Unix System Calls

     UNIX system calls are used to manage the file system, control processes,
     and to provide interprocess communication.  The UNIX system interface
     consists of about 80 system calls (as UNIX evolves this number will
     increase).  The following table lists about 40 of the more important
     system call:

 GENERAL CLASS              SPECIFIC CLASS                 SYSTEM CALL
     ---------------------------------------------------------------------
     File Structure             Creating a Channel             creat()
     Related Calls                                             open()
                                                               close()
                                Input/Output                   read()
                                                               write()
                                Random Access                  lseek()
                                Channel Duplication            dup()
                                Aliasing and Removing          link()
                                Files                          unlink()
                                File Status                    stat()
                                                               fstat()
                                Access Control                 access()
                                                               chmod()
                                                               chown()
                                                               umask()
                                Device Control                 ioctl()
     ---------------------------------------------------------------------
     Process Related            Process Creation and           exec()
     Calls                      Termination                    fork()
                                                               wait()
                                                               exit()
                                Process Owner and Group        getuid()
                                                               geteuid()
                                                               getgid()
                                                               getegid()
                                Process Identity               getpid()
                                                               getppid()
                                Process Control                signal()
                                                               kill()
                                                               alarm()
                                Change Working Directory       chdir()
     ----------------------------------------------------------------------
     Interprocess               Pipelines                      pipe()
     Communication              Messages                       msgget()
                                                               msgsnd()
                                                               msgrcv()
                                                               msgctl()
                                Semaphores                     semget()
                                                               semop()
                                Shared Memory                  shmget()
                                                               shmat()
                                                               shmdt()
     ----------------------------------------------------------------------

     [NOTE:  The system call interface is that aspect of UNIX that has
     changed the most since the inception of the UNIX system.  Therefore,
     when you write a software tool, you should protect that tool by putting
     system calls in other subroutines within your program and then calling
     only those subroutines.  Should the next version of the UNIX system
     change the syntax and semantics of the system calls you've used, you
     need only change your interface routines.]

Original Link: http://www.di.uevora.pt/~lmr/syscalls.html 

linux_interview_questions

1. How to display the 10th line of a file?
head -10 filename | tail -1

2. How to remove the header from a file? [ -i in sed will update the file ]
sed -i '1 d' filename

3. How to remove the footer from a file? [ $ is for end of the file ]
sed -i '$ d' filename

4. Write a command to find the length of a line in a file?
The below command can be used to get a line from a file.
sed –n '<n> p' filename
We will see how to find the length of 10th line in a file
sed -n '10 p' filename|wc -c

5. How to get the nth word of a line in Unix?
cut –f<n> -d' '

6. How to reverse a string in unix?
echo "java" | rev

7. How to get the last word from a line in Unix file?
echo "unix is good" | rev | cut -f1 -d' ' | rev

8. How to replace the n-th line in a file with a new line in Unix?
sed -i'' '10 d' filename       # d stands for delete
sed -i'' '10 i new inserted line' filename     # i stands for insert

9. How to check if the last command was successful in Unix?
echo $?

10. Write command to list all the links from a directory?
ls -lrt | grep "^l"

11. How will you find which operating system your system is running on in UNIX?
uname -a

12. Create a read-only file in your home directory?
touch file; chmod 400 file

13. How do you see command line history in UNIX?
The 'history' command can be used to get the list of commands that we are executed.

14. How to display the first 20 lines of a file?
By default, the head command displays the first 10 lines from a file. If we change the option of head, then we can display as many lines as we want.
head -20 filename
An alternative solution is using the sed command
sed '21,$ d' filename
The d option here deletes the lines from 21 to the end of the file

15. Write a command to print the last line of a file?
The tail command can be used to display the last lines from a file.
tail -1 filename
Alternative solutions are:
sed -n '$ p' filename
awk 'END{print $0}' filename

1. How do you rename the files in a directory with _new as suffix?
ls -lrt|grep '^-'| awk '{print "mv "$9" "$9".new"}' | sh

2. Write a command to convert a string from lower case to upper case?
echo "apple" | tr [a-z] [A-Z]

3. Write a command to convert a string to Initcap.
echo apple | awk '{print toupper(substr($1,1,1)) tolower(substr($1,2))}'

4. Write a command to redirect the output of date command to multiple files?
The tee command writes the output to multiple files and also displays the output on the terminal.
date | tee -a file1 file2 file3

5. How do you list the hidden files in current directory?
ls -a | grep '^\.'

6. List out some of the Hot Keys available in bash shell?

    Ctrl+l - Clears the Screen.
    Ctrl+r - Does a search in previously given commands in shell.
    Ctrl+u - Clears the typing before the hotkey.
    Ctrl+a - Places cursor at the beginning of the command at shell.
    Ctrl+e - Places cursor at the end of the command at shell.
    Ctrl+d - Kills the shell.
    Ctrl+z - Places the currently running process into background.

