I want to copy file from A to B use "rsync", But the file is not exist right now in A. I run the "rsync" command on B, Can i let "rsync" to wait until the file is creating in A, and copy it in the same time?
Let's assume we start with a command running on hostB like
hostB$ rsync hostA:remotepath localpath
If hostA has a normal shell, we can make rsync wait until the file exists by tweaking the helper command it normally runs on hostA. Depending on the environment, something like this might work:
hostB$ rsync --rsync-path='
while [ ! -f remotefile ]; do sleep 1; done;
sleep 5;
rsync' hostA:remotepath localpath
the while loop busywaits for remotefile to become available
then sleep 5 allows a few seconds for the contents to settle
rsync is the normal remote command
must come last
must not end with newline or semicolon or comment
Problem
Have a multi_exec.pl that shall handle timed-out execution of command provided.
And we call this multi_exec.pl at various places in our legacy applciation.
Sample call :
$grab = `multi_exec.pl -1 'bcp_cmd-1' 'bcp_cmd-2' ... 'bcp_cmd-n'`
want to understand how to achieve the below using STDOUT[ERR] re-directions
capture bcp STDOUT[ERR] of individual BCP commands on the terminal
while need to capture failure messages on STDERR from multi_exec.pl
STDOUT of multi_exec.pl needs to go to /dev/null ( don't want to capture STDOUT
)
while need to capture failure messages on STDERR from multi_exec.pl
Nothing special needs to be done for this - STDERR of the parent script as well as the individual commands will go to the terminal by default
STDOUT of multi_exec.pl needs to go to /dev/null (don't want to capture STDOUT)
capture bcp STDOUT[ERR] of individual BCP commands on the terminal
These are conflicting requirements because STDOUT of the parent script as well as the individual bcp commands will end up on the terminal by default. There is no way to bifurcate just one of them to be sent to /dev/null. You could modify multi_exec.pl so that it writes its own output to a file, if specified. If no file is specified, it shouldn't write anything to stdout at all. So, it is ensured the STDOUT of multi_exec.pl is always from bcp commands.
I am developing an application that can establish a server-client connection using QTcp*
The client sends the server a number.
The received string is checked on its length and quality (is it really a number?)
If everything is OK, then the server replies back with a file path (which depends on the sent number).
The client checks if the file exists and if it is a valid image. If the file complies with the rules, it executes a command on the file.
What security concerns exist on this type of connection?
The program is designed for Linux systems and the external command on the image file is executed using QProcess. If the string sent contained something like (do not run the following command):
; rm -rf /
then it would be blocked on the file not found security check (because it isn't a file path). If there wasn't any check about the validity of the sent string then the following command would be executed:
command_to_run_on_image ; rm -rf /
which would cause panic! But this cannot happen.
So, is there anything I should take into consideration?
If you open a console and type command ; rm -rf /*, something bad would likely happen. It's because commands are processed by the shell. It parses text output, e.g. splits commands by ; delimiter and splits arguments by space, then it executes parsed commands with parsed arguments using system API.
However, when you use process->start("command", QStringList() << "; rm -rf /*");, there is no such danger. QProcess will not execute shell. It will execute command directly using system API. The result will be similar to running command "; rm -rf /*" in the shell.
So, you can be sure that only your command will be executed and the parameter will be passed to it as it is. The only danger is the possibility for an attacker to call the command with any file path he could construct. Consequences depends on what the command does.
I have a problem with the nohup command.
When I run my job, I have a lot of data. The output nohup.out becomes too large and my process slows down. How can I run this command without getting nohup.out?
The nohup command only writes to nohup.out if the output would otherwise go to the terminal. If you have redirected the output of the command somewhere else - including /dev/null - that's where it goes instead.
nohup command >/dev/null 2>&1 # doesn't create nohup.out
Note that the >/dev/null 2>&1 sequence can be abbreviated to just >&/dev/null in most (but not all) shells.
If you're using nohup, that probably means you want to run the command in the background by putting another & on the end of the whole thing:
nohup command >/dev/null 2>&1 & # runs in background, still doesn't create nohup.out
On Linux, running a job with nohup automatically closes its input as well. On other systems, notably BSD and macOS, that is not the case, so when running in the background, you might want to close input manually. While closing input has no effect on the creation or not of nohup.out, it avoids another problem: if a background process tries to read anything from standard input, it will pause, waiting for you to bring it back to the foreground and type something. So the extra-safe version looks like this:
nohup command </dev/null >/dev/null 2>&1 & # completely detached from terminal
Note, however, that this does not prevent the command from accessing the terminal directly, nor does it remove it from your shell's process group. If you want to do the latter, and you are running bash, ksh, or zsh, you can do so by running disown with no argument as the next command. That will mean the background process is no longer associated with a shell "job" and will not have any signals forwarded to it from the shell. (A disowned process gets no signals forwarded to it automatically by its parent shell - but without nohup, it will still receive a HUP signal sent via other means, such as a manual kill command. A nohup'ed process ignores any and all HUP signals, no matter how they are sent.)
Explanation:
In Unixy systems, every source of input or target of output has a number associated with it called a "file descriptor", or "fd" for short. Every running program ("process") has its own set of these, and when a new process starts up it has three of them already open: "standard input", which is fd 0, is open for the process to read from, while "standard output" (fd 1) and "standard error" (fd 2) are open for it to write to. If you just run a command in a terminal window, then by default, anything you type goes to its standard input, while both its standard output and standard error get sent to that window.
