Ubuntu – What do the square brackets mean in an ubuntu terminal command’s synopsis

command line

For the 'find' command, the synopsis from

man find

is:

find [-H] [-L] [-P] [-D debugopts] [-Olevel] [path...] [expression]

What do the square brackets mean? I used to think they just meant an argument or flag was optional, but [path…] and [expression] can't both be optional for find, right…?

Best Answer

What do the square brackets mean?

Square brackets in a command synopsis mean optionality (i.e. whatever is listed inside of them is optional / not required in order to run the command).

I used to think they just meant an argument or flag was optional, but [path...] and [expression] can't both be optional for find, right...?

They can; running find without [path...] and [expression] just prints the list of folders / files in the current working directory's hierarchy without any condition.

The [<expr>] construct along with other symbols / constructs commonly used in command synopses (e.g. |, alternation, and {<expr>}, repetition) are the same used to define formal languages in Extended Backus-Naur Form (EBNF), so EBNF's syntax is a a good reference to understand / remember what symbols / constructs in command synopses mean.

Related Solutions

Ubuntu – What do the symbols like =, * and | in the output of “ls -F” mean

I believe you're talking about indicators presented by ls -F. From the manpage of ls:

-F, --classify
   append indicator (one of */=>@|) to entries

[...]

--indicator-style=WORD
   append indicator with style WORD to entry names: none (default), slash (-p),
   file-type (--file-type), classify (-F)

To get an overview of the meaning of these indicators, we have to dive into the info page as suggested at the bottom of the manpage (info coreutils 'ls invocation'):

`-F'
`--classify'
`--indicator-style=classify'
     Append a character to each file name indicating the file type.
     Also, for regular files that are executable, append `*'.  The file
     type indicators are `/' for directories, `@' for symbolic links,
     `|' for FIFOs, `=' for sockets, `>' for doors, and nothing for
     regular files.  Do not follow symbolic links listed on the command
     line unless the `--dereference-command-line' (`-H'),
     `--dereference' (`-L'), or
     `--dereference-command-line-symlink-to-dir' options are specified.

Above is an excerpt taken from the 'General output formatting' section. Go there directly using info coreutils 'General output formatting'.

TL;DR

* for executables
/ for directories
@ for symbolic links
| for FIFOs
= for sockets
> for doors
(nothing) for regular files

Ubuntu – What does $PATH mean

Run in a terminal:

echo $PATH

printf "%s\n" "$PATH"

what you see is a list of directories, looking like:

/home/jacob/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

If you put an executable in either one of these directories, you do not need to set the path to the executable / script, but you can run it by its name as a command.

Executables in $PATH should not have a language extension by convention (although they would work)

Editing your $PATH variable

You can (permanently) add a directory to $PATH by adding the following line to your ~/.profile file (invisible by default, press Ctrl+H in the file manager to make it visible):

export PATH=$PATH:/path/to/dir

More usefull information on environment variables

(such as $PATH) can be found here (thanks for the suggestions @Letizia)