Ubuntu – `at` command: what is the meaning of the option `-w`

at-commandcommand line

I have a script that uses at command. And sometimes I receiving emails as this one:

Date: Mon,  3 Jul 2017 14:18:00 +0300 (EEST)
From: root <root@localhost.org>
To: root@localhost.org
Subject: Output from your job      173

Another app is currently holding the xtables lock. Perhaps you want to use the -w option?

Of course I would use this option to prevent messages like this, but first I want to know what is the meaning of this option. Unfortunately -w option is not listed in man at.

Does anyone know what exactly this option means?

Best Answer

The message is from your job, not from at (and at also does not have a -w option BTW). By default, at sends STDOUT and STDERR from any job via mail, unless specified otherwise.

Presumably, your job was a iptables task -- based on the message regarding xtables. And the -w (--wait) option of iptables waits for the xtables lock to be released, as suggested in the message.

Related Solutions

Ubuntu – the meaning of “ps -aef | grep $(pwd)” command

From the man page for ps:

   -a              Select all processes except both session leaders (see getsid(2)) and
                   processes not associated with a terminal.

   -f              Do full-format listing. This option can be combined
                   with many other UNIX-style options to add additional
                   columns. It also causes the command arguments to be
                   printed. When used with -L, the NLWP (number of
                   threads) and LWP (thread ID) columns will be added. See
                   the c option, the format keyword args, and the format
                   keyword comm.
   -e              Select all processes. Identical to -A.

grep is used to print lines matching a pattern.

What it does

The command

ps -aef | grep `pwd`

prints out all the lines matching the output of the command pwd(which will be the path your current working directory), from the output of ps -aef.

e.g:

saji@geeklap:~$ pwd
/home/saji

saji@geeklap:~$ ps -aef | grep `pwd`
saji      2854  2814  0 09:51 ?        00:00:00 /usr/bin/ssh-agent /usr/bin/gpg-agent --daemon --sh --write-env-file=/home/saji/.gnupg/gpg-agent-info-geeklap /usr/bin/dbus-launch --exit-with-session gnome-session --session=ubuntu
saji      2855  2814  0 09:51 ?        00:00:00 /usr/bin/gpg-agent --daemon --sh --write-env-file=/home/saji/.gnupg/gpg-agent-info-geeklap /usr/bin/dbus-launch --exit-with-session gnome-session --session=ubuntu
saji      2879     1  0 09:51 ?        00:00:00 /usr/lib/gvfs//gvfs-fuse-daemon -f /home/saji/.gvfs
saji     14242 14148  0 15:26 pts/7    00:00:00 grep --color=auto /home/saji

As you can see the output shows the lines matching my current working directory, which is /home/saji.

Background info:
If a command is in $(...) or ..., then the command is run and the output (what is printed to the screen) is caught and substituted to where the original $() or `` string was. So the actual command run is grep pwd.

For more information refer this link.(Thanks to @minerz029 for this information).

Do check out the following link for a detailed technical answer from the man pages itself:

http://explainshell.com/explain?cmd=ps+-aef+|+grep+%60pwd%60

Ubuntu – the meaning of gcc command

Your question

What gcc -o sample sample.c actually does

The command you have pasted triggers gcc to compile sample.c. As a result you'll get an output/executable with the same sample.

The parameters (as -o in your example which defined the name of the output file ) are explained in the man page of gcc. You can open it by running man gcc

I am quoting the -o section of the man page here

-o file

       Place output in file file.  This applies to whatever sort of output
       is being produced, whether it be an executable file, an object
       file, an assembler file or preprocessed C code.

       If -o is not specified, the default is to put an executable file in
       a.out, the object file for source.suffix in source.o, its assembler
       file in source.s, a precompiled header file in source.suffix.gch,
       and all preprocessed C source on standard output.

In General

Wikipedia explains a good amount of gcc here.

I am quoting only the start - but the complete article is worth a read and offers a lot of additional links and references

The GNU Compiler Collection (GCC) is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain. The Free Software Foundation (FSF) distributes GCC under the GNU General Public License (GNU GPL). GCC has played an important role in the growth of free software, as both a tool and an example.

Originally named the GNU C Compiler, when it only handled the C programming language, GCC 1.0 was released in 1987. It was extended to compile C++ in December of that year. Front ends were later developed for Objective-C, Objective-C++, Fortran, Java, Ada, and Go among others.

GCC has been ported to a wide variety of processor architectures, and is widely deployed as a tool in the development of both free and proprietary software. GCC is also available for most embedded platforms,[citation needed] including Symbian (called gcce), AMCC, and Freescale Power Architecture-based chips. The compiler can target a wide variety of platforms, including video game consoles such as the PlayStation 2 and Dreamcast.

As well as being the official compiler of the GNU operating system, GCC has been adopted as the standard compiler by many other modern Unix-like computer operating systems, including Linux and the BSD family, although FreeBSD and OS X have moved to the LLVM system. Versions are also available for Microsoft Windows and other operating systems; GCC can compile code for Android and iOS.