Re: How do I determine the existence (or non-existence) of a file from an AWK script?

[Andrew J. Schorr]

Hi,

On Mon, Apr 10, 2023 at 05:41:22PM +0000, Alan Mackenzie wrote:
> This should be something quite simple, but I can't work out how to do it.
> 
> Given a file name in an AWK script, does a file with that name actually
> exist?  In a shell script, I'd simply do [ -f "$FOO" ].  What do I have
> to do in AWK?
> 
> The context is a script which checks certain syntactic things in a git
> commit message.  I want to extend it to check that the noted file names
> in the commit message don't have typos, or missing directory parts, that
> kind of thing.

Here are a couple of efficient solutions that don't involve running a child
process.

1. Use getline to try to read from the file. Some examples:

bash-5.1$ gawk -v fn=/tmp/bogus 'BEGIN {if ((rc = (getline x < fn)) >= 0) 
{printf "%s exists and I can read it\n", fn; close(fn)} else printf "I cannot 
read %s; ERRNO is [%s]\n", fn, ERRNO}'
I cannot read /tmp/bogus; ERRNO is [No such file or directory]

bash-5.1$ gawk -v fn=/etc/passwd 'BEGIN {if ((rc = (getline x < fn)) >= 0) 
{printf "%s exists and I can read it\n", fn; close(fn)} else printf "I cannot 
read %s; ERRNO is [%s]\n", fn, ERRNO}'
/etc/passwd exists and I can read it

bash-5.1$ gawk -v fn=/etc/shadow 'BEGIN {if ((rc = (getline x < fn)) >= 0) 
{printf "%s exists and I can read it\n", fn; close(fn)} else printf "I cannot 
read %s; ERRNO is [%s]\n", fn, ERRNO}'
I cannot read /etc/shadow; ERRNO is [Permission denied]

2. Use the filefuncs stat extension. It is documented here:
https://www.gnu.org/software/gawk/manual/html_node/Extension-Sample-File-Functions.html

Some examples:

sh-5.1$ gawk -lfilefuncs -v fn=/tmp/fubar 'BEGIN {print stat(fn, st), ERRNO; 
printf "%o\n", st["mode"]}'
-1 No such file or directory
0

bash-5.1$ gawk -lfilefuncs -v fn=/etc/passwd 'BEGIN {print stat(fn, st), ERRNO; 
printf "%o\n", st["mode"]}'
0 
100444

bash-5.1$ gawk -lfilefuncs -v fn=/etc/shadow 'BEGIN {print stat(fn, st), ERRNO; 
printf "%o\n", st["mode"]}'
0 
100000

Regards,
Andy