What does it mean in GFortran?
 

if [ $? -ne 0 ]; then. What does -ne mean? And why is there a semicolon inside? The expression seems to be from a command line argument. You can find a links for fftpack5_prb.sh in the middle of the page. Can anybody interpret it all. Thanks.

fftpack5_prb.sh is a shell script, not a program written in GFortran.

The -ne comparison operator means "not equal to". The semicolon after the closing bracket is part of the syntax for an if statement in most shells:The $? shell variable contains the exit code for the last command/executable that was run. An exit code of 0 indicates that the program/command completed without errors. In other words, the if statement seems to say: "if the previous program encountered an error and returned a non-zero exit code, do [whatever comes after then]".

Thanks. I thought it was a command line argument for a Fortran run main or something. Is it a command line written in shell script? If so where can I read about it?

fftpack5_prb.sh is a bash script, indicated by both the filename extension and the "#!/bin/bash" at the top of the script.

is standard bash syntax for checking the exit status of the previously-executed command. If the exit status is not 0, then it does something in response, usually printing an error and exiting.

suicidaleggroll, thank you. As always you are on top of the game!!!