Created Sunday 08 April 2018
Always read man pages, help pages, and info pages.
TIP: If you are new to *nix, don't expect to understand very much of the manuals when reading and trying stuff in the very first few attempts. It will really depend a lot on your background.
man man
man --help
man foo
Bash has builtin commands, like help, pwd, type, and cd.
type cd
# → cd is a shell builtin
help is a builtin command that gives help on other builtin commands. To list all builtin commands, simple do:
help
And to see the help for a builtin, you can either do man bash and search for that command or do.
help <some builtin command>
First:
man info
info --help
info info
Generally, info pages are more user-friendly and tutorial-like than man pages. To read info pages, do
info <program or command>
like
info ed
info sed
info bash
You can also open info directly into a section of an info document (if you know the name of the section, that is), something like:
info sed 'execution cycle'
From GNU Emacs, you can read the info pages with
C-h i m <command>
For example:
C-h i m sed
Info has a lot of nomenclature, concepts and commands. info info explains about commands to find stuff inside info, navigate documents, etc. It is a somewhat complex system. Yet, a powerful one.
Take a look:
man cp
Then you see:
NAME cp - copy files and directories
SYNOPSIS cp [OPTION]... [-T] SOURCE DEST cp
[OPTION]... SOURCE... DIRECTORY cp [OPTION]... -t DIRECTORY SOURCE...
Let's understand the man page syntax.
"cp" is the name of the command or program. No mistery.
Anything inside "[" and "]" means that thing is optional. In this case, [OPTION] means that command line options are optional, that is, you can do something like
cp -v foo bar
or simply
cp foo bar
Where -v is an option to the command (and since it is optional, it is up to you to use it or not) and "foo" and "bar" are files (you are making a copy of ‘foo’ and naming that copy as ‘bar’).
When you see SOMETHING followed by ..., like in [OPTION]... or SOURCE..., it means that thing may occur more than one time. If something is optional, it may occur zero or more times. If that thing
is required, then may occur one or more times. So, in the case of
cp [OPTION]... SOURCE... DIRECTORY
it means we must use cp, followed by zero or more command line options. Then, SOURCE... is required, but it can occur more than once. Finally, DIRECTORY is required, and must occur only once.
Copy three files to a backup directory. We use vi options, which mean ‘verbose’ and ‘interactive’ (read about them in the man page):
cp -vi file1 file2 file3 bkpdir/
One of the popular Scheme interpreters (repl) is “Chicken”, and its command line tool is csi (chicken scheme interpreter, for the command line repl).
csi -help
produces
usage: csi [FILENAME | OPTION ...]
Note that we have the square braces enclosing two things, and there is a "|" (the pipe character) between those two things. That character means 'OR', that is, either one thing, or the or the other. It doesn't mean “invoke csi followed by a filename followed by an option.” Nope, that is incorrect. What that means is either one of these:
csi program.scm
or
sci <some option>
but this is INCORRECT:
sci program.scm <some option>
On the other hand, if you look at the csi man page (or sci -help), you'll see that some options require a file name, like the -s (or -script) option.
The moral is that the man page shows something that can be easily misunderstood:
csi [FILENAME | OPTION ...]
Can lead one to think the syntax is:
sci program.scm -s
which is incorrect. The correct is either:
sci program.scm
or (because the option -s takes a filename)
sci -s program.scm
That is, csi filename or csi <option>, just that some options require a filename after the option itself.
help for shows help for the "for in" construct. How to make it show help for the for (( ... )) one?
<go|dfish> FernandoBasso: well, (( )) is not part of for. help '(('
<go|dfish> !((
((...)) is an arithmetic command, which returns an exit status of 0 if the expression is nonzero, or 1 if the expression is zero. Also used as a synonym for "let", if side effects (assignments) are needed. See <http://mywiki.wooledge.org/ArithmeticExpression>.
go|dfish, help lists all build in commands. It shows "for NAME..." and "for (( ..." I understood that as if those are two different for constructs.
Anyway, thanks for helping.
FernandoBasso, http://www.gnu.org/software/bash/manual/bashref.html#index-for
!cfor
C-style for loop: for (( expr; expr; expr )); do COMMANDS; done # Each expr is done in a math context. Example: for ((i=1; i<=n; i++)); do echo "$i"; done. Use in place of the non-working: for i in {1..$number}
So:
help '(('
Also see [http://www.gnu.org/software/bash/manual/bashref.html#index-for[bash's] manpage section].
for ((i = 1; i < 15; i += 3)) do echo $i ; done
If you do echo $- from inside a script, it won't show i. And ' foo ' bar.