7. How do you make an existing file empty?
cat /dev/null >  filename

8. How do you remove the first number on 10th line in file?
sed '10 s/[0-9][0-9]*//' < filename

9. What is the difference between join -v and join -a?
join -v : outputs only matched lines between two files.
join -a : In addition to the matched lines, this will output unmatched lines also.

10. How do you display from the 5th character to the end of the line from a file?
cut -c 5- filename

1. Display all the files in current directory sorted by size?
ls -l | grep '^-' | awk '{print $5,$9}' |sort -n|awk '{print $2}'

2. Write a command to search for the file 'map' in the current directory?
find -name map -type f

3. How to display the first 10 characters from each line of a file?
cut -c -10 filename

4. Write a command to remove the first number on all lines that start with "@"?
sed '\,^@, s/[0-9][0-9]*//' < filename

5. How to print the file names in a directory that has the word "term"?
grep -l term *
The '-l' option make the grep command to print only the filename without printing the content of the file. As soon as the grep command finds the pattern in a file, it prints the pattern and stops searching other lines in the file.

6. How to run awk command specified in a file?
awk -f filename

7. How do you display the calendar for the month march in the year 1985?
The cal command can be used to display the current month calendar. You can pass the month and year as arguments to display the required year, month combination calendar.
cal 03 1985
This will display the calendar for the March month and year 1985.

8. Write a command to find the total number of lines in a file?
wc -l filename
Other ways to pring the total number of lines are
awk 'BEGIN {sum=0} {sum=sum+1} END {print sum}' filename
awk 'END{print NR}' filename

9. How to duplicate empty lines in a file?
sed '/^$/ p' < filename

10. Explain iostat, vmstat and netstat?

    Iostat: reports on terminal, disk and tape I/O activity.
    Vmstat: reports on virtual memory statistics for processes, disk, tape and CPU activity.
    Netstat: reports on the contents of network data structures.

1. How do you write the contents of 3 files into a single file?
cat file1 file2 file3 > file

2. How to display the fields in a text file in reverse order?
awk 'BEGIN {ORS=""} { for(i=NF;i>0;i--) print $i," "; print "\n"}' filename

3. Write a command to find the sum of bytes (size of file) of all files in a directory.
ls -l | grep '^-'| awk 'BEGIN {sum=0} {sum = sum + $5} END {print sum}'

4. Write a command to print the lines which end with the word "end"?
grep 'end$' filename
The '$' symbol specifies the grep command to search for the pattern at the end of the line.

5. Write a command to select only those lines containing "july" as a whole word?
grep -w july filename
The '-w' option makes the grep command to search for exact whole words. If the specified pattern is found in a string, then it is not considered as a whole word. For example: In the string "mikejulymak", the pattern "july" is found. However "july" is not a whole word in that string.

6. How to remove the first 10 lines from a file?
sed '1,10 d' < filename

7. Write a command to duplicate each line in a file?
sed 'p' < filename

8. How to extract the username from 'who am i' comamnd?
who am i | cut -f1 -d' '

9. Write a command to list the files in '/usr' directory that start with 'ch' and then display the number of lines in each file?
wc -l /usr/ch*
Another way is
find /usr -name 'ch*' -type f -exec wc -l {} \;

10. How to remove blank lines in a file ?
grep -v ‘^$’ filename > new_filename

1. How to display the processes that were run by your user name ?
ps -aef | grep <user_name>

2. Write a command to display all the files recursively with path under current directory?
find . -depth -print

3. Display zero byte size files in the current directory?
find -size 0 -type f

4. Write a command to display the third and fifth character from each line of a file?
cut -c 3,5 filename

5. Write a command to print the fields from 10th to the end of the line. The fields in the line are delimited by a comma?
cut -d',' -f10- filename

6. How to replace the word "Gun" with "Pen" in the first 100 lines of a file?
sed '1,00 s/Gun/Pen/' < filename

7. Write a Unix command to display the lines in a file that do not contain the word "RAM"?
grep -v RAM filename
The '-v' option tells the grep to print the lines that do not contain the specified pattern.

