Using Shell to Check Whether a File Exists, and only if it does, Execute a Set of Commands - shell

Question

I have a few lines of code in Stata. I'd like the lines to be executed only if the .txt file to which the lines refer exist a priori. I am wondering whether there is a shell command that I can use for this that I can embed in an if statement.
For example might something like the following exist and be possible:
insheet using "file.txt" if ('file.txt')
My intent is to say insheet the file file.txt only if it exists. My concern is that the program would otherwise stop, fail, die, or whatever you call it due to a syntax error if I have that insheet statement but the file does not exist.

Answer 1

Immediate answer is No. There is nothing like that syntax for several reasons.
The if qualifier tests whether some condition is true separately for each observation and whether a file exists is not an appropriate condition for testing observation by observation.
The quite different if command tests once and once only whether something is true and might seem more appropriate. In practice it is not used for this purpose, but to learn more, see help ifcmd.
Stata has no special syntax based on paired identical single quotes ' '.
However, Stata provides a separate construct here
confirm file file.txt
In practice that is going to stop a do-file or program whenever the file does not exist and the file does not exist. A general scheme to catch the error is something like
capture confirm file file.txt
if _rc == 0 insheet using file.txt
else {
<code if the file does not exist>
}
capture is to be thought of as eating the return code from the confirm command. In general the return code _rc from any command is 0 if the command was valid and executed and some non-zero value otherwise. Sometimes one tests for a specific non-zero code. Experiment shows that file not found is return code 601. The main reason for looking up error codes (in [P] error) is to deliver official-looking error messages, but in practice knowing the zero/non-zero rule is the main detail under this heading.
The example here uses == to test for equality.
Note that insheet using file.txt is not strictly a syntax error if the file does not exist. As far as Stata's language is concerned, that is legal syntax. However, that is a fine distinction: it is an error in every ordinary sense.
(LATER) It would be possible to short-circuit the entire process
capture insheet using file.txt
if _rc != 0 {
<code if the file does not exist>
}
as in this case the non-existence of the file is the presumed explanation for any failure of the insheet command. If, however, the insheet call were more complicated, with a varlist and/or options, then failure of the command could arise for other reasons. So in general separating out a check for the existence of the file seems a better strategy.

Answer 2

The confirm command has what you're looking for.
capture confirm file "file.txt"
if !_rc { # if the file exists, confirm will return error code 0
insheet using "file.txt"
}
Alternatively, you could put a capture before the insheet command, which will catch the syntax error. Check the [P] manual for more on capture and confirm.