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Why is ./ sometimes needed?


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# 29  
Quote:
Originally Posted by wisecracker
Thanks Don...

There is this however, reference to OSX 10.14.1, default bash terminal:
Code:
Last login: Thu Jan 31 12:05:01 on ttys000
AMIGA:amiga~> man true
AMIGA:amiga~> help true
true: true
    Return a successful result.
AMIGA:amiga~> man false
AMIGA:amiga~> help false
false: false
    Return an unsuccessful result.
AMIGA:amiga~> man :
No manual entry for :
AMIGA:amiga~> help :
:: :
    No effect; the command does nothing.  A zero exit code is returned.
AMIGA:amiga~> 
AMIGA:amiga~> which true
/usr/bin/true
AMIGA:amiga~> which false
/usr/bin/false
AMIGA:amiga~> which :
AMIGA:amiga~> _

Both XUbuntu56 and I can learn from your expertise on this.
I have not copied and pasted the man pages but they do exist.

As you can see there is a manual AND a help entry for "true" and "false", do I take it the some shells, perhaps dash or ash, do not have these as builtins as it looks like I have both builtin and transient versions.
":" is builtin only so I assume that in POSIX this IS a requirement but the others are optional?

TIA.

Bazza...
The : utility is required by the standard to be a special shell built-in utility.

If you have initialized PATH to the set of directories defined by your operating system to contain all of the standard utilities, the standard requires that the false and true utilities be available either as regular shell built-ins or as stand-alone utilities in one of the directories in PATH. Furthermore, if you are using things like:
Code:
find -exec command_name "{}"

where command_name is the name of a standard utility (including both false and true, but not :) with PATH set as described above), it is required to be found even if it is supplied by your shell as a regular built-in. If, however, command_name is the name a special built-in utility (including :), the results are undefined.

And, despite what the macOS true and : man pages say the following sequence of commands (attempting to run the : and true commands) with redirections that fail:
Code:
bash
bash-3.2$ : >/unknown/unknown
bash: /unknown/unknown: No such file or directory
bash-3.2$ true >/unknown/unknown
bash: /unknown/unknown: No such file or directory
bash-3.2$ echo $?
1
bash-3.2$ : >/unknown/unknown
bash: /unknown/unknown: No such file or directory
bash-3.2$ echo $?
1
bash-3.2$

both produce non-zero exit codes and diagnostic messages written to the standard error output. This was run on macOS Mojave version 10.14.2. Before you say that the man pages are wrong, look more closely at the diagnostics. The diagnostics are coming from bash; not from the : or true utilities. The shell is performing the redirections before invoking the utilities and since the redirections are failing, the utilities are not invoked. You have to be able to run : and true before they can return a zero exit code.

Are we having fun yet? Smilie
This User Gave Thanks to Don Cragun For This Post:
# 30  
Hi Don...

I was aware that bash created the error report but didn't know that part of the standard included certain transient commands.
I appreciate the '/bin/echo' and '/usr/bin/printf' have to be around so that if 'bash' is used as 'sh' as in OSX then these have to override the builtins.
I had great fun getting '/bin/echo' to work with escape codes but it is easily possible in 'dash'.


Anyhow we have command[]eered this thread and I am now clear. Thanks to all including the OP for putting up with me.

See attachment, I have left the error in, my typo fault:
Why is  ./  sometimes needed?-escpng
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