bc

bc  is an arbitrary-precision arithmetic language in unix.
 

Syntax:
bc [-c] [-1] [file...]

bc is an interactive processor for a language that resembles C but providesunlimited precision arithmetic.It takes the input from any of the given files, then reads the standard input. bc is actually a preprocessor for the desk calculator program dc, which it  invokes automatically unless the -c option is present. In this case the dc input is sent to the standard output.

-c: Compile only.The output is sent to the standard output.
-1: Argument stands for the name of an arbitrary precision math library.

The syntax for bc programs is as follows:

here L means the letters a-z

         E means expression

         S means statement

Comments are enclosed in /* and*/

 

Names:

   simple variables:L

   array elements:L[E]

   the words ibase (for input base),  obase( for output base) and  scale

Other operands:

   arbitrarily long numbers with optional sign and decimal point(E)

   sqrt(E)

   length(E) : number of significant decimal digits

   scale(E):   number of digits right of decimal point.

   L(E,...,E)

Operators:

  +-*/%^(% is for remainder and ^ is for power)

   ++,-- (prefix and postfix)

   == <= >= ! < >

   = =+ =- =* =/ =% =^

Statements:

  E

   {S;...;S}

  if(E)S

  while (E)S

  for(E;E;E)S

  null statement

  break

  quit

Function Definitions:

  defineL(L,...,L){

   auto L,..,L

   S;...S

   return(E)

}

Functions in -1 math library:

   s(x): sine

   c(x): cosine

   e(x): exponential

   l(x) :  log

   a(x): arctangent

   j(n,x): Bessel function

All function arguments are passed by value.

The value of  a statement  that is an expression is printed unless the main operator is an assignment. Either semicolons or newlines may separate statements.Assignments to scale influences the number of digits to be retained on arithmetic operations in the manner of dc.Assignments to  ibase  or obase set the input and output number radix respectively.

The same letter may be used as an array, a function and  a simple variable simultaneously. All variables are global to the program. auto variables are pushed down during function calls.When using array  as function arguments, or when defining them as automatic variables,empty square brackets must follow the array name.

The files used for bc command are

/usr/lib/lib.b   mathematical library

/usr/bin/dc     desk calculator proper

Also,

    the bc command does not recognise the logical operators && and ||.
    the for statement must have all three expressions
    the quit statement is interpreter when read not executed.

The echo command

The echo command echoes its arguments separated by blanks and terminated by a new line on the standard output.
Syntax:
              echo [arg] 
              echo[-n][arg]


The /user/bin/sh version understands the following C-like escape sequences:
 \b  backspace
 \c  print line without newline
\n   new--line
\r    carriage return
\t    tab
\v   vertical tab
\\    backslash
\n   where n is the 8 bit character whose ASCII code is the 1,2or 3 digit octal number representing that character.

The following option is available to /usr/bin/sh users only if /usr/ucb preceeds /usr/bin in the user's path.It is available to  /usr/csh users regardless of the PATH:
-n  Do  not add newline to the output

The echo command is useful for producing diagnostics in command files, for sending known data into a pipe, and for displaying the contents of environment variables.

The next command will be bc which is an arbitrary-precision arithmetic language in unix

Choosing a Password and Password Files

Choosing a Password:


Passwords must be constructed to meet the following requirements:

• Each password must have atleast PASSLENGTH characters as set in /etc/default/passwd file.PASSLENGTH must contain a minimum of 6 characters, but only the first 8 characters are significant.
• Each  password must contain at least 2 alphabetic characters and atleast 1 numeric or special character.
• Each password must differ from the user's login name and any reverse or circular shift of that login name. For comparison purposes, an upper case letter and its corresponding lower case letter are equivalent .

• New passwords must differ from the old by atleast 3 characters.

Password Files:

The passwd  command uses the files /etc/shadow, /etc/passwd and /etc/oshadow.
The file /etc/passwd contains the following information for each user:

• login name
• dummy password
• numerical user ID
• numerical group ID
• comment
• initial working directory
• program to be used as the shell

passwd is an ASCII file . Each field within each user's entry is separarted from the next by a colon. The comment field  can contain any information that the user desires. each user is separated from the next by a new-line. If the shell field is null, /usr/bin/sh is used.

This file has user login information and general read permission. It can therefore be used , to map numerical user ID's to names.
The password field consists of the character x.Password information is contained in the file /etc/shadow.




The next command will be the echo command.

