Lecture 4 (05-29)

We’re still on these slides: https://www.cs.utoronto.ca/~trebla/CSCB09-2025-Summer/08-shell-l.pdf

Dot commands

. ./cmds.sh

• The command is literally a .
• It runs the commands of that script in the current shell as opposed to sh cmds.sh which does it in a separate shell process

You MUST include the ./ to indicate relative path. It basically wants to see some kind of slash so it knows what you’re doing (“running arbitrary files might be dangerous!“)

This means you cannot do . cmds.sh

Export

• Marks a variable as an environment variable
• export MYENVVAR="bazinga!"
• Goes byebye after you close the terminal you ran export in though

Unset

• Undoes a variable ig?

Local Variables

myfunc()
{
	# in this scenario, x and y are defined, but Y is initialized
	local x y="hello"
	x="hi" #okay now x is initialized too!
}

• Does dynamic scoping

y="goodbye"
x="bye"
myfunc()
{
	# x and y in this scope only affect these "local" x and y's (NOT the "global" ones outside of le function)
	local x y="hello"
	x="hi" #does not touch "bye" of the global x
}

Another example

z=good
 
victim() {
	# This "z" refers to the "z" depending on where this function is 
	# defined. it does NOT use the local `z` just 
	# because mitm calls it
	echo "$z"
 
	# and now this will update the global "z"; any other function
	# using the global `z` will change! 
	z="bazinga?"
}
 
mitm() {
	local z=evil
	# does not "pass" the local z into victim 
	victim
}
 
echo -n 'mitm ' ; mitm

Arrays

rgb=(red blue green)
 
rgba=( "${rgb[@]}" alpha ) # "appedns" an item to somewhere in the array!

Associative Arrays

• Basically dictionaries

declare -A mark # defines the array
mark=([bob]=9 [rob]=30) # define an array of key-pairs

Process Substitution

Pipelining is good for 1 input, 1 output (or more with chaining)

sort <(cmd1) <(cmd2)

• Basically cmd1 and cmd2’s output to stdout goes into sort!
• Lets you have two input sources!

cat <(echo bazinga) <(echo what)

• Another example of Process Substitution

Diff

time

diff old.c new.c

• It will output what lines to add, delete, and replace

Grep

• Searches a text file via a given pattern (thats regex)
• Exits with 1 if there are no matches

find finds a file