Basic Linux Tools
==================


Information about System Programs
------------------------------------

* man
* whereis
* whatis
* mandb
* which


.. code-block:: bash

    # program man
    man bash
    man python3

    man hosts # /etc/hosts config file man

    man sshd # deamon man


    man -k syslog # search man pages with "syslog"
    apropos ssh # same as man -k

    whatis bash
    whatis node

    whereis python3 # locate the sources, bins and mans of a prog



Working with Directories
-------------------------

* pwd
* cd
* ls
* mkdir
* rmdir


.. code-block:: bash

    # users home directory
    cd ~ 

    # parent directory
    cd ..

    # return to previous directory, and alternate between two Directories
    cd -


    # cd can be used in a subshell, as part of a pipeline.
    
    echo $(cd /var/log && tail dmesg)
    # or 
    echo $(cd /var/log; tail dmesg)


    # this is especially useful with the tar utility, which relies on current shell pwd

    ( cd /bits/stage/; tar -c docs -f docs.tar; mv docs.tar ~; cd - )



Working with files
------------------


* ``file``: Determine Filetype
* ``touch``: update file timestamp
* ``rm``: Remove file or directory
* ``mv``: Move File or Directory
* ``cp``: Copy file or Directory
* ``rename``: rename many file with regex
* ``cat``: concatenate files and print to stdout
* ``tac``: reverse of cat - concatenate and print files in reverse
* ``strings``: display string within binary files.
* ``xxd``: make a hexdump or do the reverse
* ``base64``: make a base64 dump or do the reverse
* ``man hier``: distro's file system hierarchy


.. code-block:: bash

    # the s flag is for special file types typically found under dev
    file /dev/*


    # touch is to update or set timestamp of a file
    touch ~/.bashrc


    # cat can be used to concatenate multiple files. Like how we do with SSL certs
    cat *.crt *.ca-bundle >> chain.crt


    # to display ASCII text embedded within binary files
    strings /bin/python3
    string /bin/node



.. code-block:: bash

    # human reasable hex dump with xxd
    echo "I love Bash, C, Python and Javascript" | xxd -ps

    # continueous hexdump
    echo "I love Bash, C, Python and Javascript" | xxd -ps

    # hex encode and decode pipeline with the -r reverse flag
    echo "I love Bash, C, Python and Javascript" | xxd -ps | xxd -r -ps

    # with cat and outiouput redirection
    mkdir /tmp/mk; cat /var/log/dmesg | xxd -ps > /tmp/mk/dmesg.hex
    tail /tmp/mk/dmesg.hex

    # encode to base64 and back
    echo "I love Bash, C, Python and Javascript" | base64 | base64 -d


The Linux Filesystem and conf
-----------------------------

/etc/init.d/
    Startup and Management scripts for system daemons and services.

    ``less /etc/init.d/postgresql``


/dev/
    The /dev in-memory directory is populated with files as the kernel is recognising hardware.
    ``file -s /dev/*``


/proc/
    It is actually a view of the kernel, or better, what the kernel manages, and is a means
    to interact with it directly

    ``cat /proc/cpuinfo``


/etc/sysctl.conf
    Runtime Kernel parameters. see also the ``sysctl`` command


Linux Processes
----------------

* ``ps`` snapshop of the current processes
* ``top`` and ``htop`` display linux process
* ``jobs``
* ``time`` command to display real, sys and user time
  
.. code-block:: bash

    time ls -lh | sort -k5n | less
    time sudo ufw status