Ksh: how to explore stdin?

Question

I want my ksh script to have a different behavior depending on whether there is something via stdin or not:

(1) cat file.txt | ./script.ksh (then do "cat <&0 >./tmp.dat" and process tmp.dat) vs. (2) ./script.ksh (then process $1 which must be a readable regular file) 

Checking for stdin to determine if it is a terminal [-t 0] does not help, because my script is being called from another script.

Running "cat <& 0> ./tmp.dat" to check the size of tmp.dat hangs, waiting for EOF from stdin if stdin is "empty" (2nd case).

How easy is it to check if stdin is "empty" or not ?!

Answer 1

EDIT: you work on HP-UX

Tested [ -t 0 ] on HP-UX and seems to work for me. I used the following setting:

/tmp/x.ksh:

 #!/bin/ksh /tmp/y.ksh 

/tmp/y.ksh:

 #!/bin/ksh test -t 0 && echo "terminal!" 

Performing /tmp/x.ksh fingerprints: terminal!

Could you confirm this on your platform and / or provide an alternative test setup that more accurately reflects your situation? Is your script ultimately spawned by cron ?

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EDIT 2

If desperate, and if Perl is available, define:

 stdin_ready() { TIMEOUT=$1; shift perl -e ' my $rin = ""; vec($rin,fileno(STDIN),1) = 1; select($rout=$rin, undef, undef, '$TIMEOUT') < 1 && exit 1; ' } stdin_ready 1 || 'stdin not ready in 1 second, assuming terminal' 

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EDIT 3

Please note that latency can be significant if your input comes from sort , ssh , etc. (All these programs can appear and set the channel using the script seconds or minutes before any data is created.) In addition, using a powerful timeout can sharply fine your script when nothing happens at the initial stage (for example, a terminal).

If potentially long timeouts are a problem, and if you can influence the way the script is called, then you can force the callers to explicitly tell your program whether to use stdin using a custom option or the standard GNU or tar method (for example, script [options [-] ] FILE ..., where FILE can be a file name, a - to indicate standard input or a combination of them, and your script will only read from standard input, if - was passed as a parameter.)

Answer 2

This strategy works for bash and most likely will work for ksh. Poll 'tty':

 #!/bin/bash set -a if [ "$( tty )" == 'not a tty' ] then STDIN_DATA_PRESENT=1 else STDIN_DATA_PRESENT=0 fi if [ ${STDIN_DATA_PRESENT} -eq 1 ] then echo "Input was found." else echo "Input was not found." fi 

Answer 3

Why not solve it in a more traditional way and use the command line argument to indicate that the data will come from stdin?

As an example, consider the difference between:

echo foo | cat -

and

echo foo > /tmp/test.txt

cat /tmp/test.txt