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*Nix related tidbits…

This is a page on various tools and techniques related to *nix administration, programming, disaster recovery - basically, odds and ends that come in handy once every so often. Anything based on a link is reiterated just for completeness, and just in case a link dies.

While probably generally applicable to other distros, the contents on this page mostly reference lessons learned whilst using Ubuntu¹ (and stated clearly when that's not the case).

Shell-based Command-line tools. So far, this section includes:

extundelete - for file system recovery
cdrecord - for burning ISO's
process I/O redirection (gdb)
fixing garbled text (gcc)

Network Various networking-related things.

NAT box methods
- with ufw
- with pf

Printing quick CUPS setup
one-liners miscellaneous single-sentence tips.

Shell-based tools and techniques

File recovery with Extundelete

Extundelete lets you recover files (directories) that you've accidentally deleted on an ext file system. The best way to use the tool is to have a live CD with the tool on it, although it does not seem to come packaged with Linux by default.

Boot the machine off of live CD.
Install extundelete. On Ubuntu, you need to edit sources.list, since it is not part of the standard repositories.

Restore the files on the partition:

# sudo extundelete --restore-all /dev/sda1

This restores everything that has been deleted in /dev/sda1, given the tool finds them. All recovered files are placed in a recovery directory that you can package and back up before rebooting the machine.

Command-line CD burner

source : http://www.yolinux.com/TUTORIALS/LinuxTutorialCDBurn.html

Many CD burners are fairly dependency-heavy, mostly thanks to their GUI. cdrecord is a command-line based CD/DVD burner with just two dependencies at ~2.3kB:

$ sudo apt-get install cdrecord
...
The following NEW packages will be installed:
  genisoimage wodim
...
After this operation, 2,306 kB of additional disk space will be used.

Now, to use:

Check for supported devices. You'll need to specify what to use when burning the image. cdrecord seems to just be a wrapper plus then some for woodim, and the two can be used interchangeably for this step.

$ wodim --devices
wodim: Overview of accessible drives (2 found) :
-------------------------------------------------------------------------
 0  dev='/dev/scd0'     rwrw-- : 'SONY' 'DVD-ROM DDU1615'
 1  dev='/dev/scd1'     rwrw-- : 'TSSTcorp' 'CDDVDW SE-208AB'
-------------------------------------------------------------------------

Burn. The meaning of the options can be listed with 'cdrecord —help'.

$ sudo cdrecord -v -eject speed=16 dev='/dev/scd1' Downloads/ubuntu-12.04-server-amd64.iso

This will spew a bunch of output, but complete with something like below:

Starting new track at sector: 0
Track 01:  684 of  684 MB written (fifo 100%) [buf  99%]  16.8x.
Track 01: Total bytes read/written: 717533184/717533184 (350358 sectors).
Writing  time:  327.327s
Average write speed  14.6x.
Min drive buffer fill was 99%
Fixating...
Fixating time:   24.715s
BURN-Free was never needed.
wodim: fifo had 11302 puts and 11302 gets.
wodim: fifo was 0 times empty and 11100 times full, min fill was 97%.

And eject the CD.

Redirecting the output of a live process.

source: http://etbe.coker.com.au/2008/02/27/redirecting-output-from-a-running-process/

Say that you have a running process whose outputs are either being piped to /dev/null or a terminal that you can't get a hold of, but you'd like to be able to see its output. Conversely, you may want to redirect something's output to /dev/null. You can force the process to change its output using gdb, the GNU debugger. This is a powerful tool that allows you to attach to a live process and inspect and manipulate the contents of its address space.

Here I have a script that I had backgrounded but is still outputting to my terminal (annoying). I will redirect the program's output to a file.

Find the PID of your process. Mine is called arrival_collector.rb, so I look for it using ps.

# ps -ef | grep collector
root     17793 11276  0 18:39 pts/1    00:00:00 ruby arrival_collector.rb -r a,h,ee,f,wknd1,wknd2 -s scott,busch_a,pubsafs,foodsci,scott,scott -v

/proc/ shows that its STDOUT is my terminal, /dev/pts/1:

# ls -l /proc/17793/fd
total 0
lrwx------ 1 root root 64 2012-02-19 18:45 0 -> /dev/pts/1
lrwx------ 1 root root 64 2012-02-19 18:45 1 -> /dev/pts/1
lrwx------ 1 root root 64 2012-02-19 18:40 2 -> /dev/pts/1
lrwx------ 1 root root 64 2012-02-19 18:45 3 -> socket:[1566783742]

Attach to the process with GDB.
```
# gdb -p [PID] [path/to/executable]
```

It wil spew a bunch of output but eventually give you a prompt:

# gdb -p 17793 /opt/grailrtls/grail3_ruby/grail3protocols/arrival_collector.rb
GNU gdb (GDB) 7.2-ubuntu
Copyright (C) 2010 Free Software Foundation, Inc.
...
Loaded symbols for /lib/libnss_files.so.2
0xb78c7424 in __kernel_vsyscall ()
(gdb)

Close STDOUT:
```
(gdb) p close(1)
$1 = 0
```

Point STDOUT to a file. Like in the link, I will use creat() to point STDOUT to a log file in /tmp/. creat() takes the path to the file and its permissions as the two arguments.
```
(gdb) p creat("/tmp/output.log", 0600)
$2 = 1
```

Exit gdb. Choose yes when asked if you want to quit.

