When I run an executable binary, what's the command called?

Question

When I type the following in the terminal

./a.out

what's the name of the command that I'm using? I have no idea how to look up command line options for this situation because I don't know what to actually look up.

`a.out` is the name of the command. – Adam Burry Oct 04 '13 at 02:02 — Adam Burry, Oct 04 '13 at 02:02

score 2 · Accepted Answer · answered Oct 04 '13 at 02:03

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The default output filename for a compiled C program is a.out. Each program parses its own arguments, so there is not canonical list of arguments. Since you have the a.out, you probably have the source code for the program. Find the function named "main" and look for code that either examines argv, or calls getopt, and maybe somebody will have provided a function (named help or usage) that displays brief program help.

Another approach would be to run the program with '-q' or '?' arguments.

Another approach would be to use 'strings' to find printable strings in the executable file,

strings a.out |less

Man pages are written separately from code, so there may not be man pages for your executable.

answered Oct 04 '13 at 02:03

ChuckCottrill

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"getopt" - that's what I needed to look up. I'm new to C and I wasn't aware you could customize flags in addition to arguments. I thought there was a set of "standard" flags that were universally applicable in addition to the ones specific to different commands. Probably because I keep seeing the same flags applied in the same ways. – Tyler Oct 04 '13 at 19:34
A (non-foolish) consistency is the sanctuary of great minds, wise enough not to waste time where it is not need. – ChuckCottrill Oct 05 '13 at 00:40
Note that there are two kinds of getopt, one for old-style unix arguments (lest we forget where Linux stole all their code from), and the other for the new style GNU options, which are typically --argument_name (notice the snakecase), and use a different getoptions function (spelled different), here is another: http://optionparser.sourceforge.net/ – ChuckCottrill Oct 05 '13 at 00:44

score 1 · Answer 2 · answered Oct 04 '13 at 02:02

./ is a location, not a command. The . represents the current directory. The / is the directory separator. a.out is a (presumably executable) file in the current directory. This is the default name of a compiled executable built by gcc.

You usually use this explicit invocation method because . is not usually part of the search path. So, if you tried to run a.out without the ./ prefix, the system would not be able to find it even though you're in the current directory. This is a security measure (to stop you unwittingly running a malicious program when you thought you were running something else), and is a good thing.

score 0 · Answer 3 · edited May 23 '17 at 10:32

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a.out is the default name of a 'c' program that has been compile without giving it a name. Usually programs are compiled with an -ofilename parameter

see Determining C executable name

edited May 23 '17 at 10:32

Community

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answered Oct 04 '13 at 01:54

Scary Wombat

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Right, but what do I do if I want to look up command line options using "man"? Like if I want to look up command line options for"ls" I would do "man ls". What do I do here? – Tyler Oct 04 '13 at 01:57
@tyler Not all commands have man-pages. Many commands provide help if invoked with no parameters, `-h`, or `--help`. But they don't have to. Anyone who compiles a program without naming it is unlikely to officially release the program. – paddy Oct 04 '13 at 02:04

score 0 · Answer 4 · answered Oct 04 '13 at 02:01

The command may not have any documentation of what its options are. If it's a precompiled binary you may have very little recourse to find out what its options are, if any. You could try to strace it to find clues, but there's no guarantee the program has a manpage or any kind of --help output.

smRaj · Answer 5 · 2013-10-04T10:05:17.607

./a.out

is by itself a command. Lets try to understand about this.

Command is basically an executable element. It can be either binary or even a shell script. In your case a.out is an executable binary element. For the command line options of a.out, please refer "argv", "argc" parameters in the main() function of the source code which is compiled and linked to get a.out.

Learn more by visiting Program Arguments

Bonus Details on Command Execution:

For a shell to execute the command, there are two well known methods.

Method 1:

Explicitly provide the path of the executable along with name of the command.

Example 1: To execute commands in present working directory by explicitly providing path:

./a.out

'.'[dot] means current directory.

Example 2: To execute commands in anywhere from anywhere by explicitly providing path

/home/username/a.out

Method 2:

Update the PATH shell environment variable to include the path of the command to be executed.

Type : env | grep -i "PATH" to know the locations currently looked over for a command to be executed if just the name of the command is provided.

If you update the PATH shell env variable to contain the location of your a.out, you can simply type:

a.out

Type below command to update the PATH shell env variable

export PATH=$PATH:/path-to-executable

You might have noticed this style.

You would have only typed "rm" and still the shell is able to execute the command. Reason is, path of command is implicit and found over reading "PATH" shell env variable.

For executing user generated commands, while Example 2 of Method 1 is sufficient in most cases, you still find the Method 2 useful at places where in you use the command extensively, day-in and day-out.

When I run an executable binary, what's the command called?

5 Answers5