Parsing arguments in bash - You Suck at Programming #002

Yo what's up everyone my name's dave and you suck at programming.

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you suck at programming #programming
#devops #bash #linux #unix #software #terminal #shellscripting #tech #stem

[SuperOriginalRecipe]

never thought i'd be learning better syntax from Bam Margera

[extrageneity]

Episode 2: Parsing arguments in bash.


In this video, Dave is demonstrating how bash processes command line arguments. As is so often the case in bash programming, it’s helpful to understand the underlying Unix/Linux concepts, this time the way a program launched from the exec() family of system calls supplies arguments to the program’s main() function:

1. argc, an integer. Indicates the number of arguments.

2. argv, a pointer to an array of strings, includes the list of arguments, with the first element in the array being the name of the executed program.


This video compares the relationship between $* and $@, both of which are used to dereference the full array of positional parameters, _except_ for that first element of argv which has the name of the script. That’s accessed as $0. The number of arguments (roughly, the equivalent of args) is accessed as $#.



In Dave’s initial cringe script, he’s looping over the unquoted $*, again showing how bash word splitting applies, even with arguments.


He begins to invoke the script with different arguments, eventually giving a quoted string that contains integral whitespace, here showing how bash can supply embedded whitespace to programs. Remember, the exec() system call family isn’t messing around with word splitting at all, the arguments are just an array of strings supplied to it, and these arguments can contain anything you want, including non-printable characters, whitespace, different character sets, whatever. This is a very early introduction to a very important concept for shell users: How do you get bash to set up the precise argv array you want when you use it to invoke other programs?



$* and $@, unquoted, are both expanding their internal concept of argv, using the first character in the bash field separator variable IFS as the separating character in the expansion. This expansion is then subject to bash word-splitting, which is governed by the whole of IFS, not just the first character. In the special case “$@“ or “${@}”, bash instead expands argv where the expansion is not subject to further word splitting. This special case is the final variation on the script Dave shows us, his based version. Along the way he also shows us how $* expands arg, still using that IFS character as the field separator, but with the expansion not subject to further word splitting by bash.



This same * vs @ distinction is also available in arrays, for example, put a copy of Dave’s based script on your system and then try:

- a=(a b c “d e”)

- based “${#a}”

- based “${a[1]}”

- based ${a[*]}

- based ${a[@]}

- based “${a[*]}”

- based “${a[@]” 


Another way of thinking about all of this is that in bash, the name of the variable containing the argv array is the empty string.


This script does not demonstrate every way of interacting with bash positional parameters. $1 through $9 access the first 9 positional parameters, brace enclosures (e.g., ${10}, ${11}, ${20}) are needed for subsequent arguments. 



The section of the bash man page which discusses all this at length is the PARAMETERS section, especially in the subsection titled “Special Parameters.” 


You can alter the set of bash positional parameters in two commonly used ways:

1. The shift command is used to remove positional parameters, deleting elements starting from $1 and shifting the others leftward in the array. Often scripts will do something like: while [ $# -gt 0 ]; do echo “arg is: \”$1\””; shift; done

2. The set command is used to reset the positional parameters completely. A common pattern in scripts is: [ $# -eq 0 ] && set defaultarg1 defaultarg2 … defaultargN

… in order to give a command a default set of arguments to act on.


Dave’s focus in these early videos is to get you thinking right away about several key concepts:

- bash word splitting, and how to control it using quoting

- how variable expansion interacts with word splitting


He’s starting here, in part, because the beating heart of any bash program is how it passes arguments to the other commands it uses exec() family system calls to invoke. But he’s also starting here for another important reason: To demonstrate how to do iterative debugging of bash scripts, not just by changing one thing at a time until you get it right, but also by being imaginative about what will happen if your script sees input you did not initially expect, especially in terms of how that input behaves if contains non-alphanumeric characters such as whitespace, newlines, or even non-printables such as terminal control sequences. 


As you learn bash using these videos as teaching aids, keep in mind that thinking like a bash programmaer involves always keeping these things in mind. Happy scripting!

[BartekChlebek]

Came for mad bash skills, left with a dope tune!

[dragonwood]

Just started learning bash. This whole channel should help a lot. Thank you.

[alilseman]

I needed this. Seriously.

When all positive thinking fails, never underestimate the power of being mocked in a rapid and condescending voice. What does this say about me—god!?

[outtacigs]

`in "$@"` is redundant, it's the default. `for arg; do echo "<$arg>"; done` is enough

[Maximonzx]

last night I went to bed at 4 am watching all your tiktoks. I truly appreciated the videos man c: Hope one day to be as good as you

[FullAnarchyDotNet]

This entire premise is amazing.
I don't even care about bash, and all this stuff, but the style is marvelous.

[MasterSergius]

I Came Looking for Copper and I Found Gold

[batchrocketproject]

60 yo programmer who sucks here. I come to learn the vocabulary and am determined my speech will become based, no one wants to be cringe. I wanted to thank you by simply leaving 'slay' but am not sure if that only works when said by 15 yo girls? Slay anyway, and thanks.

[RealViPdude]

as a very newbie bash user id watch a bash basics series in this style, i cant learn bash from other sources because its too dry.

[flipinfin]

its cuz the [@] says give me all argumets

[voidtongue]

I'm not cool enough for this channel

[bunyn]

for((a=1;a<=$#;a++)) { echo "$a: ${!a}"; }

[mlitzy]

"$*" and "$@" give me different results on GNU bash, version 5.2.21(1)-release . with "$*" I get <foo bar baz hello world> with "$@" i get the same as in the video