Lecture (05-15)

shell 🐚🐚🐚

All code used is on the website (under “code”)

Shell commandddss

Echo

sh 
sh-5.2$ echo bazinga  
bazinga

spaces seperate “units” so it doesn’t matter how many spaces there are between “units”. It simply prints out all “units” seperated by a single space

echo a     b  
a b

but you can use ' ' single quotes to literally print the spaces

• in most cases, single quotes are “stronger” than double quotes - negendran

echo bazinga'        'more bazinga
bazinga        more bazinga

Comments

# Whole Line Comment
ls -l # in-line comment

Variables

var=value

• There are no spaces otherwise it would be like, the command var with arguments = and value. var is NOT a command!

v=xyz

• This declares a variable v

echo $v
xyz
echo ${v} # also works!
xyz

Now consider this example:

v=zzz
v0=xyz
echo $v0 # This would print xyz
xyz
echo ${v}0 # This prints zzz0
zzz0

Prinitng things that are ambigious!

echo \$v
$v
echo "$v"
xxx
echo '$v $v $v'
$v $v $v

Variables seperated by spaces, when passed as an argument, treats them as two arguments! Pretend we do have a file Sales Receipt.pdf

v='Sales Receipt.pdf'
ls -l $v
# This will throw an error along the lines of 
# "Cannot access 'Sales' and Cannot access 'Receipt.pdf'
 
ls -l "$v"
# This will work, and print the contents of our file

The " " sort of treats everything inside of it (what otherwise would be a stream of characters seperated by spaces) as one big string. You can also add extra stuff to this chonky string, and then pass that!

ls -l "Prefix for the file of $v"

math

y=123
echo $((y * 4))
492

Command Substitution

Note that echo returns something, and we can “capture that”. This works with all sorts of commands!

echo Todays date is $(date)
Todays date is Thu 15 May 2025 03:40:25 PM EDT

Shell Scripts

sh will run a file and read it line by line, running le commands.

echo -n xxx
echo -n yyy
echo -n zzz

You can make a proper “executable” with the following steps

todo Why does ./bazinga throw an error saying “Permission Denied” when just trying to run it (it has a bunch of commands in it!)

• I know it’s not an executable. I want to know why the error is “Permission Denied” ALSO, why does bazinga throw the error “command not found” !?!? Why are the errors different (I thought the pointed to the same file (?))

You can do a similar thing with python This file is foo

#!/usr/bin/python3
 
print("xxx")

then you can run ./foo ANYWHERE. The language does not matter!

todo

• Thinking of "" as a STRING, what would foo bar xyz be? What does it represent? What is its “data type” ??? Or is it really just 3 seperate “strings”

Command line arguments

#!/bin/sh
 
echo "\$# = $#"

Loops

for i in "$@" ; do
    echo "'$i'"
done

If you run ./foo a 'b c d' e f the above will print

'a'
'b c d'
'e' 
'f'

BECAUSE of the " "’s around $@todo WHY

Shift

”shifts positional parameters”

while [ $# -gt 0 ] ; do
	echo "'$1'"
	shift
done

File redirection

• The 0 represents standard in.
  • 1 is for stdout and 2 is for stderr
• In some contexts you can use these numbers to represent the above standard thingys (todo is there a general term for these guys?)

Appending

echo hello > myfile
echo kitty >> myfile

the file will contain

hello
kitty`

[File] Redirection

&1 represents what stdout is already pointing to (?)

command > file 2>&1

• Redirects stdout and stderr to the same file

todo this WHOLE thing b/c i really do not understand it one bit

Sequential Composition

(You can run, on a single line, many commands) cd b09 ; ls -l ; cd ..

known as a (command) list

If you want to continue a line to two lines, you can use a backslash to look nicer

echo hello B09 \  
	students

prints hello B09 students

Exit Code / Success / Failure

$? is a special variable and stores the most recent command’s exit code

• Any non-zero number represents a failure
• This is different from programming / logic where false = 0 since the numbers stand for different kinds of errors.
  • False is 0 in logic since $0\cdot 1=0$ and $\text{true}\wedge \text{false}$ is false!

Pretend you run, a program, then

echo $?
echo $?

The second echo will print 0 since the echo ran successfully !

AND OR NOT true false

Sequential execution

mdkir foo1 && cp myfile foo

• Will run the first command; if the exit code is 0, then it will run the next command!

&& and || have the SAME precedence, which is different from regular logic / programming. aaa. Both of these are lower than ! though. phew.

Test commands

Returns 0 or non-zero. Useful for… things. Like showing if a file exists or not. That kind of thing.

• in the example given, echo sadface will NOT run if [ -d lab02 ] is true, since the || is now sufficient and no more work must be done.
  • This is EXACTLY like short-circuit evaluation

Something noah suggested:

if [ -n "$verbose" ]; then
	echo "deleting $f"
fi 
if [ -z "$dryrum" ]; then
	rm "$f"
fi

The above SHOULD be equivalent to

[ -n "$verbose"] && echo "deleting $f"
 
[ -z "$dryrum" ] && rm "$f"

You DONT need the semicolons. You can also use a new line but it’s supa ugly

if [-n "$verbose" ] ; then
	echo "deleting $f"
elif 
	echo ""
fi

EQUALS (I THINKKKK)

if [-n "$verbose" ]
	then
		echo "deleting $f"
elif 
	echo ""
fi

todo confirm plz thanknknaktnawn

SO MANYU THING SABOUT WHILE LOOPS seq 0 3 returns numbers 0 1 2 3 seperated (so it’s not a string, it’s those “seperated words” i was confused about before)

braindead

echo --help
--help