8. How to print the squares of numbers from 1 to 10 using awk command
awk 'BEGIN { for(i=1;i<=10;i++) {print "square of",i,"is",i*i;}}'

9. Write a command to display the files in the directory by file size?
ls -l | grep '^-' |sort -nr -k 5

10. How to find out the usage of the CPU by the processes?
The top utility can be used to display the CPU usage by the processes.


1. Write a command to remove the prefix of the string ending with '/'.
The basename utility deletes any prefix ending in /. The usage is mentioned below:
basename /usr/local/bin/file
This will display only file

2. How to display zero byte size files?
ls -l | grep '^-' | awk '/^-/ {if ($5 !=0 ) print $9 }'

3. How to replace the second occurrence of the word "bat" with "ball" in a file?
sed 's/bat/ball/2' < filename

4. How to remove all the occurrences of the word "jhon" except the first one in a line with in the entire file?
sed 's/jhon//2g' < filename

5. How to replace the word "lite" with "light" from 100th line to last line in a file?
sed '100,$ s/lite/light/' < filename

6. How to list the files that are accessed 5 days ago in the current directory?
find -atime 5 -type f

7. How to list the files that were modified 5 days ago in the current directory?
find -mtime 5 -type f

8. How to list the files whose status is changed 5 days ago in the current directory?
find -ctime 5 -type f

9. How to replace the character '/' with ',' in a file?
sed 's/\//,/' < filename
sed 's|/|,|' < filename

10. Write a command to find the number of files in a directory.
ls -l|grep '^-'|wc -l

1. Write a command to display your name 100 times.
The Yes utility can be used to repeatedly output a line with the specified string or 'y'.
yes <your_name> | head -100

2. Write a command to display the first 10 characters from each line of a file?
cut -c -10 filename

3. The fields in each line are delimited by comma. Write a command to display third field from each line of a file?
cut -d',' -f2 filename

4. Write a command to print the fields from 10 to 20 from each line of a file?
cut -d',' -f10-20 filename

5. Write a command to print the first 5 fields from each line?
cut -d',' -f-5 filename

6. By default the cut command displays the entire line if there is no delimiter in it. Which cut option is used to supress these kind of lines?
The -s option is used to supress the lines that do not contain the delimiter.

7. Write a command to replace the word "bad" with "good" in file?
sed s/bad/good/ < filename

8. Write a command to replace the word "bad" with "good" globally in a file?
sed s/bad/good/g < filename

9. Write a command to replace the word "apple" with "(apple)" in a file?
sed s/apple/(&)/ < filename

10. Write a command to switch the two consecutive words "apple" and "mango" in a file?
sed 's/\(apple\) \(mango\)/\2 \1/' < filename

11. Write a command to display the characters from 10 to 20 from each line of a file?
cut -c 10-20 filename

1. Write a command to print the lines that has the the pattern "july" in all the files in a particular directory?
grep july *
This will print all the lines in all files that contain the word “july” along with the file name. If any of the files contain words like "JULY" or "July", the above command would not print those lines.

2. Write a command to print the lines that has the word "july" in all the files in a directory and also suppress the filename in the output.
grep -h july *

3. Write a command to print the lines that has the word "july" while ignoring the case.
grep -i july *
The option i make the grep command to treat the pattern as case insensitive.

4. When you use a single file as input to the grep command to search for a pattern, it won't print the filename in the output. Now write a grep command to print the filename in the output without using the '-H' option.
grep pattern filename /dev/null
The /dev/null or null device is special file that discards the data written to it. So, the /dev/null is always an empty file.
Another way to print the filename is using the '-H' option. The grep command for this is
grep -H pattern filename

5. Write a command to print the file names in a directory that does not contain the word "july"?
grep -L july *
The '-L' option makes the grep command to print the filenames that do not contain the specified pattern.

6. Write a command to print the line numbers along with the line that has the word "july"?
grep -n july filename
The '-n' option is used to print the line numbers in a file. The line numbers start from 1

7. Write a command to print the lines that starts with the word "start"?
grep '^start' filename
The '^' symbol specifies the grep command to search for the pattern at the start of the line.

