Testing whether a command exists in a simple way in script.

Hi, all:

If I need to test whether a command exists using command line in
shell, then I simply do "which". For example:
# which nawk
/bin/nawk
# which gawk
gawk: Command not found.

Now I need to test whether a command exists while running a script.
The way that I can think of is still using "which" command, and direct
the output into a file. Then I can check the file to see whether it's
a path or it has "Command not found".

But I think my approach is not a good way. There must be a much easier
way to accomplish the goal. Would you please give ideas?

Thanks.

Kuhl wrote:
> Hi, all:
>
> If I need to test whether a command exists using command line in
> shell, then I simply do "which". For example:
> # which nawk
> /bin/nawk
> # which gawk
> gawk: Command not found.
>
> Now I need to test whether a command exists while running a script.
> The way that I can think of is still using "which" command, and direct
> the output into a file. Then I can check the file to see whether it's
> a path or it has "Command not found".
>
> But I think my approach is not a good way. There must be a much easier
> way to accomplish the goal. Would you please give ideas?

Try one of

which nawk && echo found

or

if which nawk
then echo found
else echo not found
fi

Suppress output of 'which' by redirecting stdout to /dev/null.

Janis

>
> Thanks.

On Wed, 27 Aug 2008 04:58:14 -0700 (PDT), Kuhl wrote:

> Now I need to test whether a command exists while running a script.
> The way that I can think of is still using "which" command, and direct
> the output into a file. Then I can check the file to see whether it's
> a path or it has "Command not found".

You could test the return code.

$ which gawk > /dev/null 2>&1
$ echo $?
1

$ which nawk > /dev/null 2>&1

$ echo $?
0

On Wednesday 27 August 2008 13:58, Kuhl wrote:

> Hi, all:
>
> If I need to test whether a command exists using command line in
> shell, then I simply do "which". For example:
> # which nawk
> /bin/nawk
> # which gawk
> gawk: Command not found.
>
> Now I need to test whether a command exists while running a script.
> The way that I can think of is still using "which" command, and direct
> the output into a file. Then I can check the file to see whether it's
> a path or it has "Command not found".
>
> But I think my approach is not a good way. There must be a much easier
> way to accomplish the goal. Would you please give ideas?

Try with

command -v nawk > /dev/null

and test the exit status (0 if found, nonzero if not)

--
All the commands are tested with bash and GNU tools, so they may use
nonstandard features. I try to mention when something is nonstandard (if
I'm aware of that), but I may miss something. Corrections are welcome.

On 2008-08-27, Kuhl wrote:
> Hi, all:
>
> If I need to test whether a command exists using command line in
> shell, then I simply do "which". For example:
> # which nawk
> /bin/nawk
> # which gawk
> gawk: Command not found.

I would use 'type' rather than an external command:

type awk

> Now I need to test whether a command exists while running a script.
> The way that I can think of is still using "which" command, and direct
> the output into a file. Then I can check the file to see whether it's
> a path or it has "Command not found".

Why would you use a file? Check the return status of the
command:

if type nawk > /dev/null
then
AWK=nawk
elif type gawk > /dev/null
then
AWK=gawk
fi

> But I think my approach is not a good way. There must be a much easier
> way to accomplish the goal. Would you please give ideas?


--
Chris F.A. Johnson, author
Shell Scripting Recipes: A Problem-Solution Approach (2005, Apress)
===== My code in this post, if any, assumes the POSIX locale
===== and is released under the GNU General Public Licence

> if type nawk > /dev/null
> then

If nawk would be not found, you'll se some errors in console sent on
stderr, rather use this:
type nawk > /dev/null 2>&1

Procek wrote:
>>if type nawk > /dev/null
>>then
>
>
> If nawk would be not found, you'll se some errors in console sent on
> stderr, rather use this:
> type nawk > /dev/null 2>&1

Depends on the shell. My ksh sends the output to stdout, my bash to
stderr, for example. Catching both channels is helpful of course.

Janis

On Wed, 27 Aug 2008 21:38:21 +0000, Chris F.A. Johnson wrote:

> On 2008-08-27, Kuhl wrote:
>> Hi, all:
>>
>> If I need to test whether a command exists using command line in shell,
>> then I simply do "which". For example: # which nawk
>> /bin/nawk
>> # which gawk
>> gawk: Command not found.
>
> I would use 'type' rather than an external command:
>
> type awk

type is indeed better than which for being a shell builtin, but it's also
better because some/all which commands will look for *csh aliases that
might not be available in a POSIX shell.

So there's more to the issue than just a little performance.