Command Completion Output – How to Redirect

autocompletebashio-redirectionpipe

On the command line, I can redirect or pipe output of a command to a file or another command using the > or | operator after the command. I have come across a less standard situation that I would like to redirect the output to a file, but I don't seem to have the oppurtunity to redirect it:

When at a new terminal,

[chiliNUT ~]$

if I press Tab without typing anything first, I get asked

display all 1725 possibilities? (y or n)

and if I then type y, I get a nice long list of different commands. How can I redirect or pipe this output to a file? I don't seem to get the chance to type > myfile.txt anywhere.

Using CentOS release 6.4 (Final).

Best Answer

You could make use of the builtin compgen:

compgen: compgen [-abcdefgjksuv] [-o option]  [-A action] [-G globpat]
[-W wordlist]  [-F function] [-C command] [-X filterpat] [-P prefix]
[-S suffix] [word]

    Display possible completions depending on the options.

    Intended to be used from within a shell function generating possible
    completions.  If the optional WORD argument is supplied, matches against
    WORD are generated.

    Exit Status:
    Returns success unless an invalid option is supplied or an error occurs.

TAB at the prompt would list commands, shell builtins, keywords, aliases and functions. So you could say:

compgen -cbka -A function | grep '^y' > myfile.txt

to get all the options that you see upon typing yTAB at the shell prompt into the file myfile.txt.

Eliminate the grep pipeline in order to get all the possible commands, functions, ... into the file:

compgen -cbka -A function > myfile.txt

Related Solutions

Can a command line program prevent its output being redirected

It's just that when stdout is not a terminal, output is buffered.

And when you press Ctrl-C, that buffer is lost as/if it has not been written yet.

You get the same behavior with anything using stdio. Try for instance:

grep . > file

Enter a few non-empty lines and press Ctrl-C, and you'll see the file is empty.

On the other hand, type:

xinput test 10 > file

And type enough on the keyboard for the buffer to get full (at least 4k worth of ouput), and you'll see the size of file grow by chunks of 4k at a time.

With grep, you can type Ctrl-D for grep to exit gracefully after having flushed its buffer. For xinput, I don't think there's such an option.

Note that by default stderr is not buffered which explains why you get a different behaviour with fprintf(stderr)

If, in xinput.c, you add a signal(SIGINT, exit), that is tell xinput to exit gracefully when it receives SIGINT, you'll see the file is no longer empty (assuming it doesn't crash, as calling library functions from signal handlers isn't guaranteed safe: consider what could happen if the signal comes in while printf is writing to the buffer).

If it's available, you could use the stdbuf command to alter the stdio buffering behaviour:

stdbuf -oL xinput test 10 > file

There are many questions on this site that cover disabling stdio type buffering where you'll find even more alternative solutions.

Best Answer

Related Solutions

Can a command line program prevent its output being redirected

Related Question