shell_scripting_notes.txt

# ===================================================================================

    Why to Write Shell Script ?

    1. Shell script can take input from user, file and output them on screen.
    2. Useful to create our own commands.
    3. Save lots of time.
    4. To automate some task of day today life.
    5. System Administration part can be also automated.

# ===================================================================================

    There are a few other variables that the system sets for you to use as well.

    $0        - The name of the Bash script. [ex: echo $SHELL or echo $0]
    $1 - $9   - The first 9 arguments to the Bash script. (As mentioned above.)
    $#        - How many arguments were passed to the Bash script.
    $@        - All the arguments supplied to the Bash script.
    $?        - The exit status of the most recently run process.
    $$        - The process ID of the current script.
    $USER     - The username of the user running the script.
    $HOSTNAME - The hostname of the machine the script is running on.
    $SECONDS  - The number of seconds since the script was started.
    $RANDOM   - Returns a different random number each time is it referred to.
    $LINENO   - Returns the current line number in the Bash script.

# ===================================================================================

    How to write shell script?

    Following steps are required to write shell script:

    1. Use any editor like vi or mcedit to write shell script.
    2. After writing shell script set execute permission for your script as follows
        syntax:
            chmod permission your-script-name

        Examples:
            $ chmod +x your-script-name
            $ chmod 755 your-script-name

        Note: This will set read write execute(7) permission for owner, for group and
              other permission is read and execute only(5).
    3. Execute your script as
        syntax:
            bash your-script-name
            sh your-script-name
            ./your-script-name

        Examples:
            $ bash bar
            $ sh bar
            $ ./bar

# ===================================================================================

    Linux Notes from Cihangir Beşiktaş

        LVM:
        ---
        Pyhsical volumes, volume groups, logical volumes

        - pvcreate, pvdisplay, pvremove
        - vgcreate, vgdisplay
        - lvcreate, lvdisplay



        Resource Limits:
        ----------------
        Resource limits of processes

         ulimit -a

        core file size          (blocks, -c) 0
        data seg size           (kbytes, -d) unlimited
        scheduling priority             (-e) 0
        file size               (blocks, -f) unlimited
        pending signals                 (-i) 63354
        max locked memory       (kbytes, -l) 64
        max memory size         (kbytes, -m) unlimited
        open files                      (-n) 1024
        pipe size            (512 bytes, -p) 8
        POSIX message queues     (bytes, -q) 819200
        real-time priority              (-r) 0
        stack size              (kbytes, -s) 8192
        cpu time               (seconds, -t) unlimited
        max user processes              (-u) 1024
        virtual memory          (kbytes, -v) unlimited
        file locks                      (-x) unlimited



        Find Large Files:
        ----------------
        Files greater than 100MB in /var/log
        find /var/log -type f -size +100000k -exec ls -lh {} \; | awk '{ print $9 ": " $5 }'



        Kernel Configuration:
        ---------------------
        There are three ways to pass options to the kernel and thus control its behaviour:

            When building the kernel.
            When starting the kernel (usually, when invoked from a boot loader).
            At runtime (through the files in /proc and /sys).

        # cat /etc/sysctl.conf
        net.ipv4.ip_forward = 1

        # sysctl kernel.sysrq=1
        # echo "1" > /proc/sys/kernel/sysrq



        Process Operations:
        -------------------
        List parent process:
        # ps -o ppid= -p <pid>
        # pstree

        Kill zombie processes:
        # kill -s SIGCHLD pid


        Monitoring
        --------------
        # top
        # vmstat
        # lsof
        # tcpdump
        # netstat
        # iptraf
        # iftop

        I/O Monitoring:
        # iostat - device level stats
        # sar
        # iotop - process level stats



        RAID:
        -----

        Raid 0: for large logical volume
        Raid 1: redundant volumes
        Raid 4: block-level striping with a dedicated parity disk
        Raid 5: block-level striping with distributed parity
        Raid 6: extends RAID 5 by adding another parity block
                it uses block-level striping with two parity blocks distributed across all member disks.



        DNS:
        ---
        TTL: https://www.dnswatch.info/articles/dns-update