CODEWITHSUNDEEP
linuxperl
adrian4aes
adrian4aes

Reputation: 869

How to see stderr output in linux

In a script in perl I have the following:

print STDERR "Access error"

I would like know where this message is printed, but I don't know how can I see the Standar error output in LInux.

Upvotes: 12

Views: 104328

Answers (4)

ikegami
ikegami

Reputation: 386621

Normally, STDOUT and STDERR are both output to your terminal.

$ perl -e'print "foo\n"; print STDERR "bar\n";'
foo
bar

But it's possible to redirect either and both.

$ perl -e'print "foo\n"; print STDERR "bar\n";' 2>stderr.log
foo

$ cat stderr.log
bar

For example, the data sent to STDERR by a CGI script usually ends up in log file specified in the web server's configuration.

It's possible for a program to get information about STDERR on a linux system.

$ perl -e'system "ls -l /proc/$$/fd"' 2>stderr.log |cat
total 0
lrwx------ ... 0 -> /dev/pts/1
l-wx------ ... 1 -> pipe:[210178663]            <-- STDOUT is a pipe
l-wx------ ... 2 -> /home/ikegami/stderr.log    <-- STDERR is a file
lr-x------ ... 3 -> pipe:[210178668]

Upvotes: 4

Gilles Qu&#233;not
Gilles Qu&#233;not

Reputation: 185760

Both the standard (STDOUT) and the error output (STDERR) are displayed on your (pseudo) terminal.

If you want to trace the outputs :

error log :

./script.pl 2> err.log

standard output log :

./script.pl > out.log

both STDERR and STDOUT in the same file :

./script.pl > out.log 2>&1

or with :

./script.pl &> out.log

A good tutorial

Upvotes: 23

user1019830
user1019830

Reputation:

You need to run the Perl script in a terminal. Depending on whether you have X on your system or not, you could use xterm or you could use a virtual console (tty1-7) to run your script. Both stderr and stdout are connected to these devices.

Upvotes: 2

Quentin
Quentin

Reputation: 944442

It is printed to wherever standard error is set to for your environment.

If you are running it from a console, then it will be mixed in with the standard output and displayed on the console (it won't be redirected if you redirect STDOUT, you have to redirect it separately).

If you are running it from CGI under Apache, then it will be dropped into your error.log file (wherever Apache is configured to save that).

If you are running it from somewhere else… well an exhaustive list is out of scope for Stackoverflow so you should try asking a more specific question ;)

Example of where it might be directed to at the console:

david@raston err $ cat err.pl
#!/usr/bin/env perl

use strict;
use warnings;
use v5.16;

say "out";
say STDERR "error";
~/tmp/err :
david@raston err $ ./err.pl
out
error
~/tmp/err :
david@raston err $ ./err.pl > stdout
error
~/tmp/err :
david@raston err $ ./err.pl 2> stderr
out

Upvotes: 11

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