q14-output-redirection

Énoncé§

Solve this question on: app-srv1

On server app-srv1 there is a program /bin/output-generator which, who would’ve guessed, generates some output. It’ll always generate the very same output for every run:

1. Run it and redirect all stdout into /var/output-generator/1.out
2. Run it and redirect all stderr into /var/output-generator/2.out
3. Run it and redirect all stdout and stderr into /var/output-generator/3.out
4. Run it and write the exit code number into /var/output-generator/4.out

Solution§

We go ahead and check the program

ssh app-srv1
root@app-srv1:~$ output-generator
...
2021/07/20 08:58:20 32668 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 08:58:20 32669 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 08:58:20 32670 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 08:58:20 32671 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 08:58:20 32672 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 08:58:20 32673 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 08:58:20 32674 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 08:58:20 32675 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 08:58:20 32676 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 08:58:20 32677 - a99947a1-2f68-475e-9326-32dbd4876f60

Investigation

What a mess! Let’s try to count the rows:

root@app-srv1:~$ output-generator | wc -l
...
2021/07/20 09:00:44 22670 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 09:00:44 22671 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 09:00:44 22672 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 09:00:44 22673 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 09:00:44 22674 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 09:00:44 22675 - a99947a1-2f68-475e-9326-32dbd4876f60
2021/07/20 09:00:44 22676 - a99947a1-2f68-475e-9326-32dbd4876f60
20342

It looks like wc -l does only count rows that are written to stdout.

We can investigate a bit further using stdout (1) and stderr (2) redirection

output-generator > /dev/null # no stdout rows
output-generator 1> /dev/null # no stdout rows (same command as above)
output-generator 1> /dev/null 2> /dev/null # no stdout or stderr
output-generator 2>&1 # stderr will be redirected into stdout

Using this we can count all lines:

root@app-srv1:~$ output-generator 2>&1 | wc -l
32678

Solution

We redirect as required:

root@app-srv1:~$ output-generator > /var/output-generator/1.out
root@app-srv1:~$ output-generator 2> /var/output-generator/2.out
root@app-srv1:~$ output-generator >> /var/output-generator/3.out 2>> /var/output-generator/3.out
root@app-srv1:~$ output-generator
root@app-srv1:~$ echo $? > /var/output-generator/4.out # get the exit code and redirect to file

Note that with $? we can access the exit code of the last command run. Which means we need to let output-generator finish properly and not interrupt it. We also can’t run another command afterwards or else $? won’t contain the output-generator exit code.

This should give us:

root@app-srv1:~$ cat /var/output-generator/1.out | wc -l
20342
root@app-srv1:~$ cat /var/output-generator/2.out | wc -l
12336
root@app-srv1:~$ cat /var/output-generator/3.out | wc -l
32678
root@app-srv1:~$ cat /var/output-generator/4.out
123