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The correct answer is (b) STANDARD output

The best I can explain: The SHELL enables one or more command ARGUMENTS to be obtained from the standard output of another command. This feature is called command substitution and it requires the command to use the standard output (STREAM).

The correct answer is (d) $cd

For explanation: PATH is the shell variable which stores the LIST of directories that the shell searches for locating commands. HOME prints our home directory while SHELL prints the absolute pathname of our LOGIN shell. cd is a command which is used for changing directories or moving through file HIERARCHY.

Correct option is (c) -txtfile

The explanation is: UNIX restricts some RULE for defining a VARIABLE. A variable name MUST BEGIN with either an underscore (_) or alphanumeric character followed by ONE or more alphanumeric or underscore characters.

Right choice is (b) set

For explanation I would say: The C shell USES the set statement to set VARIABLES. There is a restriction that there either has to be whitespace on both sides of = or none at all. For EXAMPLE, for assigning a value to a variable named count, USE the following command:

The CORRECT OPTION is (a) True

To elaborate: A variable can be protected from reassignment by readonly command. unset is also an INTERNAL command. For example, to PROTECT a variable x from reassignment USE the following command,

The correct CHOICE is (a) UNSET

The EXPLANATION: A variable can be REMOVED using the unset command. unset in an internal command. For example, if we want to undefine a variable x then,

Correct answer is (c) =

EXPLANATION: The SHELL supports variables that are useful both in the command line and shell SCRIPTS. These variables are called shell variables like pwd.A variable ASSIGNMENT is of the form variable=value(no SPACES around =). For example,

Right option is (d) bourne

Easy explanation: Whether or not we USE POSIX NOTATION for command substitution is something up to the USER. But we should make sure that we do not have to run our SHELL scripts with the Bourne shell because $(command) is not SUPPORTED by Bourne shell.

Right answer is (d) $

Explanation: POSIX recommends the use of the form $(command) instead of archaic `command` for command SUBSTITUTION. For example, to display the DATE USING command substitution use the FOLLOWING command:

Correct option is (a) command substitution

Explanation: APART from a pipeline, SHELL enables connecting of two commands in ANOTHER WAY. Shell enables one or more arguments to be obtained from the standard output of another command. This feature is called command substitution. For EXAMPLE, to display output like:

Correct CHOICE is (a) `

The explanation is: When SCANNING the command line, the SHELL looks for another metacharacter i.e. ` (the backquote) usually placed on the top-left of our keyboard, and it should not be CONFUSED with a single quote (‘).

The correct option is (a) True

To EXPLAIN I would SAY: /dev/tty can be used as an argument to some UNIX commands. Because the TERMINAL is also a file we can USE the device name /dev/tty as an argument to some commands like tee. For example,

The correct choice is (a) it is a feature of the SHELL

Explanation: TEE command is not a feature of the shell. tee command HANDLES a character stream by DUPLICATING its input. It saves one copy in a file and writes the other to standard output. It is an external command.

The correct answer is (b) False

For explanation: tee is an EXTERNAL command and not a feature of the shell. tee command handles a character STREAM by DUPLICATING its input. It saves one copy in a file and WRITES the other to standard OUTPUT.

Correct option is (c) tee

Explanation: tee command handles a CHARACTER stream by DUPLICATING its input. It saves ONE COPY in a file and WRITES the other to standard output. For example,

Right ANSWER is (a) True

To ELABORATE: There is a restriction which is applied while using a pipeline. In a pipeline, the COMMAND on the left of | must use STANDARD output (stream)and one on the right must use standard input (stream). Otherwise, an error will be generated or the output will be erroneous.

The correct option is (a) True

To explain: There is no RESTRICTION on the number of commands we can use in a pipeline EXCEPT that we should KNOW the behavioral properties of these commands to place there. For EXAMPLE, CONSIDER this command:

Right answer is (a) /dev/tty

For explanation I WOULD say: /dev/tty is another special file which is used for REPRESENTING ONE’s terminal. Consider user1 is working on terminal /dev/pts/1 and USER 2 is working on /dev/pts/2. Then, both the users can refer to their own TERMINALS with the same filename. For example, if user 1 issues the command

The CORRECT answer is (c) >

Best explanation: As we REDIRECT our output using > symbol to other files in simple commands, this technique can also be USED to redirect ERROR messages to /dev/null file. For example,

Correct answer is (d) /dev/null and /dev/tty

The explanation is: There are two special files named /dev/null and /dev/tty which is used in UNIX for special purposes. For example, if we want to check whether the program runs SUCCESSFULLY without seeing its output on the screen or may not want to SAVE output in a file either. For this purpose, a special file /dev/null is used. It accepts any STREAM without GROWING in size. The second file /dev/tty is the one used for indicating one’s terminal.

