====== GNU/Linux ======

<note>This page concerns the //Debian// distribution (//Jessie//, then //Stretch//), but may also be applied to other //GNU/Linux// (or //POSIX//) distributions.</note>

<note tip>The extended //Debian// security advisories in //RDF// format and in English : https://www.debian.org/security/dsa-long.en</note>

===== Connection through 'ssh' without password =====

To enable the connection to a host without password, launch from the machine from where you want to connect : ''ssh-copy-id [<login>@]<host>''.


===== Numlock enabled by default  =====

When installing //Debian// //Jessie// in a virtual machine, the numlock is disabled by default. To enable it by default, you should do the following :
  * Install the //numlockx// package,
  * add ''/usr/bin/numlockx on &'' to the end of file ''/etc/X11/xinit/xinitrc''.

===== Compiling for IA-32 under an AMD64 environment  =====

When compiling for //IA-32// under a //AMD64// enviroment (by passing the ''-m32'' option to both compiler and linker), the compiler will complain about missing headers. To fix this, install ''g++-multilib'' package.

===== Fetching informations =====

==== OS ====

  * ''uname -a'',
  * ''lsb_release -a'' (package //lsb-release//, with ''-'', not ''_''),
  * ''cat ''...
    * ''/proc/version'',
    * ''/etc/issue'',
    * ''/etc/issue.net'',
    * ''/etc/*release'',
    * ''/etc/*version''.

==== Hardware ====

http://www.binarytides.com/linux-commands-hardware-info/

  * ''lscpu'',
  * ''lshw -short'',
  * ''hwinfo %%--%%short'',
  * ''df -h'',
  * ''free'',
  * ''cat /proc/''...
    * ''cpuinfo'',
    * ''meminfo'',
    * ...
  * ...

==== Miscellaneous ====

  * Temperature : with package //lm-sensors// ''sensors'',
  * frequencies : with package //cpufrequtils// ''cpufreq-info''.


===== 'sudo' =====

==== Setting ====


To give a user same rights as for //root// through //sudo//, add a file in ''/etc/sudoers.d/'' with following content :

<code>
<user> ALL=(ALL) ALL
</code>

where ''<user>'' is the user's login.

''visudo'' should be used to edit this file, but it's not needed. The content of the created file is immediately take in account by //sudo//.

==== Error 'unable to resolve <hostname>' ====

To avoid this error when //sudoing//, following conditions must be met (from http://askubuntu.com/questions/59458/error-message-when-i-run-sudo-unable-to-resolve-host-none) :
  * ''/etc/hostname'' must contain //<hostname>// (''hostname //<hostname>//'' command works only until next reboot),
  * there must be an entry in ''/etc/hosts/'' where ''//<hostname>//'' points to ''127.0.0.1''.

<note important>Depending on your configuration, the modification in ''/etc/hosts'' will not survive to a //reboot//. You should then have a comment at the head of your ''/etc/hosts'' file, which indicates that you have to modify a file in ''/etc/cloud/templates'', but the given file may not be the correct one. Under //Debian//, for example, you have to modify ''hosts.debian.tmpl'' file, not ''hosts.tmpl''.</note>




===== USB =====
What follows may only concern //ARM// architectures ; it was not tested on //x86// architectures.

:!: It seems that you have to install the //ntfs-3g// package __before__ installing the //usbmount// package, or the //NTFS// devices will only be //read-only//.

  * To //automount// //USB// devices, install //usbmount// package,
  * In order to mount //NTFS// devices, add '' ntfs fuseblk'' to ''FILESYSTEMS'' in ''/etc/usbmount/usbmount.conf'' 

The devices will be accessible under ''/media/usb...''.

The ''fdisk -l'' command will show if your //USB// device is recognized (//recognized// does not means //mounted//).

:!: Trouble with //NTFS// devices auto-mounting : https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=774149.

===== Clock =====

  * To set the correct time from a server (package //ntpdate//) : ''ntpdate <server>'', with ''<server>'', a //NTP// server (''ntp.ubuntu.com'' for example),
  * to display the hardware clock : ''hwclock'',
  * to synchronize the hardware clock with system clock : ''hwclock --systohc'',
  * to synchronize the system clock with hardware clock : ''hwclock --hctosys''.

===== WiFi =====

==== Raspberry Pi/ODROID ====

For each //Wifi// network, add following in the ''/etc/wap_supplicant/wpas_supplicant.conf''

<code config>
network={
	ssid="<ssid>"
	scan_ssid=1
	psk="<key>"
	key_mgmt=<mgmnt>
}
</code>

Replace the entry with correct values. ''<mgmnt>'' can be something like ''WPA-PSK'' or ''NONE'' (without double-quotes!). ''scan--ssid'' is for hidden network, but should also work for not hidden network. ''psk'' can be omitted when the network is open.

==== Other ====

<note important>Was once used, but above should be used instead.</note>

  * Install ''wpasupplicant'' package,
  * Launch ''wpa-passphrass <SSID> <key>'', where ''<SSID>'' is the //SSID// of the WiFi network, and ''<key>'', the corresponding key. You should obtain something like :

<code config>
network={
        ssid="<SSID>"
        #psk="<key>"
        psk=3a19d8ecd12eff56bbebd67af8e4c14e1b4b5fb1a278f7e5c474ad252853a1d9
}
</code>
  *  in ''/etc/network/interfaces'', insert :
<code config>
auto wlan0
iface wlan0 inet dhcp
    wpa-ssid <SSID>
    wpa-scan-ssid 1 # If the SSID id hidden.
    wpa-psk <psk>
</code>
where ''<SSID>'' is the //SSID// of the WiFi network, and ''<psk>'' the hexadecimal string right of the uncommented above ''psk='' entry.
  * launch ''ifup wlan0''.




===== Hostname =====

To change //hostname// :
  * Until next //reboot// : ''hostname <name>'' (you must logout to see the change),
  * permanently : modify file ''/etc/hostname''.

<note>
If the //hostname// doesn't match an ID of the //SSL// certificate, //apache// will complain.
</note>

How to correctly set the //hostname// to avoid some error messages (for example ''unable to resolve host ...'' with //sudo//): https://askubuntu.com/questions/59458/

===== Configuring an account to allow backup only =====

You may want to backup the entire //www-data// account (which contains the web pages) using //scp// or //rsync//, but you get a message like ''protocol version mismatch -- is your shell clean?''. This is because the the shell for this account is a restricted one (probably ''/usr/sbin/nologin'', as you will see in ''/etc/passwd'').

To fix this :
  * install ''rssh'' package,
  * modify ''/etc/passwd'' so the shell will be ''/usr/bin/rssh'' (''bin'', not ''sbin'') for the desired account,
  * modify ''/etc/rssh.conf'' accordingly.

===== Killing a connection =====

To kill the a TCP connection without having to stop the corresponding program:

''sudo ss -K dst <IP> [dport <port>]'' 

===== Remote copy with resuming =====

An alternative to ''scp'', which allows to resume an interrupted transfer:

''rsync %%--%%append-verify [%%--%%progress] [-e ssh]  <src>  <dest>''

  *  ''-e ssh'' should be the default, but may be useful to add some parameters to ssh (put ''ssh'' and its parameters between quotes then),
  * ''%%--%%progress'' is to display the progress.


 

Can be used directly, or after an aborted ''scp''.
===== Other =====

  * //[[.:systemd:]]//,
  * //[[.:Odroid:]]//.