Print command result while executing from script

Question

I am trying to run from a shell script a C++ program that print some outputs (using std::cout), and I would like to see them in the console while the program is running.

I tried some things like this :

RES=`./program`
RES=$(./program)

But all I can do is to only display the result at the end : echo $RES...

How to display the outputs in run-time in the console, AND in the variable RES ?

Try man tee, and man tty. Then you can split the output stream so it is reproduced to the actual terminal. — BoBTFish
– BoBTFish, Commented Oct 27, 2016 at 8:14

BoBTFish · Accepted Answer · 2016-10-27 08:42:09Z

2

TTY=$(tty);
SAVED_OUTPUT=$(echo "my dummy c++ program" | tee ${TTY});
echo ${SAVED_OUTPUT};

prints

my dummy c++ program
my dummy c++ program

First we save off the name of the current terminal (because tty doesn't work in a pipeline).

TTY=$(tty)

Then we "T" the output (a letter T looks like one stream in at the bottom, 2 out at the top, and comes from the same "plumbing" metaphor as "pipe"), which copies it to the filename given; in this case the "file" is really a special device representing our terminal.

echo "my dummy c++ program" | tee ${TTY}

edited Oct 27, 2016 at 8:42

answered Oct 27, 2016 at 8:34

BoBTFish

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1 Comment

Charles Duffy Over a year ago

FYI, using all-caps names for your own variables is bad form; per the relevant POSIX standard's guidelines on environment variable naming, all-caps names are used for variables with meaning to the operating system and the shell, whereas names with at least one lower-case character are reserved for application use. Since setting a shell variable overwrites any like-named environment variable, this applies even when not explicitly exporting content to the environment.

wang sky · Accepted Answer · 2016-10-27 12:52:13Z

0

RES=( $(./program) )
echo ${RES[@]}

you can try in this way

answered Oct 27, 2016 at 12:52

wang sky

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6 Comments

Charles Duffy Over a year ago

Content is still captured, not streamed live (as the OP is asking for). Also, if your program outputs *, your echo will have not that * but a list of files in the current directory -- surely not correct behavior. Similarly, if your program outputs newlines or tabs, they'll be emitted as spaces instead by this code.

wang sky Over a year ago

can you paste your cpp output

Charles Duffy Over a year ago

An answer that only works for the OP's output isn't helpful to anyone but the OP. Part of the point of StackOverflow is to have a knowledgebase that's useful to everyone; that's why building robust answers (that work even in unexpected circumstances, such as when a program has wildcards or newlines) is important. Anyhow, for a command that generates outputs that will generate the bugs described, consider using the following in place of ./program for test purposes: printf '%s\n' '*' $'hello\t\tworld'; sleep 5; echo "post-streaming content"

Charles Duffy Over a year ago

For a fully correct implementation, one would see the first content without the 5-second delay, and the other content would have its original formatting.

wang sky Over a year ago

you can use this method , i don't know whether this is your result or not

|

Vytenis Bivainis · Accepted Answer · 2016-10-27 20:53:17Z

0

You can use a temporary file

./program | tee temp
RES=$(< temp)
rm temp

You can generate a temporary file with unique name by using mktemp.

answered Oct 27, 2016 at 20:53

Vytenis Bivainis

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Comments

wang sky · Accepted Answer · 2016-10-28 02:23:07Z

0

 res=$(sed -n 'p;' <<< $(printf '%s\n' '*' $'hello\t\tworld'; sleep 5; echo "post-streaming content")&;wait)     
 echo $res
 #output
 *
 hello           world
 post-streaming content

answered Oct 28, 2016 at 2:23

wang sky

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6 Comments

Charles Duffy Over a year ago

That still doesn't print anything -- even the first two lines -- until after the 5-second delay is complete. And &; is invalid syntax in POSIX sh and most if not all released versions of bash (because & itself is a command separator) -- which one are you testing with?

wang sky Over a year ago

bash --version GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)

Charles Duffy Over a year ago

...but are you sure that's actually your running shell? I'm guessing that you're using zsh -- unlike bash, &; is valid there.

Charles Duffy Over a year ago

Try running echo "$BASH_VERSION"; if that doesn't print anything, then echo "$ZSH_VERSION".

Charles Duffy Over a year ago

...that also explains why the * is literal for you -- zsh doesn't follow POSIX rules on expansion behavior.

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wang sky · Accepted Answer · 2016-10-28 02:46:43Z

0

[sky@kvm35066 tmp]$ cat test.sh 
#!/bin/bash
echo $BASH_VERSION
res=$(printf '%s\n' '*' $'hello\t\tworld'; sleep 5; echo "post-streaming content")
echo "$res"

[sky@kvm35066 tmp]$ bash test.sh 
4.1.2(1)-release
*
hello           world
post-streaming content

i think the result is correct, and it is what you want

answered Oct 28, 2016 at 2:46

wang sky

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