Bourne-Again Shell

Question 1

Unix shell provides ___

Answer 1

an interface that lets the user interact with the operating system by running commands

Question 2

To bash, there are three kinds of tokens:

Answer 2

1. reserved words
2. words
3. operators

Question 3

reserved words

Answer 3

words that have meaning to the shell and its programming language; e.g. if and while

Question 4

operators

Answer 4

composed of one or more metacharacters

Question 5

metacharacters

Answer 5

characters that have special meaning to the shell on their own, e.g. | and >

Question 6

words

Answer 6

sequences of characters separated by metacharacters, which include simple separators like spaces and tabs, or characters that are special to the shell language, like semicolons and ampersands

Question 7

word beginning with a dollar sign introduces ___

Answer 7

a variable or parameter reference

Question 8

all variables are ___ by default

Answer 8

global

Question 9

redirections allow the user to ___

Answer 9

control the input to and output from invoked commands

Question 10

pipeline

Answer 10

a linear list of commands in which the output of one command in the list becomes the input of the next

Question 11

Bash reads from ___ when interactive, and ___ specified as an argument otherwise

Answer 11

the terminal

the script file

Question 12

Bash uses the ___ to implement command line editing.

Answer 12

readline library

Question 13

Readline is structured as a ___.

Answer 13

read/dispatch/execute/redisplay loop

Question 14

To update the terminal display, readline must track three things:

Answer 14

1. the current contents of the buffer of characters displayed on the screen
2. the updated contents of that display buffer
3. the actual characters displayed

Question 15

In the presence of multibyte characters, ___, and the redisplay engine must take that into account.

Answer 15

the characters displayed do not exactly match the buffer

Question 16

Readline’s approach to the string-to-string correction problem is ___

Answer 16

1. identify the beginning and end of the portion of the buffer that differs
2. compute the cost of updating just that portion, including moving the cursor backward and forward
3. perform the lowest-cost update
4. clean up by removing any characters remaining at the end of the line if necessary and position the cursor in the correct spot

Question 17

Most modifications to the redisplay engine have been to ___ and ___.

Answer 17

1. have non-displaying characters in the prompt (such as to change colours)
2. cope with characters that take up more than a single byte

Question 18

When the shell is not using readline, it uses either ___ or ___ to obtain input.

Answer 18

stdio

its own buffered input routines

Question 19

The shell must ___ when reading from non-seekable devices such as pipes, but ___ when reading from files.

Answer 19

read scripts a character at a time

may buffer as many characters as it likes

Question 20

When in a locale that supports multibyte characters, the shell ___.

Answer 20

stores its input in a buffer of bytes but treats these bytes as potentially multibyte characters

Question 21

The initial job of the parsing engine is lexical analysis:

Answer 21

to separate the stream of characters into words and apply meaning to the result

Question 22

basic unit upon which the parser operates is a ___

Answer 22

word

Question 23

The lexical analyzer steps:

Answer 23

1. takes lines of input from readline or another source
2. breaks them into tokens at metacharacters
3. identifies the tokens based on context
4. passes them on to the parser to be assembled into statements and commands

Question 24

Aliasing is implemented in the lexical phase, though the parser has to ___

Answer 24

inform the analyzer when expansion is permitted

Question 25

Three types of quoting

Answer 25

backslash
single quote
double quote

Question 26

backslash

Answer 26

escapes the next character

Question 27

single quotes

Answer 27

prevents interpretation of all enclosed characters

Question 28

double quotes

Answer 28

prevents some interpretation but allows word expansions, and treats backslashes differently

Question 29

interface between parser and lexical analyzer:

Answer 29

parser encodes a certain amount of state and shares it with the analyzer to allow the sort of context-dependent analysis the grammar requires

Question 30

After parsing, but before execution, many words are subject to ___

Answer 30

word expansion

Question 31

types of variable expansion

Answer 31

1. expansion of variable string values into new words and word lists
2. act on the variable’s value itself
3. some can depend on the state of the variable, e.g. set or not set

Question 32

order of expansions

Answer 32

1. brace expansion
2. command substitution
3. tilde expansion
4. arithmetic expansion

Question 33

The results of the word expansions are ___

Answer 33

split using the characters in the value of the shell variable IFS as delimiters

Question 34

After the results are split, the shell interprets each word as a potential pattern and ___

Answer 34

tries to match it against an existing filename, including any leading directory path

Question 35

During the command execution phase, ___

Answer 35

the set of expanded words is decomposed into a command name and set of arguments, and passed to the operating system as a file to be read and executed with the remaining words passed as the rest of the elements of argv

Question 36

The input to the command execution phase is

Answer 36

the command structure built by the parser and a set of possibly-expanded words

Question 37

builtin commands are executed by the shell without ___

Answer 37

creating a new process

Question 38

When the shell executes jobs in the foreground, ___

Answer 38

it waits for the command to finish and collects its exit status

Question 39

When the shell executes jobs in the background, ___

Answer 39

it immediately reads the next command

Question 40

job control

Answer 40

the ability to move processes between the foreground and background, and to suspend and resume their execution

Question 41

compound commands

Answer 41

consist of lists of one or more simple commands and are introduced by a keyword such as if or while

Question 42

Lessons learned:

Answer 42

1. detailed change logs
2. extensive regression testing from the beginning
3. standards, including participation in standardization process
4. internal standards
5. good documentation
6. use what other software libraries that you can
7. engage the user community, but don’t take it personally :)”

Question 43

Would have done differently:

Answer 43

1. introduced formal bash compatibility levels earlier
2. more frequent releases, using some kind of public repository
3. would have written a parser by hand