CODEWITHSUNDEEP
bashstdinfile-descriptorio-redirection
Ondra Žižka
Ondra Žižka

Reputation: 46904

Bash redirecting stdin as a standard input

In my script, sometimes I need a script to be passed to SSH, and sometimes let the user interact.

I tried this trick:

if [ "" != "$SSH_SCRIPT" ] ; then
  ...
else
  SSH_SCRIPT="&0";
fi

And then:

ssh $SSH_IGNORE_STDIN $SSH_TTY $noKeyCheck -i $AWS_KEY $USER_@$HOST_ "$COMMAND_SSH" <$SSH_SCRIPT;

Which I assumed would work because cat <&0 works.

But when I use a variable after <, it works differently - it complains that file &0 was not found. Looks like <&n is a syntax construct. Reproducible this way:

$ FOO='&0';
$ cat <$FOO
bash: &0: No such file or directory

Is there a way to parametrize redirection by file descriptor number?

Enclosing the whole command to bash -c "..." isn't probably a good idea because it makes the whole thing more complicated.
Uploading the command to the remote server and running it there is also a bit overkill.

So far I have resorted to:

    if [ "" == "$SSH_SCRIPT" ] ; then
        ssh $SSH_IGNORE_STDIN $SSH_TTY $noKeyCheck -i $AWS_KEY $USER_@$HOST_ "$COMMAND_SSH";
    else
        ssh $SSH_IGNORE_STDIN $SSH_TTY $noKeyCheck -i $AWS_KEY $USER_@$HOST_ "$COMMAND_SSH" <$SSH_SCRIPT;
    fi

Upvotes: 0

Views: 127

Answers (1)

chepner
chepner

Reputation: 532408

bash will recognize /dev/stdin as the argument to a redirection whether or not your file system exposes such a file.

if [ -z "$SSH_SCRIPT" ]; then
    SSH_SCRIPT=/dev/stdin
fi

ssh ... < "$SSH_SCRIPT"

Upvotes: 2

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