8. In the text file, some lines are delimited by colon and some are delimited by space. Write a command to print the third field of each line.
awk '{ if( $0 ~ /:/ ) { FS=":"; } else { FS =" "; } print $3 }' filename

9. Write a command to print the line number before each line?
awk '{print NR, $0}' filename

10. Write a command to print the second and third line of a file without using NR.
awk 'BEGIN {RS="";FS="\n"} {print $2,$3}' filename

11. How to create an alias for the complex command and remove the alias?
The alias utility is used to create the alias for a command. The below command creates alias for ps -aef command.
alias pg='ps -aef'
If you use pg, it will work the same way as ps -aef.
To remove the alias simply use the unalias command as
unalias pg

12. Write a command to display todays date in the format of 'yyyy-mm-dd'?
The date command can be used to display todays date with time
date '+%Y-%m-%d'

Bash Notes

Bash Notes:


bash  [options] [arguments]
man bash [ for more informations ]
positional parameters $1, $2, etc. But its good to use the positional parameters as ${1}, ${2}
The name of the script is available as ${0}

Filename Metacharacters:

* -> Match any string of zero or more characters.
? -> Match any single character.
[abc...] -> Match any one of the enclosed characters; a hyphen can specify a range (e.g., a-z, A-Z, 0–9).
[!abc...] -> Match any character not enclosed as above.
~ -> Home directory of the current user.
~name -> Home directory of user name.
~+ -> Current working directory ($PWD).
~- -> Previous working directory ($OLDPWD).






Adding the given number:

i=0; for x in {1..10}; do i=$(( $i + $x)); echo $i; done
 

With the extglob option on:

?(pattern) Match zero or one instance of pattern.
*(pattern) Match zero or more instances of pattern.
+(pattern) Match one or more instances of pattern.
@(pattern) Match exactly one instance of pattern.
!(pattern) Match any strings that don’t match pattern.

This pattern can be a sequence of patterns separated by "|" meaning that the match applies to any of the patterns.
The extglob also can be used at egrep and awk.

Examples:
ls new* : List new and new.1
cat ch? : Match ch9 but not ch10
vi [D-R]* : Match files beginning with D through R
pr !(*.o|core) | lp : Print files non-object and non-core files
pre{X,Y[,Z...]}post : Expands to preXpost, preYpost, and so on.

# Expand textually; no sorting
$ echo hi{DDD,BBB,CCC,AAA}there
hiDDDthere hiBBBthere hiCCCthere hiAAAthere

# Expand, then match ch1, ch2, app1, app2
$ ls {ch,app}?

# Expands to mv info info.old
$ mv info{,.old}

# Simple numeric expansion
$ echo 1 to 10 is {1..10}
1 to 10 is 1 2 3 4 5 6 7 8 9 10

*) Renaming all the .html files to .htm file in a single directory:
rename 's/\.html/\.htm/' *.html
[ how about renameing .htm file to .php  -> rename 's/\.htm/\.php/' *.htm ]
[ rename is using sed type replacement, "\" is used for the escape of . ]

for i in *.html; do mv "${i}" "${i/.html}".htm; done   [ There should not be a space at "${i/.html}".htm ]

find /home/amitmund/test/rename_Test/ -type f -iname "*.html" -exec rename 's/\.html/\.htm/' *.htm {} \;

Quoting:

; Command separator.
& Background execution.
() Command grouping.
| Pipe.
< > & Redirection symbols.
* ? [ ] ~ + - @ ! Filename metacharacters.
" ' \ Used in quoting other characters.
` Command substitution.
$ Variable substitution (or command or
 arithmetic substitution).
# Start a comment that continues to the end of the line.
space tab newline Word separators.

" "  Everything between " and " is taken literally. [ Except few ]
$ Variable (or command and arithmetic) substitution will occur.
` Command substitution will occur.
" This marks the end of the double quoted string.
\ The character following a \ is taken literally. Also known as escape character.

Examples
$ echo 'Single quotes "protect" double quotes'
Single quotes "protect" double quotes
$ echo "Well, isn’t that \"special\"?"
Well, isn’t that "special"?
$ echo "You have `ls | wc -l` files in `pwd`"
You have
43 files in /home/bob
$ echo "The value of \$x is $x"
The value of $x is 100


cmd & -> Execute cmd in background.
cmd1 ; cmd2 -> Command sequence; execute multiple cmds on the same line.
{ cmd1 ; cmd2 ; } -> Execute commands as a group in the current shell.
(cmd1 ; cmd2) -> Execute commands as a group in a subshell.
cmd1 | cmd2 -> Pipe; use output from cmd1 as input to cmd2.
cmd1 `cmd2` -> Command substitution; use cmd2 output as arguments to cmd1.
cmd1 $(cmd2) -> POSIX shell command substitution; nesting is allowed.
cmd $((expression)) -> POSIX shell arithmetic substitution. Use the result of expression as argument to cmd.
cmd1 && cmd2 -> AND; execute cmd1 and then (if cmd1 succeeds) cmd2.
cmd1 || cmd2 -> OR; execute either cmd1 or (if cmd1 fails) cmd2.
! cmd -> NOT; execute cmd, and produce a zero exit status if cmd exits with a nonzero status.

