Shell – Using dc with Standard Input or Heredoc

dchere-documentshell

dc can read command from a file or/and from standard input.
If I want to print user input :

cat essai_dc
[enter a number : ]
n
?
p

dc essai_dc 
  enter a number : 4
  4

Now, if I try with a heredoc :

dc <<EOF
> [enter a number : ]
> n
> ?
> p
> EOF  

enter a number : dc: stack empty

I get the same with standard input :

cat essai_dc | dc
enter a number : dc: stack empty

The command ? get the p and execute it but the stack is empty.
Is it possible to get it to work (tell dc to wait for the input)

Best Answer

? gets its input from standard input which is the here document here. You'd need to feed the script to dc using a different file descriptor. On systems with /dev/fd/n, that could be with:

dc /dev/fd/3 3<< 'EOF'
[enter a number : ]
n
?
p
EOF

Or you can use ksh-style process substitution (which generally uses /dev/fd/n underneath):

dc <(cat << 'EOF'
[enter a number : ]
n
?
p
EOF
)

Or doing away with the here-document and the call to the (generally) external cat utility:

dc <(printf %s \
'[enter a number : ]
n
?
p
'
)

Some dc implementations like GNU dc allow passing the contents of the dc script as argument with -e, so you could use command substitution:

dc -e "$(cat << 'EOF'
[enter a number : ]
n
?
p
EOF
)"

Or directly:

dc -e '[enter a number : ]
n
?
p'

Related Solutions

shell io-redirection cat here-document – How to Use Output Redirection with Here-Documents and Cat?

There are multiple ways to do this. The simplest is probably this:

cat <<EOF | sh
touch somefile
echo foo > somefile
EOF

Another, which is nicer syntax in my opinion:

(
cat <<EOF
touch somefile
echo foo > somefile
EOF
) | sh

This works as well, but without the subshell:

{
cat <<EOF
touch somefile
echo foo > somefile
EOF
} | sh

More variations:

cat <<EOF |
touch somefile
echo foo > somefile
EOF
  sh

Or:

{ cat | sh; } << EOF
touch somefile
echo foo > somefile
EOF

By the way, I expect the use of cat in your question is a placeholder for something else. If not, take it out, like this:

sh <<EOF
touch somefile
echo foo > somefile
EOF

Which could be simplified to this:

sh -c 'touch somefile; echo foo > somefile'

or:

sh -c 'touch somefile
echo foo > somefile'

Redirecting output instead of piping

sh >out <<EOF
touch somefile
echo foo > somefile
EOF

Using cat to get the equivalent of echo test > out:

cat >out <<EOF
test
EOF

Multiple Here Documents

( cat; echo ---; cat <&3 ) <<EOF 3<<EOF2
hi
EOF
there
EOF2

This produces the output:

hi
---
there

Here's what's going on:

The shell sees the ( ... ) and runs the enclosed commands in a subshell.
The cat and echo are simple enough. The cat <&3 says to run cat with file descriptor (fd) 0 (stdin) redirected from fd 3; in other words, cat out the input from fd 3.
Before the (...) is started, the shell sees the two here document redirects and substitutes fd 0 (<<EOF) and fd 3 (3<<EOF2) with the read-side of pipes
Once the initial command is started, the shell reads its stdin until EOF is reached and sends that to the write-side of the first pipe
Next, it does the same with EOF2 and the write-side of the second pipe

Shell Script Breaks with Heredoc in PHPmyadmin Installation – Troubleshooting Guide

You're doing:

bash << 'EOT'
some-command-that-reads-stdin
EOT

But some-command-that-reads-stdin's stdin will be that here document as well, as it is started by bash so inherits the same stdin.

You could do:

bash /dev/fd/3 3<< 'EOT'
some-command-that-reads-stdin
EOT

So stdin is left untouched, and bash gets the code from that here document on another fd.

Best Answer

Related Solutions

shell io-redirection cat here-document – How to Use Output Redirection with Here-Documents and Cat?

Shell Script Breaks with Heredoc in PHPmyadmin Installation – Troubleshooting Guide

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