Right option is (a) True

To explain: UNIX has a special file that simply accepts any stream WITHOUT GROWING in size. This file is /dev/null. This file is used for redirecting error messages away from the terminal so that they don’t appear on the SCREEN. This file is pseudo-device because like other device FILES, it’s not ASSOCIATED with any physical device.

Right answer is (d) cat foo 2>errorfile

The best explanation: Redirecting standard error REQUIRES the 2> symbol. Suppose if foo doesn’t EXIST then an error message ‘cannot open foo’ will be generated but we can send this message to another FILE. But here > and >> cannot be used. We have to USE 2> symbol as the file descriptor number 2 represents the standard error stream.

The correct option is (a) True

To EXPLAIN I would say: When WC is used WITHOUT any arguments it will read the DATA from the default source which is a keyboard. In the above COMMAND, redirection symbol < is present. The following steps are performed:

1. On seeing the <, the shell opens the disk file, sample.txt for reading.

2. wc performs its execution and displays the output.

Right OPTION is (C) standard error

The best EXPLANATION: The standard error (or stream) is used for representing error messages that emanate from the COMMAND or shell. This stream is also connected to the display as error messages are DISPLAYED on the terminal.

Right option is (b) STANDARD output

Easy explanation: Standard output is the file (or STREAM)which is used for representing output, and is connected to the display. Each COMMAND which uses the display for its output will find this file always OPEN and available. The file will be automatically closed after the command has completed its EXECUTION.

Correct answer is (a) True

Explanation: Redirection is a process in which switching of the standard stream of data is PERFORMED so that it comes from a SOURCE other than its DEFAULT source or so that it goes to some destination other than its default destination.

Right CHOICE is (d) rm ‘*’ and rm \*

The explanation: To suppress the nature of wildcard * we can use either escaping or quoting. The \ symbol will suppress the feature of wildcard and will remove the file NAMED *. SIMILARLY using quoting we can TURN off the meaning of the META character.

The correct answer is (a) True

The best explanation: When a command ARGUMENT is enclosed in single quotes, the MEANING of all enclosed special characters is turned off. But when we enclose the same in double quotes, we cannot PROTECT the $ and ` (BACKQUOTE). For EXAMPLE,

Correct answer is (b) \

The best I can explain: The newline character is also special, it marks the end of the COMMAND line. Some command lines that uses several arguments can be long ENOUGH to OVERFLOW to the next line. To split the WRAPPED line into two lines, MAKE sure you input a \ before pressing [Enter].

Right ANSWER is (a) True

The explanation is: SOMETIMES we may need to INTERPRET the \ itself literally. To do so we need another \ before it. For example,

The correct answer is (a) RM my\ document.txt

For explanation: APART from metacharacters, there are other characters that are special LIKE the space CHARACTER. The shell uses it to delimit COMMAND line arguments. So to remove the file my document.txt we can use escaping. The backslash will make the shell to ignore the space. Hence file will be removed easily.

Correct option is (a) escaping

Explanation: We KNOW that SHELL uses some special characters to match filenames or perform other search and replace operations. But if the filename itself contains those special characters, then it could be a great nuisance. For DEALING with such files we use escaping and quoting.

Escaping means providing a backslash (\) before the wild card so that its special meaning could be removed. For EXAMPLE, if we want to remove a file named chap*, then using the command rm chap* will DELETE all the file beginning with a prefix ’chap’. In this situation, we can use the following command,

Right option is (a) True

The explanation is: As we know that CD command is used for changing directories, the command cd * will WORK only when there is only one file in the CURRENT directory and that file should ALSO be a directory. Otherwise, this command is invalid.

Correct answer is (d) / and .

The best I can explain: There are two THINGS which * and ? cannot match. First is, they cannot match filenames STARTING with a DOT (.). Second is, they cannot match / in the pathname. For example, we cannot use cd/usr?localto SWITCH to /usr/local. It will generate an error.

Right ANSWER is (c) !

Explanation: We can use the ! as the FIRST symbol for NEGATING the CHARACTER CLASS. For example,

Correct CHOICE is (C) ls.???*

Best explanation: The * doesn’t MATCH all files beginning with a (.) dot. So if we NEED to lists all the HIDDEN filenames in our directory having at least three characters after the dot (.) we can use the following command,

Right answer is (c) $

The explanation: The ‘$’ sign REPRESENTS the shell PROMPT while all the other CHARACTERS belong to a category of shell wildcards.