Redirection Forms
File descriptor Name Common abbreviation Typical default
0 Standard input stdin Keyboard
1 Standard output stdout Screen
2 Standard error stderr Screen

Simple redirection
cmd > file -> Send output of cmd to file (overwrite).
cmd >> file -> Send output of cmd to file (append).
cmd < file -> Take input for cmd from file.
cmd <> file -> Open file for reading and writing on the standard input. The contents are not destroyed.*
cmd >| file -> Send output of cmd to file (overwrite), even if the shell’s noclobber option is set.


Redirection using file descriptors:
cmd >&n -> Send cmd output to file descriptor n.
cmd m>&n -> Same as previous, except that output that would normally go to file descriptor m is sent to file descriptor n instead.
cmd >&- -> Close standard output.
cmd <&n -> Take input for cmd from file descriptor n.

Multiple redirection
cmd 2>file -> Send standard error to file; standard output remains the same (e.g., the screen).
cmd > file 2>&1 -> Send both standard output and standard error to file.
cmd >& file -> Same as previous.
cmd &> file -> Same as previous. Preferred form.

cmd &>> file -> Append both standard output and standard error to file.
cmd > f1 2> f2 -> Send standard output to file f1 and standard error to file f2.
cmd | tee files -> Send output of cmd to standard output (usually the terminal) and to files. See tee(1).
cmd 2>&1 | tee files -> Send standard output and error output to screen and files.
cmd |& tee files ->Same as previous.

/dev/stdin -> A duplicate of file descriptor zero.
/dev/stdout -> A duplicate of file descriptor one.
/dev/stderr -> A duplicate of file descriptor two.

$ sed 's/^/XX /g' << EOF #Here document is sed's input
This is often how a shell archive is "wrapped",
bundling text for distribution. You would normally
run sed from a shell program, not from the command line.
EOF


${#var} Use the length of var.
${#*} Use the number of positional parameters.
${#@} Same as previous.

Built-in Shell Variables:

$# -> Number of command-line arguments.
$- -> Options currently in effect (supplied on command line or to set). The shell sets some options automatically.
$? -> Exit value of last executed command.
$$ -> Process number of the shell.
$! -> Process number of last background command.
$0 -> First word; that is, the command name.
$n -> Individual arguments on command line. If its more then 9 then we can use ${n}.
$*, $@ -> All arguments on command line ($1 $2 ...).
"$*" -> All arguments on command line as one string ("$1 $2..."). The values are separated by the first character in $IFS.
"$@" -> All arguments on command line, individually quoted ("$1" "$2" ...).
$_ -> Temporary variable;

BASHPID -> The process ID of the current Bash process. In some cases, this can differ from $$.
BASH -> The full pathname used to invoke this instance of Bash. [ e.g. echo $BASH ]
EUID -> Read-only variable with the numeric effective UID of the current user.
OLDPWD -> Previous working directory (set by cd).
PPID -> Process number of this shell’s parent.
PWD -> Current working directory (set by cd).
SECONDS[=n] -> Number of seconds since the shell was started.
BASH_ENV -> If set at startup, names a file to be processed for initialization commands.
IFS='chars' -> Input field separators; default is space, Tab, and newline.

PS1=string Primary prompt string; default is $.
PS2=string Secondary prompt (used in multiline commands); default is >.
PS3=string Prompt string in select loops; default is #?.

Array:

message=(how are you message?)
echo ${message[0]}
echo ${message[1]}
echo ${message[2]}
echo ${message[3]}
echo ${message[*]}
echo ${#message[*]}  -> 4  [ display total number of elements in the array message ]

${name[i]} Use element i of array name. i can be any arithmetic expression as described under let.
${name} Use element 0 of array name.
${name[*]} Use all elements of array name.
${name[@]} Same as previous.
${#name[*]} Use the number of elements in array name.
${#name[@]} Same as previous.


