web/content/drafts/bash.md

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title	description	summary	draft	tags	author	showToc
Shell Scripting [BASH]			true		TrudeEH	true

Bash Language

Strings

• "" Defines a string which supports substitutions ($ and \, for example).
• '' Defines a string, but preserves its actual value (substitutions are treated as regular characters).
• ANSI Escape Sequences apply when using "".

Comments

# comment

Commands

A shell command consists of the command itself, followed by its arguments.

command "arg1" "arg2"

If the first word of a command is a reserved word, bash handles the command, otherwise, it searches for an executable on the system's $PATH, a list of directories where a binary could be located.

Reserved Words

if	then	elif	else	fi	time
for	in	until	while	do	done
case	esac	coproc	select	function
{	}	[[	]]	!

List of Commands

• command1 ; command2 Execute command2 after command1, sequentially.
• command1 & Execute command1 asynchronously in a subshell.
• command1 && command2 AND: Only execute command2 if command1 returns 0 (success).
• command1 || command2 OR: Only execute command2 if command1 returns a non-zero exit value (failure).

Loops

until

until test-commands; do 
	...
done

Execute the code in ... for as long as test-commands return non-zero.

while

while test-commands; do 
	...
done

Execute ... for as long as test-commands return 0.

for

Expand words and execute ... for each member in the resultant list, with name bound to the current member.

Iterate through List

for item in list; do
	echo $item
done

C-like Loop

for (( i=1; i<=10; i++ )); do  
 echo "Loop number:" $i  
done

Infinite Loop

for (( ; ; )); do
 echo "Press Ctrl+C to stop..."
done

Conditional Constructs

if

if test-commands; then
  ...
elif more-test-commands; then
  ...
else
	...
fi

Execute the first ... if test-commands returns 0, and evaluate the next condition otherwise. This process repeats until else is found, or one of the tests evaluates to a 0.
Once any ... executes, the remaining if construct is skipped.

case

case word in
  p1 | p2 | p3) ... ;;
  p4 | p5 )
	  ... 
	;;
  *) ... ;;
esac

Execute the ... corresponding to the first pattern (pX) that matches the word.
The | operator separates multiple patterns, and each clause can end with ;;, ;& or ;;&. It's common to use * as the default case, since the pattern will always match.

Using ;& instead of ;; would cause the next ... to be executed as well, and ;;& would test the next clause, instead of immediately exiting.

select

PS3="Enter a number: "

select option in entry1 entry2 entry3 entry4 entry5
do
    echo "Selected character: $option"
    echo "Selected number: $REPLY"
done

The select command generates a menu, displaying each entryX in a list. The user is then prompted to select an option (in this case, a number from 1-5), and the resultant $option and $REPLY are then provided as variables.

Output:

1) entry1
2) entry2
3) entry3
4) entry4
5) entry5
Enter a number:

((...))

The arithmetic expression is evaluated according to the rules described below (see Shell Arithmetic TODO link to shell arithmetic).

[...](...)

Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below in Bash Conditional Expressions.

Combine Expressions

• ( expression ) Returns the value of expression. (Can be used to override precedence).
• ! expression NOT an expression. (true if expression is false).
• exp1 && exp2 AND - true if both expressions are true.
• exp1 || exp2 OR - true if either expressions are true.

Grouping Commands

Bash allows for commands to be grouped as a single unit. That way, if the group is redirected, the output of every command in the list is passed to a single stream.

• ( list ) Create a subshell (variables created inside it can't be accessed outside).
• { list; } No subshell is created.

Functions

fname() {
	...
}

function fname {
	...
}

A function can store a block of code (compound command), so it can be reused by calling its name:

fname

Any variables defined inside the function

Arguments

fname() {
	echo $1 $2
}

fname "a" "b"

Scope

var1='A'
var2='B'

fname () {
  local var1='C'
  var2='D'
  echo "var1: $var1, var2: $var2" # C, D
}

echo "$var1, var2: $var2"       # A, B
fname                           # C, D
echo "var1: $var1, var2: $var2" # A, D

Defining a variable inside the function overwrites the global scope. To prevent this, use the local keyword.

return

fname() {
	return 1;
}

fname
echo $? # 1

Use the return command to exit the function and return a value.

Variables (Parameters)

name="Trude"
echo $name # Trude

name+=" EH"
echo $name # Trude EH
echo ${name}

Variables can be of any type, and grow to any needed size.

Special Variables

• $* Expands to every positional parameter: $1$2$3.
• $@ Expands to every positional parameter, separated by spaces: "$1" "$2" "$3".
• $# Number of positional arguments.
• $? Exit status of last command / pipeline.
• $- Current option flags set by set, or by the shell itself.
• $$ Process ID of the shell. In a subshell, it expands to the process ID of the parent shell.
• $! Process ID of the latest job placed into the background.
• $0 Name of the shell or script.

Shell Expansions

Brace Expansion

echo a{d,c,b}e # ade ace abe

Tilde Expansion

• ~ = $HOME
• ~+ = $PWD
• ~- = $OLDPWD

Shell Parameter Expansion

Builtins ---------------------

Bash doesn't come with any programs such as cat, grep and ls by default, for example. Those