Check If There Is an Stdout Redirection in Bash Script

Check if there is an stdout redirection in bash script

You can check if stdout is a terminal. When stdout is redirected or piped it will not be a terminal. You can use the test command with the -t option to get this information:

if [ -t 1 ] ; then
    # stdout is a terminal
else
    # stdout isn't a terminal
fi

From man test:

  -t FD  file descriptor FD is opened on a terminal

Test stdout and stderr redirection in bash script

You should be able to use the -t test switch to tell if the output streams are ttys or not:

if [ -t 1 ] ; then
  echo stdout is a terminal
else
  echo stdout is not a terminal
fi

Use -t 0 for stdin.

Use -t 2 for stderr.

Redirecting stdout and stderr to variable within bash script

If you want to redirect for the entirety of your code, instead of using blocks, use the exec command. That is:

stdout_file="/some/path/$1/timestamp.stdout"         # Creating job-id specific folders
stderr_file="/some/path/$1/timestamp.stderr"
exec >"$stdout_file" 2>"$stderr_file"

# ...all code below this point has stdout going to stdout_file, and stderr to stderr_file

Shell redirection: Command doesn't appear to output to stdout or stderr

@Gordon Davisson is right on the money. It appears the output was being sent directly to /dev/tty, which apparently is a thing people do... Thanks, Gordon!

Update: Upon further tinkering and input from others, it looks like the important part is using script to capture all the output. Additional redirection (ie. 2>&1) is not necessary, but can be done if need be. Continue below to see it in action.

Here's my basic code:

The binary app that sends data to /dev/tty is called 'binApp', and let's say it takes an arbitrary number of command line arguments. My wrapper/watcher script will be run with binApp and all its args, like so:

WrapScript binApp arg1 arg2 ... argn

WrapScript:

#!/bin/bash

args=("$@")
cmd=`echo "${args[@]:0}"`

run() {
    script -q -c "$cmd" 2>&1 | while read -r line; do
        if [ "$line" = "$badthing" ]; then
            echo "restarting..."
            break
        fi
        echo "$line"
    done
}

badthing="ERROR"

while true; do
    run
    sleep 0.5
done

The sleep 0.5 allows user to press "Ctrl+c" twice to exit.

Redirect stdout / stderr to bash script

You can use xargs to move data from a pipe into an argument:

java -jar build/libs/bot-kt-1.1.3.jar 2>&1 | xargs ./script.sh

to pass everything as a single argument:

java -jar build/libs/bot-kt-1.1.3.jar 2>&1 | xargs --null ./script.sh

Check if script has been redirected

You can use -t option of bash:

 -t fd  True if file descriptor fd is open and refers to a terminal.

And check like this:

if [[ -t 1 ]]; then
 # console is attached
else 
 # Redirected to somewhere
fi

Redirect stderr and stdout in Bash

Take a look here. It should be:

yourcommand &> filename

It redirects both standard output and standard error to file filename.

Redirect STDOUT by variable in Bash

bash will recognize a file name of /dev/stdout as standard output, when used with a redirection, whether or not your file system has such an entry.

# If an argument is given your script, use that value
# Otherwise, use /dev/stdout
out=${1:-/dev/stdout}

zip bar.zip foo > "$out"

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