Shell Programming

Question 1

Sequential Logic

Answer 1

to execute commands in the order in which they appear

Question 2

Decision Logic

Answer 2

to execute commands only if a certain condition is satisfied

Question 3

Looping Logic

Answer 3

to repeat a series of commands for given number of times

Question 4

Case Logic

Answer 4

to replace “if then/else if/else” statements when making numerous comparisons

Question 5

Positional Parameters

Answer 5

special variables used when shell scrit or shell function is called with argument parameters

Question 6

$#

Answer 6

number of input parameters

Question 7

$0

Answer 7

Name of the script

Question 8

$1, $2, $3, …. , $9

Answer 8

first, second, third, and 9th argument parameter

Question 9

$@

Answer 9

List of input parameters

Question 10

$*

Answer 10

List of input parameters as space separated string

Question 11

shift

Answer 11

shifts the positional parameters by one towards the beginning and drops $1 from the list. After a shift, $2 becomes $1 and so on (note if more than 9 arguments, they cannoted be directly accessed by $1 to $9 . Must use shift command

Question 12

Example of Positional Variables

Answer 12

cat myinputs.sh
    #!/bin/sh
    echo Total number of inputs: $#
    echo First input: $1
    echo Second input: $2

$ chmod u+x myinputs.sh
$ ./myinputs.sh ONE TWO BUCKLE MY SHOE
Total number of inputs: 5
First input: ONE
Second input: TWO

Question 13

Decision Logic: Conditional

Answer 13

if test-command
then
    execute command
elif test-command
then
    execute this
    and execute this command
else
    execute default command
fi

Question 14

What is a TEST command?

Answer 14

testis a built in bash commands. that returns true (0) ir false (non-zero) for a given set of arguments

Question 15

Arguments structure for TEST

Answer 15

Argument structure is:
test item1 comparator item2

or

test option item1

• Instead of calling test, can use square brackets:
[ item1 comparator item2 ]
[ option item1 ]

you must have spaces around the brackets

Question 16

test Options for File Inquiry

Answer 16

[ -d filename ] Test if filename is a directory
[ -f filename ] Test if filename is not a directory
[ -s filename ] Test if filename has non zero length
[ -r filename ] Test if filename is readable
[ -w filename ] Test if filename is writable
[ -x filename ] Test if filename is executable
[ -o filename ] Test if filename is owned by the user
[ -e filename ] Test if filename exists
[ -z filename ] Test if filename has zero length
All these conditions return exit status code ($?) of true (0) if satisfied and false (1) otherwise

Question 17

[ -d filename ]

Answer 17

Test if filename is a directory

Question 18

[ -f filename ]

Answer 18

Test if filename is not a directory

Question 19

[ -s filename ]

Answer 19

Test if filename has non zero length

Question 20

[ -r filename ]

Answer 20

Test if filename is readable

Question 21

[ -w filename ]

Answer 21

Test if filename is writable

Question 22

[ -x filename ]

Answer 22

Test if filename is executable

Question 23

syntax for Combining Tests

Answer 23

&& represents AND
|| represents OR

Syntax:
if cond1 && cond2 || cond3 …
An alternative form is to use a compound statement in test using
–a and –o keywords. For example:
if [ cond1 –a cond2 –o cond3 … ]
where cond1, cond2, cond3 are either commands returning a
value or test conditions of the form:
test arg1 op arg2 or [ arg1 op arg2 ]

Question 24

Looping Logic

Answer 24

A loop is a block of code that is repeated a number of times. Either:
– A pre-determined number of times determined by a list of items
in the loop count ( for loops), or
– Until a particular condition is satisfied ( while and until
loops)

To provide flexibility to the loop constructs there are also
two statements for control:
– continue : skips to the next item in a for loop
– break : exits out of a loop

Question 25

for Each Loops

Answer 25

for arg in list_args
do
     command1
     command2
     command3
done

where the value of variable arg is set to the values provided in
list_args one at a time and the block of statements is
executed. This is repeated until the list is exhausted.

