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Asked: 2017-12-06 01:19:28 +0800 CST

Spaces in commands with redirection

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program 2> error.log 
program &> filename 
program >> filename 2>&1
program 2>&1

I was able to figure out what these lines mean.

However I don't quite understand where I should put the spaces. I also worry that it actually doesn't matter where to put spaces. Thanks for reading.

command-line

2 Answers

  1. Best Answer

    Yes, spacing between words and redirections doesn't matter. That's the short answer.

    The details lie in the fact that shell ( for simplicity let's just refer to bash only) treates certain characters and groups of characters as "words" and "metacharacters". From bash 4.3 manual:

    metacharacter

    A character that, when unquoted, separates words. One of the following:

    |  & ; ( ) < > space tab

    And

    word A sequence of characters considered as a single unit by the shell. Also known as a token.

    So when we do:

    $ echo "hello world">/dev/null

    or 

    $ echo "hello world" > /dev/null

    that's still 3 words ("hello world" can be considered a single shell word because it's quoted), with one > meta character and couple spaces. Shell will see it, and perform redirection first (so it looks for meta characters first), and then runs commands in accordance with its standard behavior.

    Order of redirections, however, matters a lot, especially when you're duplicating file descriptors with something like 2>&1. Say you want to get send both stderr and stdin to same location. Here's a wrong way to do it:

    $ stat ./non-existent file 2>&1  > /dev/null
stat: cannot stat './non-existent': No such file or directory
stat: cannot stat 'file': No such file or directory

    You're making file descriptor 2 output to same location as 1, which is your terminal, but it was already doing so. That's why stderr shows up.

    If you redirect stdout first, and only then change where 2 points - then it'll work:

    $ stat ./non-existent file > /dev/null 2>&1
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  2. Spacing doesn't matter except a few special cases.

    1. You have a digit before the standard output redirection.
      According to the Bash manual:

    if the file descriptor number is omitted, and the first character of the redirection operator is ‘<’, the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is ‘>’, the redirection refers to the standard output (file descriptor 1).

    So, for example, echo test 2 > output.txt will be interpreted as echo test 2 1> output.txt, i.e. redirect the stdout of the command echo test 2 to the file output.txt which will create the file containing the text "test 2".
    On the other hand, echo test 2> output.txt will be interpreted as "redirect the stderr (file descriptor 2) of the command echo test to the file output.txt", which will be empty because the stderr of the command is empty.
    This behavior is not limited to digits 1 and 2. So echo test 5> output.txt will also create an empty file.

    1. When using process substitution in the form <(...) or >(...), space between the left parentheses and the > or < is not allowed.

    Note that no space may appear between the < or > and the left parenthesis, otherwise the construct would be interpreted as a redirection.

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