Bash – echo \\* – bash backslash escape behavior, is it evaluated backwards

bashechoescape-charactersquotingshell

So in bash,
When I do

echo \*
*

This seems right, as * is escaped and taken literally.

But I can't understand that,
when I do

echo \\*
\*

I thought the first backslash escaped the second one thus two backslash "\\" will give me one "\" in literal. and * followed carrying its special meaning.
I was expecting:

echo \\*
\file1 file2 file3

ANSWER SUMMARY:
Since \ is taken literally, echo \* will behave just as echo a*, which will find any file that starts with literal "a".

Follow up question,
If I want to print out exactly like

\file1 file2 file3

What command should I use?
e.g. like the following but I want no space

echo \\ * 
\ file1 file2 file3

Best Answer

If you don't have a file in the current directory whose name starts with a backslash, this is expected behaviour. Bash expands * to match any existing file names, but:

If the pattern does not match any existing filenames or pathnames, the pattern string shall be left unchanged.

Because there was no filename starting with \, the pattern was left as-is and echo is given the argument \*. This behaviour is often confusing, and some other shells, such as zsh, do not have it. You can change it in Bash using shopt -o failglob, which will then give an error as zsh does and help you diagnose the problem instead of misbehaving.

The * and ? pattern characters can appear anywhere in the word, and characters before and after are matched literally. That is why echo \\* and echo \\ * give such different output: the first matches anything that starts with \ (and fails) and the second outputs a \, and then all filenames.

The most straightforward way of getting the output you want safely is probably to use printf:

printf '\\'; printf "%s " *

echo * is unsafe in the case of unusual filenames with - or \ in them in any case.

Related Solutions

Number of Backslashes Needed for Escaping Regex Backslash on Command-Line

For the unquoted example, each \\ pair passes one backslash to grep, so 4 backslashes pass two to grep, which translates to a single backslash. 6 backslashes pass three to grep, translating to one backslash and one \c, which is equal to c. One additional backslash does not change anything, because it is translated \c -> c by the shell. Eight backslashes in the shell are four in grep, translated to two, so this does not match anymore.

For the example in double quotes, note what follows your second quote from the bash manpage:

The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or newline.

I.e. when you give an odd number of backslashes, the sequence ends in \c, which would be equal to c in the unquoted case, but when quoted, the backslash looses its special meaning, so \c is passed to grep. That is why the range of "possible" backslashes (i.e. those that make up a pattern matching your example file) slides down by one.

Shell – Escape Sequences with echo -e in Different Shells

Because you used sh, not bash, then echo command in sh doesn't have option -e. From sh manpage:

echo [-n] args...
            Print the arguments on the standard output, separated by spaces.
            Unless the -n option is present, a newline is output following the
            arguments.

And it doesn't have \e, too:

        If any of the following sequences of characters is encountered
        during output, the sequence is not output.  Instead, the specified
        action is performed:

        \b      A backspace character is output.

        \c      Subsequent output is suppressed.  This is normally used at
                the end of the last argument to suppress the trailing new‐
                line that echo would otherwise output.

        \f      Output a form feed.

        \n      Output a newline character.

        \r      Output a carriage return.

        \t      Output a (horizontal) tab character.

        \v      Output a vertical tab.

        \0digits
                Output the character whose value is given by zero to three
                octal digits.  If there are zero digits, a nul character
                is output.

        \\      Output a backslash.

        All other backslash sequences elicit undefined behaviour.

Best Answer

Related Solutions

Number of Backslashes Needed for Escaping Regex Backslash on Command-Line

Shell – Escape Sequences with echo -e in Different Shells

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