Question 26

Iterating through for loops

Answer 26

Common programming practice to iterate through index

for (int index = 0; index <= 10; index++)...

In bash script, we can use sequence expression

{istart..iend[..incr]} where [..incr] is an
optional step size

for index in {1..10}; do…

Question 27

In terms of output to terminal:
$ ls -l > flist.txt; grep -e “-rwxrw—-“ flist.txt
is equivalent to
$ ls -l | grep -e “-rwxrw—-“

Answer 27

Instead of a program writing data to a file on disk and the next reading it, keep information in a buffer and pass it along

Difference in resulting file system and speed!
1. First command creates a file (flist.txt) in the local directory
2. Writing to/reading from a file is slow compared to accessing
memory

Question 28

man ls > ls.txt; wc < ls.txt

Standard IO redirect to (disk)

Answer 28

1. ‘man ls’ is sent to stdout buffer
2. then the buffer put the command to ‘ls.txt’ which is on the DISK
3. then the txt file (DISK) is put into the stdin buffer
4. The ‘wc’ command is then executed on the stdin buffer
5. finally outputs to the stdout bufffer

Question 29

man ls | wc

piping

Answer 29

1. man ls is put into stdout buffer memory
2. stdout buffer moves to stdin buffer
3. ‘wc’ executes on the information coming from stdin buffer
4. wc takes its output to the stdout buffer and is done.

Disk is never accessed, all data is accessed through buffer

Question 30

grep -e

Answer 30

-e option is to exclude

Question 31

chmod can be set with :

Answer 31

incremental permission change (+) or (-)
chmod ug+x myscript.sh
chmod 0-rwx myscript.sh

setting permission with “=”

perform multiple setting with “,”

Question 32

$ grep -i “juliet” shakespeare.txt > names.log
$ grep -i “romeo” shakespeare.txt&raquo_space; names.log
$ sort < names.log > names_sort.log
$ uniq < names_sort.log > names_uniq.log
$ wc –l < names_uniq.log

Answer 32

grep -i : case insensitive option, “juliet” and create names.log and second line just appends to the names.log for “romeo”

sort will put all the output in order by line

uniq will delete duplicate any lines where romeo and juliet are in the same line and leaving only 1 instance isntead of 2

uniq will delete any two lines that are the same next to each other.

wc -l : prints out to

Question 33

&laquo_space;HERE or <

Answer 33

will direct input (terminal or script) to stdin

Question 34

2> &1

Answer 34

redirect stderr to stdout

& means “same place as”

Question 35

Examples:
string1='15L Fina Review!!'
string2="Good luck on finals!!"
var2='$string1'
var3="$string2"

echo var2
echo $var2
echo $var3

Answer 35

Output:

var2
$string1
Good luck on finals!!

line1: without the quotes it literally echos
line2:

Question 36

Example:

var1=7
var2=expr $var1 + 7
var3=expr $var1+7
var4=”expr $var1 + 7”

echo $var2
echo $var3
echo $var4
var3=

Answer 36

Output:

12
5+7
expr 5 + 7

line1: runs the expression and command in actually adding it. You do need spaces for EXPR cmd
line2: doesn’t uses spaces so it just prints out the string
line3: it doesn’t know to treat expr as a command since its using “ “ instead of ` `

Question 37

Example:

var1=7
var3=$[$var1+8]
var5=$(($var1 + 7))

echo $var3
echo $var5

Answer 37

Output:

15
14

bracketts are for older syntax, but (( )) is more associated with expr

when brackets don’t need spaces
when (( )) NEED spaces since its interpreted like ` `

Question 38

echo is sueful when trying to debug scripts

echo [options] stringtoprint

using “ “ is weakly quotes

using ‘ ‘ is strongly quoted

In weakly quoted strings, references to variables are replaced y the value of those variables before the output

Answer 38

common options

• e : expand \ special characters
• n : do not output a new-line at the end

”” : will dereference the variable and give the value

’ ‘ : will be taken literally nothing is treated as special

` ` : enclosed string is treated as a command and shell attempts to execute it, if successful the primary output from command replaces the string