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I am confused about the backslash character \ and single quotations inside grep. From my understanding it preserves the literal meaning. For example

if I do echo 'This is \., it gives me This is \.

However, if I want to match any actual .(period) I need to use '\.' to escape the special meaning of . in regular expression. For example if a have a file demo.txt taken from example from this website

Problem output

My question is how is the ' ' not preserving the literal value in this case for grep? This is proving to be very confusing for me. Any explanations would be very helpful.

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Single quotes protect characters from interpretation by the shell, but grep still treats the argument as a regular expression.

You can turn this behavior off by running grep with the -F or --fixed-strings option:

grep -F 'purchase.' demo.txt

The single quotes are not necessary as none of those p, u, r, c, h, a, s, e or . characters are special to the shell:

grep -F purchase. demo.txt

Or you could use grep without -F but escape the . regular expression operator with \:

grep 'purchase\.' demo.txt

The single quotes are so that \ (which is also special for the shell; a quoting operator there) is passed literally to grep. Or you can use the \ shell quoting operator to quote itself:

grep purchase\\. demo.txt

You can also use the [set] regex operator to match on a literal .:

grep 'purchase[.]' demo.txt

Again, [set] is also a shell operator (a globbing operator there) so needs to be quoted as well (here with single quotes again).

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