Mark Needham

Thoughts on Software Development

Unix: Redirecting stderr to stdout

without comments

I’ve been trying to optimise some Neo4j import queries over the last couple of days and as part of the script I’ve been executed I wanted to redirect the output of a couple of commands into a file to parse afterwards.

I started with the following script which doesn’t do any explicit redirection of the output:

#!/bin/sh
 
./neo4j-community-2.2.3/bin/neo4j start

Now let’s run that script and redirect the output to a file:

$ ./foo.sh > /tmp/output.txt
Unable to find any JVMs matching version "1.7".
 
$ cat /tmp/output.txt
Starting Neo4j Server...WARNING: not changing user
process [48230]... waiting for server to be ready.... OK.
http://localhost:7474/ is ready.

So the line about not finding a matching JVM is being printed to stderr. That’s reasonably easy to fix:

#!/bin/sh
 
./neo4j-community-2.2.3/bin/neo4j start 2>&1

Let’s run the script again:

$ ./foo.sh > /tmp/output.txt
 
$ cat /tmp/output.txt
Unable to find any JVMs matching version "1.7".
Starting Neo4j Server...WARNING: not changing user
process [47989]... waiting for server to be ready.... OK.
http://localhost:7474/ is ready.

Great, that worked as expected. Next I extended the script to stop Neo4j, delete all it’s data, start it again and execute a cypher script:

#!/bin/sh
 
./neo4j-community-2.2.3/bin/neo4j start 2>&1
rm -rf neo4j-community-2.2.3/data/graph.db/
./neo4j-community-2.2.3/bin/neo4j start 2>&1
time ./neo4j-community-2.2.3/bin/neo4j-shell --file foo.cql 2>&1

Let’s run that script and redirect the output:

$ ./foo.sh > /tmp/output.txt
Unable to find any JVMs matching version "1.7".
 
real	0m0.604s
user	0m0.334s
sys	0m0.054s
 
$ cat /tmp/output.txt
Unable to find any JVMs matching version "1.7".
Another server-process is running with [50614], cannot start a new one. Exiting.
Unable to find any JVMs matching version "1.7".
Another server-process is running with [50614], cannot start a new one. Exiting.
+---------+
| "hello" |
+---------+
| "hello" |
+---------+
1 row
4 ms

It looks like our stderr -> stdout redirection on the last line didn’t work. My understanding is that the ‘time’ command swallows all the arguments that follow whereas we want the redirection to be run afterwards.

We can work our way around this problem by putting the actual command in a code block and redirected the output of that:

#!/bin/sh
 
./neo4j-community-2.2.3/bin/neo4j start 2>&1
rm -rf neo4j-community-2.2.3/data/graph.db/
./neo4j-community-2.2.3/bin/neo4j start 2>&1
{ time ./neo4j-community-2.2.3/bin/neo4j-shell --file foo.cql; } 2>&1
$ ./foo.sh  > /tmp/output.txt
 
$ cat /tmp/output.txt
Unable to find any JVMs matching version "1.7".
Another server-process is running with [50614], cannot start a new one. Exiting.
Unable to find any JVMs matching version "1.7".
Another server-process is running with [50614], cannot start a new one. Exiting.
Unable to find any JVMs matching version "1.7".
+---------+
| "hello" |
+---------+
| "hello" |
+---------+
1 row
4 ms
 
real	0m0.615s
user	0m0.316s
sys	0m0.050s

Much better!

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Written by Mark Needham

August 15th, 2015 at 3:55 pm

Posted in Shell Scripting

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