The passwd command- continued

To recollect the syntax of the passwd command is:
              passwd[name]
              passwd[-l |-d ][-f][-n min][ -x max][-w warn] name
              passwd -s [-a]
              passwd -g [name]


The following options can be used only by a privileged user:

• -l: Locks password entry for name.
• -d: Deletes password for name. The login name will not be prompted for password.
• -n: Sets minimum field for name.  The  min field contains the minimum number of days between password changes for  name.  If value of  min  >value of  max, the user may not change the password.Always use this option with the -x option, unless max is set to -1(ageing turned off ). In that case, min need not be set.
• -x: Sets maximum field for name. The max  field  contains the number of days that the password is valid for name. The ageing of name  will be turned off if max is set to -1.
• -w: Sets  warn  field for name. The warn  field contains the number of days the user will be warned before the password expires.
• -a:  Shows password attributes for all entries. Use only with -s option ; name  must not be provided.

• -f:  Forces the user to change password at the next login by expiring the password for name.

 Any user may use the -s option to show password attributes for one's own login name.
The format of the output will be:
name status mm/dd/yy min max warn

 Or, if password ageing information is absent,
name status

Super users may change any password; hence; passwd does not prompt privileged users for the old password. Privileged users are not forced to comply with password ageing and password construction requirements. A privileged user can create a null password by entering a carriage return in response to the prompt for a new password.

The passwd command exits with one of the following values:
0 SUCCESS
1 Permission denied
2 Invalid combination of options
3 Unexpected failure. Password file unchanged.
4 Unexpected failure .Password file is missing.
5 Password file(s) busy .Try again later
6 Invalid argument to option.


If root deletes a password for a user with the passwd -d command and password ageing is in effect for that user, the user will not be allowed to add a new password until the NULL password has been aged.This is true even if  the PASSREQ flag in /etc/login/default  is set to YES. This results in a  user without a password. It is recommended that the -f option be used with the  -d option so that the user is forced to change the password at the next login.


The next post will list the rules for choosing a password and the details of the files used by the passwd command.

The passwd command

The passwd command changes the password or lists attributes assosciated with the user's login name. Additionally, privileged users may use this command to install or change passwords and attributes assosciated with any login name.

Syntax:
              passwd[name]
              passwd[-l |-d ][-f][-n min][ -x max][-w warn] name
              passwd -s [-a]
              passwd -g [name]

  The behaviour of this command is extremely system-dependent.It asks you to enter your old  password, and if that is typed correctly,prompts you to enter your new password twice.The password may be chosen by a unix system or customized by the user. Some system administrators implement password aging which for security reasons forces the user to change the password after a certain period of usage.

Unix updates the corresponding files /etc/passwd  and /etc/shadow  for every change in password.
/etc/passwd  lists information about users.A user can read this file but not write into it. Each field of information is separated from the next by a ':' .An /etc/passwd file  dispalys login name, encrypted password,user ID, group Id, comment , default  working directory, default working shell.

There is another password file present in  /bin  directory called /bin/passwd . It is this file that actually gets executed when we change the password. Thus, /bin/passwd  is  an executable file which permits changing of password, whereas /etc/passwd  contains the information about each user.
 I shall discuss the options of the passwd command in the next post.

The uname command

The command uname prints the name of the current unix system.

Syntax:
uname[-amnprsv]
uname[-S system_name]
uname prints the current system name of the unix system to standard output.It is mainly useful to determine which system one is using.The options cause selected information returned by uname and/or sysinfo command to be printed:

• -a : Print all information.
• -m: Print the machine hardware name.
• -n:  Print the node name(name by which the system is known to the communication  network)   .This is the default setting.
• -p: Print the processor type of the current host.
• -r: Print the operating system release.
• -s:Print the name of the operating system.
• -v: Print the version of the operating system.

 On your computer specifying a system name argument to the -S option may change the node name. The system name argument is restricted to SYS_NMLN  characters.SYS_NMLN is an implementation specific value defined in <sys/utsname.h>  file. Only the super-user is allowed to use this capability.

The next post will talk about the passwd command.

The tty command

The unix command tty displays the name of the terminal type.tty is the controling terminal interface.
The file /dev/tty is, in each process a synonym for the control terminal assosciated with the process group of that process. It can be used for programs that demand the name of a file for output, when typed output is desired .

Syntax:
            tty[-l][-s]
 The tty command prints the path name of the user terminal.
-l prints the synchronous line number to which the user's terminal is connected, if it is on an active
    synchronous line.
-s inhibits the printing of the terminal path name, allowing one to just test the exit code.
    The exit codes are as follows:
 2 if invalid options were specified
 0 if standard input is a terminal.
 1 otherwise

The error message  not an active synchronous line,  appears if the standard input is not a synchronous terminal and -1 is specified.

The error message  not a tty, appears if the standard input is not a terminal and -s is not specified.