Most used Linux Commands

Here I document the Linux commands I use most often for fast pickup then needed.

(Many years ago I liked to call myself a console cowboy, however without continous Linux work I am no more the Linux Fu Master I newer really were and have now resorted to creating an ad-hoc cheat sheet)

• System :
  • shell> lsb_release -a : shows Ubuntu version and codename
  • shell> egrep -c ' lm ' /proc/cpuinfo : shows whether CPU is 64 bit
    • 0: not 64 bit
    • 1 or more : 64 bit
  • shell> uname -m : shows whether the kernel is running 64 bit
    • i386 or i486 or i586 or i686 : 32 bit kernel
    • x86_64 : 64 bit kernel
  • shell> lscpu : shows cores, threads per core and other cpu properties
  • shell> fdisk -l : list all the physical disks, their partitions and their mounts
• Package management
  • shell> dpkg -l : list all packages installed
    • shell> dpkg -l | grep virt : list all packages that have "virt" in name or description
• File management
  • shell> ls -l : list view of current directory.
  • shell> df -h : show used & available disk space.
  • shell> scp sourcefile targetDestination : copy a file from one server to another server (destination does NOT contain the filename)
    • shell> scp root@123.123.123.123:/var/VMs/VHDs/MySql . :  copy the MySql VM from a server with IP 123.123.123.123 using the root user (on 123.123.123.123) to current directory (current directory is referenced using a dot '.')
    • shell> scp MySql root@123.123.123.123:/var/VMs/VHDs : copy the Mysql file from current directly to the /var/VMs/VHDs directory on server with IP 123.123.123.123 using the root user on 123.123.123.123.
  • shell> cp filename1 filename2 : copy filename1 to filename2
  • shell> chown user:group filename : sets user & group for a specific file
  • shell> rm filename : deletes the file
  • shell> mv filename1 filename2 : rename filename1 to filename2
• Network
  • shell> ifconfig : display interface information.
  • shell> nano /etc/network/interfaces : edit the interface configuration file.
  • shell> ifdown eth0 && ifup eth0 : restart the eth0 network
  • shell> /etc/init.d/networking restart : restart the interface card (all networks will go down)
  • servername :
    • shell> hostname :
    • shell> hostname --fqdn :
• Bind9  : (DNS Server for Ubuntu)
  • shell> apt install bind9 : the bind9 DNS server package.
  • shell> apt install dnsutils : package for testing and troubleshooting DNS issues.
  • shell> dpkg -l | grep bind : check your bind9 packages.
  • Configuration files :
    • /etc/bind/named.conf : specifies which other configuration files to use, typically :
      • /etc/bind/named.conf.options : dns options like whether to listen to v6 etc.
      • /etc/bind/named.conf.default-zones : default zones such as localhost, its reverse and the root hints.
      • /etc/bind/named.conf.local : references to local zone files (in the same /etc/bind/ directory).
  • Roles
    • primary server : bind9 reads the data for a zone from a file on its host and is authoritative for that zone
    • secondary server : bind9 gets the zone data from a nameserver that is authoritative for the zone.
    • caching server : bind9 will find the answer to name queries and remember the answer when the domain is queried again. I think that if you do NOT make your bind9 a caching server, then it will NOT be able to answer on requests other than the domains for which bind9 is either a primary or secondary server. To make bind9 act as a caching server uncomment forwarders in /etc/bind/named.conf.options and set one or multiple name servers to fetch results from typically your ISP dns server(s) or you can use 8.8.8.8 by Google eg. 
      

      forwarders {
          8.8.8.8;
      };

