strange behaviour of grep in UNIX

Question

When I type the command

$ grep \\\\h junk  

then the shell should interpret \\\\h as \\h - as the two pairs of \\ each become \ . grep in turn, should interpret \\h as \h as \\ becomes \, so grep should search for the pattern \h in junk, which it is doing successfully.

But it's not working for \\\\$. Why?

Answer 1

To return lines with a dollar sign, another way without escapes is:

grep "[$]" file

To find lines with the literal \$:

grep "[\][$]" file

Answer 2

Returns only lines with the dollar sign in it.

grep "\\$" application/bootstrap.php

Because $ is a regex command as well ( end of line ), you have to quote it to get the double slashes past your shell and into grep.

Answer 3

Grep always uses \ to quote the next character, so \h simply becomes h in your example, since \h is not "special":

$ cat file
\a
a

$ grep \\a file
\a
a

$ grep \\\\a file
\a

Answer 4

Assuming you're trying to match the character '$', the pattern:

[$]

is the easiest way to specify it without getting tangled up in multiple levels of character-escapes. Or if you're trying to match the two characters: '\$', then

[\\][$]

will do it.

Answer 5

\h is not a special escape for grep, so '\h' should find a \h in a file, which it does. The shell removes the first \ before grep sees it, as \ is special (type ls \\ and it says ls: \: So such file or directory ). So to match "\h" in a file, use grep \h, or '\h' to avoid the first \ --> \ reduction by the shell.

Now \$ : \ is reduced to \ by the shell, and grep sees \$ and because $ is special in regexes (like +,.,* etc), \$ is a literal dollar sign, as a simple test file will show you. See the man page for re_format (7). So this matches all lines with a dollar sign.

So if you want to match a literal "\$" in a file, we need to think: \$ is unescaped by the shell to literal $, and \ to \: try ls \\\$' and you get "ls: \$: No such file or directory". So ingrep \\$ file, grep sees \$ and interprets that as $ again... So we want grep to see \\\$ (which it will unescape again to \$), so we can put single quotes around that and avoid headaches, or escape all the special characters for the shell too:grep \\\\$ file`. This works.

That's why I almost always put single quotes around the first grep argument, so I only have to think about what grep will do, and not about the shell as well.

Answer 6

Your shell is interpreting \\ as \, and $ as a first character of some control sequences, like variable names. Therefore \\\\ is interpreted by your shell as literal \\ (as you think), but $ needs to be escaped to be interpreted as literal character. Use \\\\\\\$ (seven backslashes) or '\\\$' (text in apostrophes is interpreted literally by shell).

Answer 7

Why can't you just grep '\\$'? Single quotes prevent almost all interpolation.