Special Prompt Strings:
Bash processes the values of PS1, PS2, and PS4 for the following
special escape sequences:
\a An ASCII BEL character (octal 07).
\A The current time in 24-hour HH:MM format.
\d The date in “weekday month day” format.
\D{format} The date as specified by the strftime(3) format format. The braces are required.
\e An ASCII Escape character (octal 033).
\h The hostname, up to the first period.
\H The full hostname.
\j The current number of jobs.
\l The basename of the shell’s terminal device.
\n A newline character.
\r A carriage return character.
\s The name of the shell (basename of $0).
\t The current time in 24-hour HH:MM:SS format.
\T The current time in 12-hour HH:MM:SS format.
\u The current user’s username.
\v The version of Bash.
\V The release (version plus patchlevel) of Bash.
\w The current directory, with $HOME abbreviated as ~.
\W The basename of the current directory, with $HOME abbreviated as ~.
\! The history number of this command (stored in the history).
\# The command number of this command (count of commands executed by the current shell).
\$ If the effective UID is 0, a #; otherwise, a $.
\@ The current time in 12-hour a.m./p.m. format. Variables | 35
\nnn The character represented by octal value nnn.
\\ A literal backslash.
\[ Start a sequence of nonprinting characters, such as for highlighting or changing colors on a terminal.
\] End a sequence of nonprinting characters.


Arithmetic Expressions:

$(( expr )) Use the value of the enclosed arithmetic expression.
++ -- Auto-increment and auto-decrement, both prefix and postfix
+ - Unary plus and minus
! ~ Logical negation and binary inversion (one’s complement)
** Exponentiationa
* / % Multiplication, division, modulus (remainder)
+ - Addition, subtraction
<< >> Bitwise left shift, bitwise right shift
< <= > >= Less than, less than or equal to, greater than, greater than or equal to
== != Equality, inequality (both evaluated left to right)
& Bitwise AND
^ Bitwise exclusive OR
| Bitwise OR
&& Logical AND (short circuit)
|| Logical OR (short circuit)
?: Inline conditional evaluation

Command editing keystrokes: [ Line edit mode ]

set -o vi or set -o emacs [ NOTE: First words as if its vi and second one if it set to emacs ]

h CTRL-b Move back one character.
l CTRL-f Move forward one character.
b ESC-b Move back one word.
w ESC-f Move forward one word.
X DEL Delete previous character.
x CTRL-d Delete character under cursor.
dw ESC-d Delete word forward.
db ESC-h Delete word backward.
xp CTRL-t Transpose two characters.

function name () { commands; }

amitmund@amitmundlaptop:~$ kill -l
 1) SIGHUP     2) SIGINT     3) SIGQUIT     4) SIGILL     5) SIGTRAP
 6) SIGABRT     7) SIGBUS     8) SIGFPE     9) SIGKILL    10) SIGUSR1
11) SIGSEGV    12) SIGUSR2    13) SIGPIPE    14) SIGALRM    15) SIGTERM
16) SIGSTKFLT    17) SIGCHLD    18) SIGCONT    19) SIGSTOP    20) SIGTSTP
21) SIGTTIN    22) SIGTTOU    23) SIGURG    24) SIGXCPU    25) SIGXFSZ
26) SIGVTALRM    27) SIGPROF    28) SIGWINCH    29) SIGIO    30) SIGPWR
31) SIGSYS    34) SIGRTMIN    35) SIGRTMIN+1    36) SIGRTMIN+2    37) SIGRTMIN+3
38) SIGRTMIN+4    39) SIGRTMIN+5    40) SIGRTMIN+6    41) SIGRTMIN+7    42) SIGRTMIN+8
43) SIGRTMIN+9    44) SIGRTMIN+10    45) SIGRTMIN+11    46) SIGRTMIN+12    47) SIGRTMIN+13
48) SIGRTMIN+14    49) SIGRTMIN+15    50) SIGRTMAX-14    51) SIGRTMAX-13    52) SIGRTMAX-12
53) SIGRTMAX-11    54) SIGRTMAX-10    55) SIGRTMAX-9    56) SIGRTMAX-8    57) SIGRTMAX-7
58) SIGRTMAX-6    59) SIGRTMAX-5    60) SIGRTMAX-4    61) SIGRTMAX-3    62) SIGRTMAX-2
63) SIGRTMAX-1    64) SIGRTMAX   


Some Commands:

times : Print accumulated CPU times.
ulimit : Manage various process limits.
unalias : Remove previously defined aliases.
until : Syntax for a loop that runs until a condition becomes true.
wait : Wait for a process or job to complete. ( e.g.: wait [ID] )

Networking Fundamentals

Network Fundamentals:

The Seven OSI Layers:

7. Application
6. Presentation
5. Session
4. Transport
3. Network
2. Link
1. Physical

http://en.wikipedia.org/wiki/OSI_model


Private IP Address: -

10.0.0.0  -  10.255.255.255
172.16.0.0  -  172.31.255.255
192.168.0.0  - 192.168.255.255

Loopback address: 127.0.0.1

Network Class:

