Ubuntu – command output redirection using ‘-< <( … )'

bashredirectsyntax

I needed to extract a shasum. This works, but can anyone explain why?

sed 's/^.*= //' -< <(openssl dgst -sha256 filename)

I'm familiar with the $( ) construct, but can't find docs for <( ), coupled with -<, which I assume is redirecting to the sed STDIN.

I know there are easier ways, but this construct eludes me.

Best Answer

The

<(openssl dgst -sha256 filename)

construct is a process substitution. It creates a file (or FIFO) behind the scenes and passes its name back to the command sequence.

is a regular file redirection, redirecting the contents of the behind-the-scenes file to stdin and

is a placeholder recognized by sed to indicate that its input is coming from stdin.

Since sed is perfectly capable of reading from files, the -< seems unnecessary in this context;

sed 's/^.*= //' <(openssl dgst -sha256 filename)

should work just as well.

Related Solutions

Batch Command with Output Redirection – Bash Scripting Guide

You forgot to quote $f, so the filename probably got wordsplit due to whitespace in the filename. I also suggest using ./*.dvi as this avoid problems if a filename starts with a - (which will make most commands treat it as a set of options instead of a filename)

for f in ./*.dvi; do catdvi -e 1 -U "$f" > "${f%.dvi}.txt"; done

Or perhaps combine the two loops

for f in ./*.tex; do 
    base=${f%.tex};
    latex "$f" &&
    catdvi -e 1 -U "$base.dvi" > "$base.txt";
done

Ubuntu – Toggle Bash redirection with a variable

IMHO it would be more elegant to use the presence / value of the variable to conditionally close the file descriptors e.g.

$ cat myscript.sh
#!/bin/bash

if [ -n "$MUTE" ]; then
    exec &>-
fi

echo somestuff
echo someerr >&2

then

$ ./myscript.sh
somestuff
someerr

but

$ MUTE=yes ./myscript.sh
$

If you really want to toggle the redirection, you could consider creating a shell function that duplicates the file descriptor(s) before closing them, and then restores the duplicates to re-enable the original streams e.g.

#!/bin/bash

function mute {
  case "$1" in
    "on") 
      exec 3>&1-
      exec 4>&2-
    ;;
    "off") 
      exec 1>&3-
      exec 2>&4-
    ;;
    *)
  esac
}


# Demonstration: 

echo "with mute on: "
mute on
ls somefile
ls nofile

mute off
echo "with mute off: "
ls somefile
ls nofile

Result:

$ ./mute.sh
with mute on: 
with mute off: 
somefile
ls: cannot access nofile: No such file or directory

Best Answer

Related Solutions

Batch Command with Output Redirection – Bash Scripting Guide

Ubuntu – Toggle Bash redirection with a variable

Related Question