Simple projects to practice my bash skills and take notes.

Notes on Bash Programming

Operators

Operator	Example	Explanation
-	${< var >-< arg >}	Returns arg if var not set
:-	${< var >-< arg >}	Returns arg if var not set OR empty
=	${< var >=< arg >}	Returns arg and sets var if var not set
:=	${< var >:=< arg >}	Returns arg and set var if var not set OR var empty
+	${< var >+< arg >}	Returns arg if var is set if not returns nothing
?	${< var >?< message >}	Crashes process if var is not set

Scope

Variables are scoped to process.

export will make a variable available to child processes.

Arithematic

• expr < arg1> < mathematical operator > < arg2>
• It should be noted that * will be interpreted and expanded.
• Safer way is
• $(( < arg1> < operator > < arg2> )) for return value.
• (( < var > = < arg1> < operator > < arg2> )) for assignment.

Declare

Declare allows us to declare a bash variable, and type.

• declare -g < var > Makes var global
• declare -a < var > Types var to array
• declare -A < var > Types var to associatve array
• declare -i < var > Types var to integar, if set to arithematic string will evaluate string
• declare -l < var > Types var to all lowercase string
• declare -u < var > Types var to all uppercase string
• declare -r < var > Makes the var readonly and cannot be unset
• declare +x < var > Makes var available to child process

Output Redirect

• 0 STDIN - Standard In
• 1 STDOUT - Standard Out
• 2 STDERR - Standard Error
• dest < src Redirects standard in
• src > dest Redirects standard out
• M>&N Redirects one stream to other stream

Processes

command & will background command.

wait < pid > will halt execution until process with given pid finishes. If pid is not provided execution will halt until all children process have finished.

Pattern Matching

• * matches any string of any length
• ? matches any single char
• [chars] matches any character in chars.
• [a-b] matches the specified range of characters.
• {< string1>,< string2>} is a set of strings. If multiple sets are combined it will produce the result of multiplying the sets.

Control Structures

If Then Else

if < condition >
then
 < block >
fi

Case

case < arg > in 
 < pattern1 >)
  < case body >
  ;;
 < pattern2 >)
  < case body >
  ;;
  *)
  < default case body >
 ;;
esac

While

while < condition >
do
 < block >
done

Iterating over Elements in Array

for < var > in < array >
do
 < block >
done

Note to loop through associative array ${array[@]} will return the values of the array and ${!array[@]} will return the keys of the array.

Conditionals

Test command

File Tests

test will perform a conditional check

Test also can be expressed as [ args ]

Option	Example	True Condition
-e	test -e < file >	file/directory exists.
-f	test -f < file >	file exists and is regular file.
-d	test -d < file >	file exists and is directory.
-L	test -L < file >	file exists and is symbolic link.
-h	test -h < file >	file exists and is symbolic link.
-r	test -r < file >	file exists and is readable.
-w	test -w < file >	file exists and is writable.
-x	test -x < file >	file exists and is executable.
-s	test -s < file >	file exists and is not empty.
-nt	test < file1> -nt < file2>	file1 is newer than file2.
-ot	test < file1> -ot < file2>	file1 is older than file2.
-ef	test < file1> -ef < file2>	file1 and file2 refrence the same inode.

String Tests

Option	Example	True Condition
-z	test -z < string >	string is empty.
-n	test -n < string >	string is not empty.
=	test string1 = string2	the strings are equal.
!=	test string1 != string2	string1 is not equal to string2
<	test string1 < string2	string1 is lexographically before string2
>	test string1 > string2	string1 is lexographically after string2.

(Escape > and < to prevent interpretation.)

(> and < only sort on only pure ASCII.)

Numeric Tests

Option	Example	True Condition
-eq	test int1 -eq int2	int1 == int2
-ne	test int1 -ne int2	int1 != int2
-le	test int1 -le int2	int1 <= int2
-lt	test int1 -lt int2	int1 < int2
-ge	test int1 -ge int2	int1 >= int2
-gt	test int1 -gt int2	int1 > int2

Boolean Operators

Option	Example	True Condition
-a	test test1 -a test2	test1 AND test2
-o	test test1 -o test2	test1 OR test2
!	test !test1	NOT test1
()	test (test1 -a test2) -o test3	Groups expressions

Option Tests

Option	Example	True Condition
-o	test -o OPT option	shell option is set
-v	test -v var	var has been set.
-V	test -V var	var is set and a named variable.

Functions

Functions have access to parent variables and reference args via $n for the nth argument.

Can be defined using either

function name {
 < function body >
}

or

name () {
 < function body >
}