read command in bash does not wait in operations with different user

Question

I'm calling bash A via different user within bash B like this, and i want to stuck on line "*" untill user enters "exit".

# bash B
su test<<endOfTestOperation
./bashA.sh
endOfTestOperation

# bash A
# Start a process &
read LINE            <------ I want to wait here until user inputs "exit"
if [ "$LINE" == "exit" ]; then
    # end the process 
    exit
fi

If i call bash A only, it works fine. if i put "read" after "endOfTestOperation", it also works. So i guess the problem roots in using different permission or forked process.

[supplemental description] What i'm trying to do is: start a tcpdump (root permission required, so bash b is ran with sudo), then start a tcpserver progress, then end the tcpserver on demanding of user input in stdin.

chepner · Accepted Answer · 2014-03-04 14:18:59Z

1

You are redirecting standard input for su from a here document. The command that su runs will inherit its standard input from su, so it is also the here document, not the terminal. Since su consumes the entire here document in reading the command to run, there is nothing left for bashA.sh to read, and so the read command fails immediately rather than waiting for input.

You'll need to post more detail about what you are actually trying to do if you want a workaround.

answered Mar 4, 2014 at 14:18

chepner

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1 Comment

Martian Puss Over a year ago

If i run bashA.sh without su test<<, it waits at "read" as expected. i'm considering this alternative. Thanks for your explanation.

John B · Accepted Answer · 2014-03-04 11:44:03Z

0

Using an if statement will not continue to check if the input is "exit", it will only check once.

You could use an until loop to make sure user input is "exit".

You can find the process of the most recent command sent to the background by using $!.

background command &
pid=$!
until [[ $LINE == exit ]]; do
    read LINE
done && { kill -9 $pid; exit; }

edited Mar 4, 2014 at 11:44

answered Mar 4, 2014 at 10:52

John B

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