  • Zone : it gives meaning to call top domains like eg. menulab.com a namespace. Information about such a namespace (top domain) is stored in zones. On Ubuntu the zones are listed in /etc/bind/named.conf.local and each zone listing have a reference to a zone file, which then contains the namespace information (the zone file listing within named.conf.local may contain some additional management information, eg. if the zone can be transferred to another registrar or another person)
  • Zone file characters :
    • $ : signifies the beginning of a directive, eg.
      • $TTL : start of zone file and sets the default Time-To-Live (in seconds) for all resource records, specifying the time that a zone resource record is valid (should be cached by caching nameservers, eg. in your router)
      • $INCLUDE :
      • $ORIGIN : eg. $ORIGIN menulab.com. : means that any named used in a resource record NOT ending in a dot '.' are appended with menulab.com. Note that if $ORIGIN is not specified, then often the zone name is used by default - therefore often it is not necessary to specify the $ORIGIN directive.
    • @ : namespace variable either $ORIGIN or zone name (if $ORIGIN is not specified). @ is commonly used to avoid writing the zone namespace in resource records.
    • . : dot '.' is used to end a name signifying that it should NOT be appended $ORIGIN (or zone name).
    • ( ) : groups data that crosses a line boundary (line terminations are not recognized within parentheses, however comments ending are)
    • ; : start of comment (end of line will stop the comment).
    • \ : used to escape charaters with special meaning, eg. \. would translate to an actual dot.
  • Zone file format :
    • $ORIGIN : optional start of zone file specifying the namespace.
    • $TTL : start of dns node tree and the starting point of the dns zone file if no $ORIGIN is specified and default expiration time of all resource records in case they don't specify their own TTL's.
    • Resource Records : (a zone file can have multiple records of different types, see record types under format below)
      • Format :
        • host label : hostname of a record.
        • ttl : Time To Live : how long time other nameservers should cache this record
        • record class :
          • IN : INternet : default
          • CH : CHaosnet) : used for querying dns server versions.
          • HS : HeSiod : uses dns functionality to provide access to database of information that change infrequently.
        • record type :
          • SOA : Start of Authority : required option for each zone. SOA contains
            • name of the zone.
            • primary nameserver containing the original zone file (typically also specified as one of the authoritative nameservers using the NS record type).
            • email address on the entity responsible for administering the zone file (registrars show this as the administrator email for a domain). Note that the email address uses a dot '.' instead of add '@'.
            • serial number specifying date and increases by 1 for each update to the zone file.
            • minimum-ttl : negative response caching time by the nameserver.
            • For zones on which this nameserver is a secondary nameserver :
              • time-to-refresh : how long time (in seconds) the nameserver should wait before checking for an update in the primary zone file (comparing serial numbers).
              • time-to-retry : how long time to wait before retrying if cannot check the primary zone.
              • time-to-expire : how long time before the nameserver considers it's copy of the zone file to be expired and must stop answering requests (if it cannot update it's copy from the primary nameserver).
          • A : IPv4 Address : returns an IPv4 IP address, most commonly used to map hostnames to an IP address.
          • AAAA : IPv6 Address : returns an IPv6 address, same function as IPv4 but with 128 bit address space instead of 32 bit.
          • CNAME : Canonical NAME : also called an alias. In the example below www expands to www.menulab.com. (per $ORIGIN) and CNAME specifies that the canonical name for www.menulab.com. is menulab.com. Therefore if there are a forward lookup request for www.menulab.com return the result for menulab.com instead.
            $ORIGIN menulab.com.
            menulab.com    IN    A    27.254.33.97
            www    IN    CNAME    menulab.com.
          • MX : Mail eXchange : maps a domain name to a list of message transfer agents for that domain.
          • NS : Name Server : specifies the authoritative nameserver (there can be more than one authoritative nameserver). Below dns.webmodelling.com is tha authoritative nameserver for menulab.com : 
            menulab.com.    IN    NS    dns.webmodelling.com.
          • PTR : PoinTeR : used for reverse lookup (IP to domain)
          • SRV :
          • TXT : Text : orignally for arbitrary human-readable text in a dns record. Now this record more often carries machine-readable data, eg. SPF, DKIM, DMARC, DNS-SD etc.
      • Example :
        Note how dns.webmodelling.com & dns2.webmodelling.com both are authoritative (specified using NS records), but only dns.webmodelling.com is primary (specified in SOA record)
        

        $TTL    3600
        @    IN    SOA    dns.webmodelling.com. rasmus.topiqs.online. (
                          202006021; serial todays date + todays serial
                          7200; refresh);
        www    86400    A    5.231.220.25
        toipqs.online.    86400    A    5.231.220.25
        topiqs.online.    3600    MX    5    ALT1.ASPMX.L.GOOGLE.COM.
        topiqs.online.    2880    NS    dns.webmodelling.com.
        topiqs.online.    2880    NS    dns2.webmodelling.com.
        topiqs.online.    86400    TXT    "google-site-verification=8MsO8jbf_z0YLe..."

  • cmd>  nslookup menulab.com dns.webmodelling.com : forward lookup (of menulab.com) requesting a specific nameserver (dns.webmodelling.com).
    • server : p396-static.nasbkkst2.c.csloxinfo.net : reverse lookup of nameserver (here different from dns.webmodelling.com)
    • Address : 202.183.164.142 : IP address of nameserver used (that is : IP of dns.webmodelling.com)
    • Name : menulab.com : the domain we make a forward lookup for.
    • Address : 27.254.33.57 : the IP of menulab.com
  • cmd> nslookup topiqs.online :
  • cmd> nslookup -type=soa topiqs.online :
  • cmd> nslookup -type=ns topiqs.online :
  • cmd> nslookup -type=mx topiqs.online :
  • cmd> nslookup -type=any topiqs.online : get all dns records for a domain.
  • cmd> nslookup 5.231.220.25 : reverse lookup.
  • cmd> nslookup -debug topiqs.online : detailed information for both dns request & response.
  • shell> dig topiqs.online any : get all dns resource records for a domain, however the nameserver may not supply dns records for any sub-domains.
• KVM/QEMU
  • shell> egrep -c '(vmx|svm)' /proc/cpuinfo : whether the host CPU supports virtualization (cpu vendor : vmx=intel, svm=amd)
    • 0 : no support
    • 1 or more : support
  • shell> kvm-ok : shows whether KVM acceleration can be used
  • shell> lsmod | grep kvm : shows whether the kvm module have been loaded (and whether it is intel or amd).
    • If the kvm module have not been loaded, it can possible be loaded using the modprobe command :
      shell> modprobe kvm_intel : (in case of intel cpu)
  • shell> virt-viewer vmName : pass local screen for VM to remote client (the remote windows client should have Xming installed and the SSH connection should forward window X)
  • Virsh
    • shell> virsh list --all : shows all virtual machines, their IDs and whether they are running
    • shell> virsh start vmName : startup this VM
    • shell> virsh shutdown vmName : stop this VM
    • shell> virsh destroy vmName : stop this VM
    • shell> virsh reboot vmName : reboots this VM
    • shell> virsh autostart vmName : set this VM to startup automatically on host boot.
    • shell> virsh autostart --disable vmName : set this VM to NOT startup automatically.
    • shell> virsh dumpxml vmName : write xml definition file for this VM to screen
      • shell> virsh dumpxml vmName | grep memory : shows the memory allocation
    • shell> virsh define /path/to/VM/xml/file : creates a new VM domain and writes an xml definition file vmName (as specified in the source xml file) in /etc/libvirt/qemu
    • shell> undefine vmName : deletes the xml definition file in /etc/libvirt/qemu for this VM and also removes the VM domain
    • shell> virsh net-list --all : show all virtual networks
    • shell> virsh net-info virtualNetworkName : shows info about a particular virtual network
    • shell> virsh net-dumpxml virtualNetworkName : write the network definition