But you can ask the shell to change where any or all of those file descriptors point before launching the command; that's what the redirection (<, <<, >, >>) and pipe (|) operators do.
The pipe is the simplest of these... command1 | command2 arranges for the standard output of command1 to feed directly into the standard input of command2. This is a very handy arrangement that has led to a particular design pattern in UNIX tools (and explains the existence of standard error, which allows a program to send messages to the user even though its output is going into the next program in the pipeline). But you can only pipe standard output to standard input; you can't send any other file descriptors to a pipe without some juggling.
The redirection operators are friendlier in that they let you specify which file descriptor to redirect. So 0<infile reads standard input from the file named infile, while 2>>logfile appends standard error to the end of the file named logfile. If you don't specify a number, then input redirection defaults to fd 0 (< is the same as 0<), while output redirection defaults to fd 1 (> is the same as 1>).
Also, you can combine file descriptors together: 2>&1 means "send standard error wherever standard output is going". That means that you get a single stream of output that includes both standard out and standard error intermixed with no way to separate them anymore, but it also means that you can include standard error in a pipe.
So the sequence >/dev/null 2>&1 means "send standard output to /dev/null" (which is a special device that just throws away whatever you write to it) "and then send standard error to wherever standard output is going" (which we just made sure was /dev/null). Basically, "throw away whatever this command writes to either file descriptor".
When nohup detects that neither its standard error nor output is attached to a terminal, it doesn't bother to create nohup.out, but assumes that the output is already redirected where the user wants it to go.
The /dev/null device works for input, too; if you run a command with </dev/null, then any attempt by that command to read from standard input will instantly encounter end-of-file. Note that the merge syntax won't have the same effect here; it only works to point a file descriptor to another one that's open in the same direction (input or output). The shell will let you do >/dev/null <&1, but that winds up creating a process with an input file descriptor open on an output stream, so instead of just hitting end-of-file, any read attempt will trigger a fatal "invalid file descriptor" error.
nohup some_command > /dev/null 2>&1&
That's all you need to do!
Have you tried redirecting all three I/O streams:
nohup ./yourprogram > foo.out 2> foo.err < /dev/null &
You might want to use the detach program. You use it like nohup but it doesn't produce an output log unless you tell it to. Here is the man page:
NAME
detach - run a command after detaching from the terminal
SYNOPSIS
detach [options] [--] command [args]
Forks a new process, detaches is from the terminal, and executes com‐
mand with the specified arguments.
OPTIONS
detach recognizes a couple of options, which are discussed below. The
special option -- is used to signal that the rest of the arguments are
the command and args to be passed to it.
-e file
Connect file to the standard error of the command.
-f Run in the foreground (do not fork).
-i file
Connect file to the standard input of the command.
-o file
Connect file to the standard output of the command.
-p file
Write the pid of the detached process to file.
EXAMPLE
detach xterm
Start an xterm that will not be closed when the current shell exits.
AUTHOR
detach was written by Robbert Haarman. See http://inglorion.net/ for
contact information.
Note I have no affiliation with the author of the program. I'm only a satisfied user of the program.
Following command will let you run something in the background without getting nohup.out:
nohup command |tee &
In this way, you will be able to get console output while running script on the remote server:
sudo bash -c "nohup /opt/viptel/viptel_bin/log.sh $* &> /dev/null" &
Redirecting the output of sudo causes sudo to reask for the password, thus an awkward mechanism is needed to do this variant.
If you have a BASH shell on your mac/linux in-front of you, you try out the below steps to understand the redirection practically :
Create a 2 line script called zz.sh
#!/bin/bash
echo "Hello. This is a proper command"
junk_errorcommand
The echo command's output goes into STDOUT filestream (file descriptor 1).
The error command's output goes into STDERR filestream (file descriptor 2)
Currently, simply executing the script sends both STDOUT and STDERR to the screen.
./zz.sh
Now start with the standard redirection :
zz.sh > zfile.txt
In the above, "echo" (STDOUT) goes into the zfile.txt. Whereas "error" (STDERR) is displayed on the screen.
The above is the same as :
zz.sh 1> zfile.txt
Now you can try the opposite, and redirect "error" STDERR into the file. The STDOUT from "echo" command goes to the screen.
zz.sh 2> zfile.txt
Combining the above two, you get:
zz.sh 1> zfile.txt 2>&1
Explanation:
FIRST, send STDOUT 1 to zfile.txt
THEN, send STDERR 2 to STDOUT 1 itself (by using &1 pointer).
Therefore, both 1 and 2 goes into the same file (zfile.txt)
Eventually, you can pack the whole thing inside nohup command & to run it in the background:
nohup zz.sh 1> zfile.txt 2>&1&
You can run the below command.
nohup <your command> & > <outputfile> 2>&1 &
e.g.
I have a nohup command inside script
./Runjob.sh > sparkConcuurent.out 2>&1
I have a program that spits out both standard error and standard out, and I want to run my pager less on the standard error, but ignore standard out. How do I do that?
Update:
That's it ... I didn't want to lose stdout ... just keep it out of pager
program 2>&1 >log | less
then later
less log
You could try redirecting standard out to /dev/null, but redirecting standard error to where standard out used to go.
Example in ksh/bash:
program 2>&1 >/dev/null | less
Here the redirection 2>&1, which sets file descriptor 2 (stderr) to point to the same stream as file descriptor 1 (stdout), gets evaluated before the redirection >/dev/null , which sets file descriptor 1 to point to /dev/null. The effect is that what you write to stderr gets sent to stdout, and what you write to stdout gets thrown away.