Redirect STDERR / STDOUT of a process AFTER it's been started, using command line?
Short of closing and reopening your tty (i.e. logging off and back on, which may also terminate some of your background processes in the process) you only have one choice left:
- attach to the process in question using gdb, and run:
- p dup2(open("/dev/null", 0), 1)
- p dup2(open("/dev/null", 0), 2)
- detach
- quit
e.g.:
$ tail -f /var/log/lastlog &
[1] 5636
$ ls -l /proc/5636/fd
total 0
lrwx------ 1 myuser myuser 64 Feb 27 07:36 0 -> /dev/pts/0
lrwx------ 1 myuser myuser 64 Feb 27 07:36 1 -> /dev/pts/0
lrwx------ 1 myuser myuser 64 Feb 27 07:36 2 -> /dev/pts/0
lr-x------ 1 myuser myuser 64 Feb 27 07:36 3 -> /var/log/lastlog
$ gdb -p 5636
GNU gdb 6.8-debian
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Attaching to process 5636
Reading symbols from /usr/bin/tail...(no debugging symbols found)...done.
Reading symbols from /lib/librt.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/librt.so.1
Reading symbols from /lib/libc.so.6...(no debugging symbols found)...done.
Loaded symbols for /lib/libc.so.6
Reading symbols from /lib/libpthread.so.0...(no debugging symbols found)...done.
[Thread debugging using libthread_db enabled]
[New Thread 0x7f3c8f5a66e0 (LWP 5636)]
Loaded symbols for /lib/libpthread.so.0
Reading symbols from /lib/ld-linux-x86-64.so.2...(no debugging symbols found)...done.
Loaded symbols for /lib64/ld-linux-x86-64.so.2
(no debugging symbols found)
0x00007f3c8eec7b50 in nanosleep () from /lib/libc.so.6
(gdb) p dup2(open("/dev/null",0),1)
[Switching to Thread 0x7f3c8f5a66e0 (LWP 5636)]
$1 = 1
(gdb) p dup2(open("/dev/null",0),2)
$2 = 2
(gdb) detach
Detaching from program: /usr/bin/tail, process 5636
(gdb) quit
$ ls -l /proc/5636/fd
total 0
lrwx------ 1 myuser myuser 64 Feb 27 07:36 0 -> /dev/pts/0
lrwx------ 1 myuser myuser 64 Feb 27 07:36 1 -> /dev/null
lrwx------ 1 myuser myuser 64 Feb 27 07:36 2 -> /dev/null
lr-x------ 1 myuser myuser 64 Feb 27 07:36 3 -> /var/log/lastlog
lr-x------ 1 myuser myuser 64 Feb 27 07:36 4 -> /dev/null
lr-x------ 1 myuser myuser 64 Feb 27 07:36 5 -> /dev/null
You may also consider:
- using
screen
; screen provides several virtual TTYs you can switch between without having to open new SSH/telnet/etc, sessions - using
nohup
; this allows you to close and reopen your session without losing any background processes in the... process.
Capture STDIN / STDERR / STDOUT of a process AFTER it's been started, using command line?
Solved in Linux (apparently Linux-specific):
reptyr -s PID
attaches a process to another terminal and/or exposes its input and output as pipes.
How to redirect output of an already running process
See Redirecting Output from a Running Process.
