CODEWITHSUNDEEP
bashshellprocess
hesson
hesson

Reputation: 1862

How to echo to running process in shell?

If I have a C++ program that expects input to cin from the console during run-time, how can I automate this input from a shell script? I also need to store its output in a file but that part is easy. I have researched different ways including echo-ing it to the file descriptor of the pid in /proc, but nothing seems to work.

Here is what I have so far:

#!/bin/sh
g++ -o runme source.cpp
echo <<EOT | ./runme > output
expected program input
more expected program input
even more
EOT

Note that each line of input requires an "enter key-press" to be read by cin in the program, which I am assuming should happen since the program input in the script is separated by new-lines. Here, the program gets executed but the same output is produced in the file regardless of what I put before the EOT so it's not being entered to cin of the program as expected.

Upvotes: 0

Views: 3101

Answers (2)

chepner
chepner

Reputation: 532538

Just FYI, echo does not read from its standard input (which a here document provides); you would want to use cat instead:

cat <<EOF | ./runme > output
...
...
...
EOF

But this is a useless use of cat, since you can just connect the here document to runme's standard input directly as shown by Thomas Nyman's answer.

Upvotes: 1

Thomas Nyman
Thomas Nyman

Reputation: 213

The form of I/O redirection you want is called a here document and does not involve echo at all:

./runme <<EOF
expected program input
more expected program input
even more
EOF

With output redirection:

./runme <<EOF > output
expected program input
more expected program input
even more
EOF

If you insist on using echo, in principle you could do:

echo -e "expected program input\nmoreexpected program input\neven more\n" | ./runme

where the -e option enables the interpretation of backslash escapes, such as the newlines indicated by \n in the argument of echo.

Upvotes: 5

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