Bash – What’s the purpose of 1> in exec 1> >(logger -s -t tagname) 2>&1

bashexecio-redirectionshell

I just stumbled over

exec 1> >(logger -s -t $(basename $0)) 2>&1

which is used to redirect the output of the current script to the system logger (in case you've never seen this, but are interested check out https://stackoverflow.com/questions/8888251/understanding-bash-exec-12-command to broaden your shell knowledge).

I am wondering why the 1> is necessary. It seems necessary because exec >(logger -s -t test) 2>&1 fails due to

bash: /dev/fd/63: Permission denied
bash: exec: /dev/fd/63: cannot execute: Permission denied

Omitting 1> is however what I'd do intuitively because exec >[some redirection target] already should be sufficient for the redirection according to the question linked above. 2>&1 then redirects the stderr to stdout as usual.

I'm using bash 4.4.19.

Best Answer

It is necessary (the extra >, not the 1, 1> could be simply written >). The >(...) process substitution will expand to something like /dev/fd/13 (a file name), and then > will redirect the standard output into it. Thence > >(...).

Related Solutions

Bash – What does exec 3<&1 do

From bash manpage:

Duplicating File Descriptors
       The redirection operator

              [n]<&word

       is used to duplicate input file descriptors.  If word expands to one or
       more  digits,  the file descriptor denoted by n is made to be a copy of
       that file descriptor.  If the digits in word  do  not  specify  a  file
       descriptor  open for input, a redirection error occurs.  If word evalu‐
       ates to -, file descriptor n is closed.  If n  is  not  specified,  the
       standard input (file descriptor 0) is used.

       The operator

              [n]>&word

       is  used  similarly  to duplicate output file descriptors.  If n is not
       specified, the standard output (file descriptor 1)  is  used.   If  the
       digits  in word do not specify a file descriptor open for output, a re‐
       direction error occurs.  As a special case, if n is omitted,  and  word
       does not expand to one or more digits, the standard output and standard
       error are redirected as described previously.

I did some debugs with strace:

sudo strace -f -s 200 -e trace=dup2 bash redirect.sh

For 3<&1:

dup2(3, 255)                            = 255
dup2(1, 3)                              = 3

For 3>&1:

dup2(1, 3)                              = 3

For 2>&1:

dup2(1, 2)                              = 2

It seems that 3<&1 do exactly the same as 3>&1, duplicating stdout to file descriptor 3.

Bash Exec Command – What Does ‘exec {fd}

Rather than having to pick a file descriptor and hope it's available:

exec 4< /dev/watchdog  # Was 4 in use? Who knows?

this notation asks the shell to pick a file descriptor that isn't currently in use, open the file for reading on that descriptor, and assign the number to the given variable (fd).

$ exec {fd}< /dev/watchdog
$ echo $fd
10

Best Answer

Related Solutions

Bash – What does exec 3<&1 do

Bash Exec Command – What Does ‘exec {fd}

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