Unmatched ( or \(
hamiltont$ grep -e 'show\('  test.txt 
grep: Unmatched ( or \(

I am just assuming there is some proper way to enclose the regex with single/double quotes. Any help?

FWIW, grep --version returns grep (GNU grep) 2.5.1

Answer 1

The command line including the arguments is processed by the shell before it is executed. You can use echo to see what the shell does:

$ echo grep -e show\(  test.txt 
grep -e show( test.txt

$ echo grep -e "show\("  test.txt 
grep -e show\( test.txt

$ echo grep -e 'show\('  test.txt 
grep -e show\( test.txt

So without quotes the backslash gets removed making the "(" a normal character for grep (grep uses basic regex by default, use -E to make grep use extended regex).