Firstly I run the command
cat > foo1
in one session and test that data from stdin is copied to the file. Then in another session I redirect the output.Firstly find the PID of the process:
$ ps aux | grep cat
rjc 6760 0.0 0.0 1580 376 pts/5 S+ 15:31 0:00 cat
Now check the file handles it has open:
$ ls -l /proc/6760/fd
total 3
lrwx—— 1 rjc rjc 64 Feb 27 15:32 0 -> /dev/pts/5
l-wx—— 1 rjc rjc 64 Feb 27 15:32 1 -> /tmp/foo1
lrwx—— 1 rjc rjc 64 Feb 27 15:32 2 -> /dev/pts/5
Now run GDB:
$ gdb -p 6760 /bin/cat
GNU gdb 6.4.90-debian
[license stuff snipped]
Attaching to program: /bin/cat, process 6760
[snip other stuff that's not interesting now]
(gdb) p close(1)
$1 = 0
(gdb) p creat("/tmp/foo3", 0600)
$2 = 1
(gdb) q
The program is running. Quit anyway (and detach it)? (y or n) y
Detaching from program: /bin/cat, process 6760
The
p
command in GDB will print the value of an expression, an expression can be a function to call, it can be a system call… So I execute aclose()
system call and pass file handle 1, then I execute acreat()
system call to open a new file. The result of thecreat()
was 1 which means that it replaced the previous file handle. If I wanted to use the same file for stdout and stderr or if I wanted to replace a file handle with some other number then I would need to call thedup2()
system call to achieve that result.For this example I chose to use
creat()
instead ofopen()
because there are fewer parameter. The C macros for the flags are not usable from GDB (it doesn’t use C headers) so I would have to read header files to discover this – it’s not that hard to do so but would take more time. Note that 0600 is the octal permission for the owner having read/write access and the group and others having no access. It would also work to use 0 for that parameter and run chmod on the file later on.After that I verify the result:
ls -l /proc/6760/fd/
total 3
lrwx—— 1 rjc rjc 64 2008-02-27 15:32 0 -> /dev/pts/5
l-wx—— 1 rjc rjc 64 2008-02-27 15:32 1 -> /tmp/foo3 <====
lrwx—— 1 rjc rjc 64 2008-02-27 15:32 2 -> /dev/pts/5
Typing more data in to
cat
results in the file/tmp/foo3
being appended to.If you want to close the original session you need to close all file handles for it, open a new device that can be the controlling tty, and then call
setsid()
.
Capture program stdout and stderr to separate variables
The easiest way to do this is to use a file for the stderr output, e.g.:
$output = & myprogram.exe 'argv[0]', 'argv[1]' 2>stderr.txt
$err = get-content stderr.txt
if ($LastExitCode -ne 0) { ... handle error ... }
I would also use $LastExitCode to check for errors from native console EXE files.
How do I get both STDOUT and STDERR to go to the terminal and a log file?
Use "tee" to redirect to a file and the screen. Depending on the shell you use, you first have to redirect stderr to stdout using
./a.out 2>&1 | tee output
or
./a.out |& tee output
In csh, there is a built-in command called "script" that will capture everything that goes to the screen to a file. You start it by typing "script", then doing whatever it is you want to capture, then hit control-D to close the script file. I don't know of an equivalent for sh/bash/ksh.
Also, since you have indicated that these are your own sh scripts that you can modify, you can do the redirection internally by surrounding the whole script with braces or brackets, like
#!/bin/sh
{
... whatever you had in your script before
} 2>&1 | tee output.file
How to redirect Windows cmd stdout and stderr to a single file?
You want:
dir > a.txt 2>&1
The syntax 2>&1
will redirect 2
(stderr) to 1
(stdout). You can also hide messages by redirecting to NUL
. More explanation and examples are on the Microsoft documentation page Redirecting error messages from Command Prompt: STDERR/STDOUT.
ProcessBuilder: Forwarding stdout and stderr of started processes without blocking the main thread
For Java 7 and later, see Evgeniy Dorofeev's answer.
For Java 6 and earlier, create and use a StreamGobbler
:
StreamGobbler errorGobbler =
new StreamGobbler(p.getErrorStream(), "ERROR");
// any output?
StreamGobbler outputGobbler =
new StreamGobbler(p.getInputStream(), "OUTPUT");
// start gobblers
outputGobbler.start();
errorGobbler.start();
...
private class StreamGobbler extends Thread {
InputStream is;
String type;
private StreamGobbler(InputStream is, String type) {
this.is = is;
this.type = type;
}
@Override
public void run() {
try {
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line = null;
while ((line = br.readLine()) != null)
System.out.println(type + "> " + line);
}
catch (IOException ioe) {
ioe.printStackTrace();
}
}
}
Related Topics
Reusing Custom Makefile for Static Library with Cmake
Bash: Loop Until Command Exit Status Equals 0
Where to Put Svn Repository Directory in Linux
What Is The Linux Equivalent to Msvc++'s Option /D1Reportsingleclasslayout
How to Allow Jenkins to Access The Files That Only Root or Some Specific Programs Have Access To
What Is The Equivalent of /Proc/Cpuinfo on Freebsd V8.1
(13)Permission Denied: Access to /Cgi-Bin/Test.Cgi Denied
How to Check If Jboss Is Running on Unix Server
Enable Dynamic Debug for Multiple Files at Boot
How to Store Output from Printf with Formatting in a Variable
How to Find The Reason for a Dead Process Without Log File on Unix
How to Sleep 10 Seconds Before Running a Linux Command
How to Create a Multi Partition Sd Disk Image Without Root Privileges
How to Let Users Run a Script with Root Permissions
Powershell's Equivalent to Linux's: Ls -Al