-O in Linux Terminal (Dash O); What Does It Mean

-o in Linux terminal (Dash o); What does it mean?

In most cases, -o will stand for output, but it's not a defined standard. It can potentially mean anything the developer wanted it to mean.

The only way someone can know which commands is to use a command line option of --help, -h, or something -? to display a simple list of commands. Again because the developer of the program chooses the possible input arguments and their meaning might differ from program to program.

The safest way to know is typically to run

man gcc

replacing the second part with the program name you want.

man <program name>

This lists a full guide for the program with a lot more detail and is usually well formatted to read on a terminal. Just press Q when you’re finished reading it.

What is the -o option in GCC?

See https://man7.org/linux/man-pages/man1/gcc.1.html. Under the Synopsis section, you will see the -o option.

Regarding

Well OK, the compiler knows where to look for stdio.h, but the executable must be linked to stdio.o, mustn't it? Where does the stdio.o file reside?

The answer to the first question is "No". Since the answer to the first question is "No", the second question is not relevant.

The functions and variables declared in stdio.h need not be in stdio.o. They are usually in a library (.a or .so) which are found in one of the directories where the linker looks for library files.

In general, there is no rule that each .h file has a corresponding .o file. It is possible to have the functions and variables declared in a .h file to be implemented in multiple .c files that will result in multiple .o files. It is also possible to have functions and variables declared in multiple .h files to be implemented in one .c file. How these are organized varies from project to project.

Each .c file, on the other hand, has a corresponding .o file (I haven't seen any platforms where multiple .c files can be compiled to create one .o file). All the .o files resulting from compiling the .c files are linked together to create an executable.

What's the magic of - (a dash) in command-line parameters?

If you mean the naked - at the end of the tar command, that's common on many commands that want to use a file.

It allows you to specify standard input or output rather than an actual file name.

That's the case for your first and third example. For example, the cdrecord command is taking standard input (the ISO image stream produced by mkisofs) and writing it directly to /dev/dvdrw.

With the cd command, every time you change directory, it stores the directory you came from. If you do cd with the special - "directory name", it uses that remembered directory instead of a real one. You can easily switch between two directories quite quickly by using that.

Other commands may treat - as a different special value.

What does set -e mean in a bash script?

From help set :

  -e  Exit immediately if a command exits with a non-zero status.

But it's considered bad practice by some (bash FAQ and irc freenode #bash FAQ authors). It's recommended to use:

trap 'do_something' ERR

to run do_something function when errors occur.

See http://mywiki.wooledge.org/BashFAQ/105

What is the meaning of -n, -z, -x, -L, -d, etc... in Shell Script?

IMHO best way is you could simply do man test for all these details. It is very well explained there. As follows is the text from man page. For BASH conditional expressions look for link https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html too once.

   -b FILE
          FILE exists and is block special

   -c FILE
          FILE exists and is character special

   -d FILE
          FILE exists and is a directory

   -e FILE
          FILE exists

   -f FILE
          FILE exists and is a regular file

   -g FILE
          FILE exists and is set-group-ID

   -G FILE
          FILE exists and is owned by the effective group ID

   -h FILE
          FILE exists and is a symbolic link (same as -L)

   -k FILE
          FILE exists and has its sticky bit set

   -L FILE
          FILE exists and is a symbolic link (same as -h)

   -O FILE
          FILE exists and is owned by the effective user ID

   -p FILE
          FILE exists and is a named pipe

   -r FILE
          FILE exists and read permission is granted

   -s FILE
          FILE exists and has a size greater than zero

   -S FILE
          FILE exists and is a socket

   -t FD  file descriptor FD is opened on a terminal

   -u FILE
          FILE exists and its set-user-ID bit is set

   -w FILE
          FILE exists and write permission is granted

   -x FILE
          FILE exists and execute (or search) permission is granted

For expressions in man test it is given:

   ( EXPRESSION )
          EXPRESSION is true

   ! EXPRESSION
          EXPRESSION is false

   EXPRESSION1 -a EXPRESSION2
          both EXPRESSION1 and EXPRESSION2 are true

   EXPRESSION1 -o EXPRESSION2
          either EXPRESSION1 or EXPRESSION2 is true

   -n STRING
          the length of STRING is nonzero

   STRING equivalent to -n STRING

   -z STRING
          the length of STRING is zero

   STRING1 = STRING2
          the strings are equal

   STRING1 != STRING2
          the strings are not equal

   INTEGER1 -eq INTEGER2
          INTEGER1 is equal to INTEGER2

   INTEGER1 -ge INTEGER2
          INTEGER1 is greater than or equal to INTEGER2

For conditional expressions info go for man bash it gives info as follows too.

CONDITIONAL EXPRESSIONS
Conditional expressions are used by the [[ compound command and the test and [ builtin commands to test file attributes and
perform string and
arithmetic comparisons. Expressions are formed from the following unary or binary primaries. If any file argument to one of
the primaries is
of the form /dev/fd/n, then file descriptor n is checked. If the file argument to one of the primaries is one of /dev/stdin,
/dev/stdout, or
/dev/stderr, file descriptor 0, 1, or 2, respectively, is checked.

   Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link, rather 

than the link
itself.

   When used with [[, The < and > operators sort lexicographically using the current locale.

What are the options with - (dash) on Linux terminal command mean?

Every U*x system comes with manual pages, and they are easy to find on the web as well.

man chmod documents the chmod command, including its options; man man documents the man command itself, etc.

GNU ships documentation in a system called Info which is less ubiquitous but more featureful (clickable links for cross-references, footnotes, etc). Typically, you will find a brief man page which directs you to Info for the full documentation.

On Linux systems, man intro is a gentler introduction to get you started with the system. (There is no intro command; the man page is simply an introduction to Life in Linux.)

The tutorials you have consulted don't seem very convincing if they failed to mention the existence of standard, high-quality on-line documentation within the system itself.

How to repeat a dash (hyphen) in shell

This throws an error:

$ printf '-%.0s' {1..100}; echo ""
bash: printf: -%: invalid option
printf: usage: printf [-v var] format [arguments]

This works fine under bash:

$ printf -- '-%.0s' {1..100}; echo ""
----------------------------------------------------------------------------------------------------

For other shells, try:

printf -- '-%.0s' $(seq 100); echo ""

The problem was the printf expects that - starts an option. As is common among Unix/POSIX utilities in this type of situation, -- signals to printf to expect no more options.

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