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Looking from a programmer's point of view then shell script is just another programming language, where one has to learn and conform to the rules of the language. However, I have to admit that this syntax is the weirdest style I have ever seen in a rather commonly used language. Did the shell take this syntax from an older language that it descents from? Is there a special implication / meaning in the syntax?
As an example, here is a little snippet that I take from another post on SO
case "$1" in start) start ;; stop) stop ;; restart) stop start ;; status) check_status ;; *) echo "Usage: $0 {start|stop|restart|status}" exit 1 ;; esac Looking at this, firstly I can see that case ends with esac, which is its reversed form (like if ending in fi). Secondly I understand that each case is followed by a ). Fair enough, but why on earth do I need two ; at the end of each statement? I would also say that the ) without an accompanying ( is ugly.
I'm looking for more information about the historical aspect of the language, but I'm open for technical reasons as well.
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The bash case statement is the simplest form of the if elif else conditional statement. The case statement simplifies complex conditions with multiple different choices. This statement is easier to maintain and more readable than nested if statements.
esac statement is to give an expression to evaluate and to execute several different statements based on the value of the expression. The interpreter checks each case against the value of the expression until a match is found. If nothing matches, a default condition will be used.
Zsh, also known as the Z shell, extends functionality of the Bourne Shell (sh), offering newer features and more support for plugins and themes. Starting with MacOS Catalina in 2019, Zsh became the default login and interactive shell in Mac machines.
Per request:
for ...; do ...; done' and not 'for ...; do ...; od'? There was a sound reason for it - but the Algol-like reversed keyword to mark the end was used elsewhere.Answer:
By reputation, the Bourne shell source code was written in idiosyncratic C with macros to make it look like Algol. This made it hard to maintain.
With respect to the main question - about why no opening bracket (parenthesis) around the alternatives in the case statement - I have a couple of related theories.
First of all, back when the Bourne shell was written (late 1970s), much editing was done with 'ed', the standard text editor. It has no concept of skipping to a balanced parenthesis or other such notations, so there was no requirement for a leading parenthesis. Also, if you are writing a document, you might well marshal your arguments with:
a) ...blah... b) ...more... c) ...again... The opening parenthesis is often omitted - and the case statement would fit into that model quite happily.
Of course, since then, we have grown used to editors that mark the matching open parenthesis when you type a close parenthesis, so the old Bourne shell notation is a nuisance. The POSIX standard makes the leading parenthesis optional; most more modern implementations of POSIX-like shells (Korn, Bash, Zsh) will support that, and I generally use it when I don't have to worry about portability to machines like Solaris 10 where /bin/sh is still a faithful Bourne shell that does not allow the leading parenthesis. (I usually deal with that by using #!/bin/ksh as the shebang.)
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The reason of using ;; is that a single ; can be used to write multiple statements in one line, like:
restart) stop; start;; ... If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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