Lecture 3 (05-22)

“test commands” are different from other commands

• the [] square brackets are simply to make things look nice
  • [ is literally the command name
  • ] is literally an argument to the command lol

[!'('"$x"' > '"$y"')']

• This “command” is still subject to all the syntax of other shell commands
• The ( character we want to pass as an argument rather than something that means something to the shell
  • so '(' or alternatively \( is there for that reason
  • Think of '' as “not for you shell; this goes into the command’s arguments!”

We be on slide 14 btw () does grouping

Things to note: { grep foo file1 ; ls} > file2

• The intention is to shove the output of both commands into file2
• HOWEVER THIS IS WRONG. It willtodo what does it do The solution is to add a ; { grep foo file1 ; ls ; } > file2

alternatively you can do this:

{
	grep foo file1
	ls
} > file2

Grouping with () (subshells)

() groups, but is different from {}

• () creates a new shell process (so the results are NOT saved)
• doing ( x=hello ) will NOT create a new x
• running echo $x after will NOT work!

( x=hello! ; echo $x ) ; echo $xtodo card this (be like, “what this return?“)

BUT $((5 + 1)) this is run in the current shell (NOT a subshell)

todo Try to rewrite this WITHOUT SEMICOLONS:

if mkdir abc ; mkdir def ; then
	echo 'success!'
else
	echo 'saddace :('
fi

• Also what does mkdir abc return? The exit code? #todo idek what does does? Why is there two args!?

seq 5 -1 1 starts at 5, increases by -1, goes to 1

Wildcards / Patterns (to match filenames)

slide 31/56

the * represents “any part of” a filename.

• for each path, we check, does it have *.pyin it? If so, we select it.

? matches one character

• if we have foo and goo, then ?oo would match both of them
• So would ?o? or even ???

Exit codes thingys

• The exit code comes from the last executed command

getopts

• GETS OPERATIONS getopts a-thingy variable-name

Each time you run it, it progresses to the next flag. Only when the exit code is 1 do we know we’ve gone through all our options

• I think it stops when it doesn’t see an argument within the a-thingy ? idek lol

Also note something like sh toyscript -n -- -v implies -- and -v are arguments not options, but -n is still an option

If you add an option thata-thingy doesn’t have, it will be like illegal option -k

OPTIND is your operation index. In the table it starts at 2 because when we go to the next row we know ‘okay, we’re looking at the second argument!’

• Really, it starts at 1 but at the end of the first getopts call, if the exit code is not 0, it should be increased to 2
• If you want to go thru the while loop again, all you have to do is set OPTIND=1 Also just so you know ORTARG and OPTIND are global variables

Functions (what the flip)

sh toyscript -n -M -etc -etc
 
myfunction () {
	while getopts pq myflag ; do
		...
	done
	
	echo "$1"
	return # This exits out of a function. alternatively you can specify a value
}

• Two things:
  1. Since toyscript runs getopts it will chance the OPTIND but never “reset it” so we would need to do that manually otherwise that while loop will NOT run!

Here Document

To store multi-line hard-coded text into stdin

• EOF or _EOF_ or whatever works. Nothing special about these terms; the shell will understand what you’re doing.
• If you start with EOF, your here document must also end with EOF