Class A: 00000000 - 01111111 -> ( 0 - 127 )
Class B: 10000000 - 10111111 -> ( 128 - 191 )
Class C: 11000000 - 11011111 ->  ( 192 - 223 )
Class D: 11100000 - 11101111 -> ( 224 - 239 )
Class E: 11110000 - 11110111 -> (240 - 247 )

0 -> 00000000
1 -> 00000001
2 -> 00000010
3 -> 00000011
4 -> 00000100
5 -> 00000101
6 -> 00000110
7 -> 00000111
8 -> 00001000

SettingUpAWSapi

Setting Up AWS api:  [ Amazon command line tools ] :

I am using ubuntu 12.10 and for the long time I was getting error while setting up AWS api in my system:

export JAVA_HOME=/usr/lib/jvm/java-7-openjdk-amd64/jre

export EC2_HOME=~/ec2

export EC2_PRIVATE_KEY=<Private_key_path>

export EC2_CERT=<Cert_path>

For other aws api:

export AWS_ELB_HOME=~/elb

export AWS_CLOUDWATCH_HOME=~/cw

PATH=$PATH:$JAVA_HOME/bin:$EC2_HOME/bin

Note:
You can create different scripts, where you specify different EC2_CERT and EC2_PRIVATE_KEY to access your different aws account, example [dev, qa or prod]

For other api call: 

http://aws.amazon.com/developertools/

For AMI CLI: 
http://awsiammedia.s3.amazonaws.com/public/tools/cli/latest/IAMCli.zip

For ElasticBeanstalk CLI:

https://s3.amazonaws.com/elasticbeanstalk/cli/AWS-ElasticBeanstalk-CLI-2.6.0.zip

For ELB CLI:

http://ec2-downloads.s3.amazonaws.com/ElasticLoadBalancing.zip

For AutoScalling CLI:

http://ec2-downloads.s3.amazonaws.com/AutoScaling-2011-01-01.zip

For S3 CLI:

http://code.google.com/p/lits3/downloads/detail?name=LitS3-Commander-0.8.4.zip

Even after above setting I was getting java related error... and I stared trying solving the same issue, checked some on-line links and found lots of people have similar issue. [ I was my mistake that I was keeping all the aws command in a same directory. ]

I removed all the jre and java related package from my ubuntu system and reinstall those pkg again but the similar issue.

The error was something like:

ec2-describe-regions

Error: Could not find or load main class com.amazon.aes.webservices.client.cmd.DescribeRegions

Then I started looking around the JAVA CLASSPATH... again spent some more time. But the good part that I understand that in hard way... after working for long hours. As a SRE/DEVOPS person, when started from Linux System Administrator, understanding java error took some time :)

Started looking the aws command code:

amitmund@amitmundlaptop:~/ec2/bin$ cat ../ec2-api-tools-1.6.0.0/bin/ec2-describe-regions

#!/usr/bin/env bash

# Copyright 2006-2010 Amazon.com, Inc. or its affiliates.  All Rights Reserved.  Licensed under the

# Amazon Software License (the "License").  You may not use this file except in compliance with the License. A copy of the

# License is located at http://aws.amazon.com/asl or in the "license" file accompanying this file.  This file is distributed on an "AS

# IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific

# language governing permissions and limitations under the License.

__ZIP_PREFIX__EC2_HOME="${EC2_HOME:?EC2_HOME is not set}"

__RPM_PREFIX__EC2_HOME=/usr/local/aes/cmdline

"${EC2_HOME}"/bin/ec2-cmd DescribeRegions "$@"

 And then followed the ec2-cmd:

amitmund@amitmundlaptop:~/ec2/bin$ cat ec2-cmd

#!/usr/bin/env bash

# Copyright 2006-2009 Amazon.com, Inc. or its affiliates.  All Rights Reserved.  Licensed under the

# Amazon Software License (the "License").  You may not use this file except in compliance with the License. A copy of the

# License is located at http://aws.amazon.com/asl or in the "license" file accompanying this file.  This file is distributed on an "AS

# IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific

# language governing permissions and limitations under the License.

# This script "concentrates" all of our Java invocations into a single location

# for maintainability.

# 'Globals'

__ZIP_PREFIX__EC2_HOME="${EC2_HOME:-EC2_HOME is not set}"

__RPM_PREFIX__EC2_HOME=/usr/local/aes/cmdline

LIBDIR="${EC2_HOME}/lib"

# Check our Java env

JAVA_HOME=${JAVA_HOME:?JAVA_HOME is not set}

# If a classpath exists preserve it

CP="${CLASSPATH}"