(gdb) quit
A debugging session is active.

        Inferior 1 [process 17793] will be detached.

Quit anyway? (y or n) y
Detaching from program: /usr/bin/ruby1.8, process 17793

Now when you check /proc/ you should see that your STDOUT is directed att the file you created:

# ls -l /proc/17793/fd
total 0
lrwx------ 1 root root 64 2012-02-19 18:45 0 -> /dev/pts/1
lrwx------ 1 root root 64 2012-02-19 18:45 1 -> /tmp/output.log
lrwx------ 1 root root 64 2012-02-19 18:40 2 -> /dev/pts/1
lrwx------ 1 root root 64 2012-02-19 18:45 3 -> socket:[1566783742]

you can confirm this with tail -f (or the fact that your program has stopped outputting to terminal).

Fixing garbled gcc and man page output.

On some machines, gcc and man pages might produce garbled text. This is usually caused by xterm not supporting UTF-8, or from mismatch in locale information if the garbling is happening when you are working on a remote machine. In either case (for people working with US English), setting LANG to en_US or C fixes things:

$ export LANG=en_US

Set it the same for both local and remote machines if it's happening over SSH.

Networking-related odds and ends

Various non-experimental network setups, usually done for convenience.

NAT boxes.

with `ufw`

source: https://nowhere.dk/articles/tip_nat_with_ubuntus_ufw_firewall ufw is your standard Linux firewall, and comes with Ubuntu server edition. Turning a multi-interface Linux box into a router is a matter of the following steps:

configure IP forwarding

edit /etc/default/ufw :

DEFAULT_FORWARD_POLICY="ACCEPT"

and /etc/ufw/sysctl.conf :

net.ipv4.ip_forward=1

set up IP masquerading in ufw

edit /etc/ufw/before.rules, just after the header :

# nat Table rules
 *nat
 :POSTROUTING ACCEPT [0:0]

 # Forward traffic through ppp0 - Change to match you out-interface
 -A POSTROUTING -s 192.168.1.0/24 -o ppp0 -j MASQUERADE

 # don't delete the 'COMMIT' line or these nat table rules won't
 # be processed
 COMMIT

The address block after -s should match the address block behind the NAT firewall.

restart ufw:
```
sudo ufw disable && sudo ufw enable
```

with `pf`

pf is the OpenBSD packet filter, a piece of software intended for heavy-duty packet filtering/firewalls and comes with some Berkeley UNIX variants². Assuming you have IP forwarding enabled, the following configuration in /etc/pf.conf should give you a NAT firewall:

ext_if="bge0"
int_if="em0"
external_addr="192.168.203.155"
internal_net="192.168.1.0/24"
nat on $ext_if from $internal_net to any -> ($ext_if)
pass in all 
pass out all

ext_if is the interface facing the external network, and int_if is the interface connected to your NATed net. Once saved, start pf:

sudo pfctl -e -f /etc/pf.conf

If it throws errors, make sure that the kernel module (pf.ko or something similar) is loaded.

Printing setup with CUPS.

ref: http://linuxcourse.rutgers.edu/howto/cups_printing.html

CUPS stands for Common UNIX Printing System, and is fairly standard as a means to print from UNIX and UNIX-like things. We'll describe the steps needed for quick CUPS printing setup (bare-bones) in Ubuntu 11.04

install packages.

cups cupsys-driver-gutenprint libcupsys2 libcupsimage2

The default port that CUPS serves its admin GUI on is tcp:631. You can see listening if you do netstat -na -4 | grep 631:
```
$ netstat -na -4 | grep 631
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN     
```

If you see it, point your browser to localhost:631. You should get a shiny-white welcome page. Yes, it was made by Apple.

Add your printer from the Administration tab → Add Printer. It will ask for the root user and password (OK if you are on sudoers). You will be asked to provide info on the printer, and choose the drivers.

If all goes well, you will be able to see the printer's status with lpstat.

$ lpstat -p -d
printer Phaser_5500DT is idle.  enabled since Sat 28 Apr 2012 01:14:59 AM EDT
no system default destination

If you want to print, you can do so by invoking lp:

$ lp -o sides=two-sided-long-edge -d Phaser_5500DT [filename]

one-liners.

in bash, if seq doesn't exist, try gseq.
options for mounting a *BSD filesystem in Linux: -t ufs -o ufstype=ufs2 (not '44bsd' like the man pages say)
a dependency fix not requiring '-f install' with apt-get: 'build-dep [package]', then install as usual.

^{1. Me being the person that I am will have likely tried these on FreeBSD and maybe lubuntu to see that they work there as well.} ^{2. See 1.}

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