Shell Programming

Question 1

Shell Programming Topics

Answer 1

• Variables and Environment
• Parameters and Quoting
• Grouping
• Exit Values
• Testing
• Repeating
• Miscellaneous

Question 2

Shell Programming:

Variables and

Environment

Answer 2

• Shell supports local variables
  
  • local to a script
• Assigning:
  
  • name = value
• Use:
  
  • Reference using ‘$’
  • echo $name
  • Be careful of blanks(unassigned variables)
• You can export a variable to make it available to any other scripts invoked by the original script:
  
  • %export MODE = “smart”
  • %script2
  • This exports allows script2 to make use of the MODE variable

Question 3

Variables Inherited

from the Environment

Answer 3

File Descriptors:

• stdin
• stdout
• stderr

Locations:

• current working directory

Variables:

• any exported variables

Question 4

Shell Scripts:

Parameters

Answer 4

Parameters are passed to a script like command line arguments:

$myScript arg1 arg2 “some string”

The parameters are referenced in the script as numerical variables:

$@ - set of parameters

$1 - first argument

$2 - second argument, etc

Dollar signs trigger parameter substitution

Quotes or backslashes suppress parameter substitution

Question 5

Shell Scripts:

Command Substitution

Answer 5

Dollar signs with parenthesis run a command and substitute the stdout

form: $( command )

example:

“ab$(echo hello world)yz”

Will be seen as:

“abhello worldyz”

Text can also use older backquote system

Question 6

Shell Scripts:

Grouping:

Combine Commands on

one line

Answer 6

Simple: separate with a semicolon

Example:

thing = “blah blah”; echo $thing

This assigns the variable thing,

then echos it

Question 7

Shell Scripts:

Grouping:

Establish a New Context

Answer 7

Use parenthesis to establish a new context.

example:

(cd /home/sarah; pwd; echo “done”)

This changes the working directory, but only within the new context.

Commands only affect the current context.

Only inner context changed working directory.

Question 8

Shell Scripts:

Exit

Answer 8

Use command:

exit <status></status>

Status returns 0 on success (True)

nonzero on failure (False)

Question 9

Shell Scripts:

Comments

Answer 9

Use the #

example:

echo $myVar #stupid comment

Question 10

Shell Scripts:

Testing:

If Statements

Answer 10

do stuff

Nothing around condition, must end with “fi”,

explicitly state “then” and “else”

Form:

if condition

then

else

fi

Question 11

Shell Scripts:

Testing:

Bracket Program

Answer 11

The Bracket Program provides some Testing Functions

• Surround test with brackets
• [-d file] -> true if file exists and is directory
• [-f file] -> true if file exists and is not a directory
• [string1 = string2] -> Strings are identical
• [string1 != string2] -> Strings are different
• [file1 -nt file2] -> file1 is newer than file2
• other flags:
  
  • -e
  • -r
  • -w

Question 12

Shell Scripts:

Repeating:

For Loop

Answer 12

Form:

for x in <set></set>

do

….

done

Question 13

Shell Scripts:

Arithmetic:

Evaluate an expression

Answer 13

Use the “let” command prior to doing math:

let n=1

let n=($n + 2)*3; echo $n

let ++n ; echo $n

Output:

9

10

Question 14

Shell Scripts:

Including a file

Answer 14

Use the “.” command

This includes a file into a script as if it were typed there.

.external_script

Question 15

Shell Scripts:

While-Do Loop Form

Answer 15

perform actions

while [condition]

do

done

Question 16

Shell Scripts:

Double Parentheses Construct

(( ))

Answer 16

Similar to the let command,

the (( … )) construct permits arithmetic expansion and evaluation.

In its simplest form, a=$(( 5 + 3 )) would set a to 5 + 3, or 8.

Also a mechanism for allowing C-style manipulation of variables in Bash, for example, (( var++ ))

Question 17

Accessing the Clipboard:

Copy and Paste

(Mac OS only)

Answer 17

Use the utilities:

pbcopy

pbpaste

Copy the result of pwd command to clipboard as

$ pwd | pbcopy

Use the content in the clipboard as

$ cd $(pbpaste)