# Check for cygwin bash so we use the correct path separator

case "`uname`" in

    CYGWIN*) cygwin=true;;

esac

# ---- Start of Cygwin test ----

cygprop=""

# And add our own libraries too

if [ "${cygwin}" == "true" ] ; then

    cygprop="-Dec2.cygwin=true"

    # Make sure that when using Cygwin we use Unix

    # Semantics for EC2_HOME

    if [ -n "${EC2_HOME}" ]

    then

        if echo "${EC2_HOME}"|egrep -q '[[:alpha:]]:\\'

        then

            echo

            echo " *INFO* Your EC2_HOME variable needs to specified as a Unix path under Cygwin"

            echo

        fi

    fi

# ---- End of Cygwin Tests ----

    for jar in "${LIBDIR}"/*.jar ; do

        cygjar=$(cygpath -w -a "${jar}")

        CP="${CP};${cygjar}"

    done

else

    for jar in "${LIBDIR}"/*.jar ; do

        CP="${CP}:${jar}"

    done

fi

CMD=$1

shift

"${JAVA_HOME}/bin/java" ${EC2_JVM_ARGS} ${cygprop} -classpath "${CP}" "com.amazon.aes.webservices.client.cmd.${CMD}" $EC2_DEFAULT_ARGS "$@"

 then I found it is looking for classpath and the jar files from the same directory's lib folder from were I have copied the aws commands.

 Later I created a "lib" directory in the same location where I created the "bin" directory for all the aws command line tools and started coping the "jar" files from all the original directory to this "lib" directory and its started working. :)

so.... where I was doing mistake?

I have downloaded all the aip from Amazon site... and it around few different zip files from windows and linux environment...

e.g:

AutoScaling-2010-08-01.zip

CloudWatch-2010-08-01.zip

ec2-api-tools.zip  RDSCli.zip

AWSCloudFormation-cli.zip 

ec2-ami-tools.zip        

IAMCli.zip

But what I did, to set the a single EC2_HOME directory, I extracted all these directory and copied all the aws command from those directories "bin" directory and copied them to ~/ec2/bin and not the related "lib" directory where all the related jar files were there.  Because of that I was getting the following error:

Error: Could not find or load main class com.amazon.aes.webservices.client.cmd.DescribeRegions

puppetlabsImpLinks

Important Links from Puppetlabs.com:


http://puppetlabs.com/resources/webinarshttp://puppetlabs.com/resources/webinars


http://docs.puppetlabs.com/
https://puppetlabs.com/misc/pdf-doc/
http://info.puppetlabs.com/download-learning-puppet-VM.html
http://docs.puppetlabs.com/references/latest/type.html
http://docs.puppetlabs.com/references/
https://puppetlabs.com/blog/
https://puppetlabs.com/resources/overview-2/
http://docs.puppetlabs.com/learning/ral.html
https://puppetlabs.com/blog/facter-part-1-facter-101/

configuring linux system as router

echo 1 > /proc/sys/net/ipv4/ip_forward

/etc/sysctl.conf:

net.ipv4.ip_forward = 1

configure A for NAT

Now that we have a connection from A to B, we can tell A to share internet connection with B.

• Go to computer A and share its internet connection with B by typing the two commands :

modprobe iptable_nat
echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A FORWARD -i eth1 -j ACCEPT

Now you do a ping test from remote system

ping 8.8.8.8  [ should be working ]

then check ping www.google.com   [ if not working then check for /etc/resolve.conf ]

• Run this script on the host A :

#!/usr/bin/env bash
modprobe iptable_nat
echo 1 > /proc/sys/net/ipv4/ip_forward
ifconfig eth1 192.168.0.1 netmask 255.255.255.0
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A FORWARD -i eth1 -j ACCEPT

• Run this script on the host B where xx.xx.xx.xx is your dns server :

#!/usr/bin/env bash
ifconfig eth0 down
ifconfig eth0 192.168.0.2 netmask 255.255.255.0
route del -net default 2>/dev/null
route add default gw 192.168.0.1 2>/dev/null
echo "nameserver xx.xx.xx.xx" > /etc/resolv.conf

few more links:

http://how-to.wikia.com/wiki/How_to_set_up_a_NAT_router_on_a_Linux-based_computer

http://martybugs.net/wireless/router.cgi

http://gohgarry.wordpress.com/2010/04/16/how-to-setup-linux-os-as-router/

http://forum.codecall.net/topic/48438-configuring-your-linux-server-as-a-router/#axzz2J0AHaM3O

http://glonek.co.uk/linux-router/basic-linux-router/