Bash – Closing a File Descriptor: >&- vs <&-

bashio-redirectionshell

In the bash tutorial I am reading, it says that if you open a file descriptor for reading, i.e.

exec 3< echolist

Then you must close it like this,

exec 3<&-

However, if you open a file descriptor for writing, it must be closed like this:

exec 3>&-

Yet when I look on the internet, I see people opening files and then closing them with this:

exec 3>&-

NOTE: when, according to the tutorial, they should be using exec 3<&1.

So my question is, can all file descriptors be closed via exec n>&- where n is the file descriptor number? Regardless if it was opened for reading, writing, or both?

Best Answer

You can close file descriptor using both <&- and >&-, bash will parse two syntax as the same.

From file y.tab.c in bash source code:

5385   /* Hack <&- (close stdin) case.  Also <&N- (dup and close). */                
5386   if MBTEST(character == '-' && (last_read_token == LESS_AND || last_read_token == GREATER_AND))
5387     return (character);

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Bash – Significance of arrows symbols in duplicating/closing file descriptors under bash

It doesn't matter because both 4>&1 and 4<&1 do the same thing: dup2(1, 4) which is the system call to duplicate a fd onto another. The duplicated fd automatically inherits the I/O direction of the original fd. (same for 4>&- vs 4<&- which both resolve to close(4), and 4>&1- which is the dup2(1, 4) followed by close(1)).

However, the 4<&1 syntax is confusing unless for some reason the fd 1 was explicitly open for reading (which would be even more confusing), so in my mind should be avoided.

The duplicated fd shares the same open file description which means they share the same offset in the file (for those file types where it makes sense) and same associated flags (I/O redirection/opening mode, O_APPEND and so on).

On Linux, there's another way to duplicate a fd (which is not really a duplication) and create a new open file description for the same resource but with possibly different flags.

exec 3> /dev/fd/4

While on Solaris and probably most other Unices, that is more or less equivalent to dup2(4, 3), on Linux, that opens the same resource as that pointed to by the fd 4 from scratch.

That is an important difference, because for instance, for a regular file, the offset of fd 3 will be 0 (the beginning of the file) and the file will be truncated (which is why for instance on Linux you need to write tee -a /dev/stderr instead of tee /dev/stderr).

And the I/O mode can be different.

Interestingly, if fd 4 pointed to the reading end of a pipe, then fd 3 now points to the writing end (/dev/fd/3 behaves like a named pipe):

$ echo a+a | { echo a-a > /dev/fd/0; tr a b; }
b+b
b-b

$ echo a+a | { echo a-a >&0; tr a b; }
bash: echo: write error: Bad file descriptor
b+b

Opening more than 10 file descriptors with ksh

In ksh, you can only use single digit for explicit file descriptor. With ksh93r and above, you can open more than 10 file descriptor by using the form:

{var}>filename

(bash and zsh copied this feature later).

ksh will pick available file descriptor greater than or equal to 10, store file descriptor number in variable var:

$ exec {var1}>/tmp/test1
$ echo "$var1"
10
$ exec {var2}>/tmp/test2
$ echo "$var2"
11

Best Answer

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Bash – Significance of arrows symbols in duplicating/closing file descriptors under bash

Opening more than 10 file descriptors with ksh

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