嵌入式外壳不支持重定向：exec 2>>（logger -t myscript）

Question

I'm trying to run the command from this question :

exec 2> >(logger -t myscript)

It works great on my desktop linux system, however, on my embedded linux device the same command presents the following error:

-sh: syntax error near unexpected token `>'

So I'm guessing my shell doesn't like part of the command syntax - most likely this portion:

exec 2> > (logger -t myscript)

In fact, while I understand that the 2> is redirecting stderr I don't actually understand the syntax of the second > character in this case, is it another way of representing a pipe?

If I can understand what it is doing then perhaps I can modify my command to work with my limited shell on the embedded linux device.

Answer 1

The syntax in question only works with bash (or other shells with ksh extensions). In the error

-sh: syntax error near unexpected token `>'

...you're trying to use that syntax with /bin/sh .

Be sure your script starts with #!/bin/bash , and that you invoke it with bash yourscript rather than sh yourscript .

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A bit more explanation:

• >(foo) gets replaced with a filename (of the form /dev/fd/## if supported, or a named pipe otherwise) which receives output from a process named foo . This is the part that requires bash or ksh extensions.
• exec <redirection> applies a redirection to the current shell process (thus, exec 2>stderr.log redirects all stderr from the current command and its children to the file stderr.log ).

Thus, exec 2> >(foo) modifies the stderr file descriptor (of your current shell session) to go to the stdin of command foo ; in this case, foo is logger -t myscript , thus sending the process's stderr to syslog.

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To perform the same operation on a more limited (but still POSIX-compliant) shell:

# note: if any security concerns exist, put the FIFO in a directory
#       created by mktemp -d rather than hardcoding its name

mkfifo /tmp/log.fifo                 # create the FIFO
logger -t myscript </tmp/log.fifo &  # start the reader in the background first!
exec 2>/tmp/log.fifo                 # then start writing
rm -f /tmp/log.fifo                  # safe to delete at this point

Answer 2

>( command ) is a bash construct called "process substitution". man bash says:

Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files.

Your shell doesn't seem to be bash, anyway.