I am trying to create a linux shell script that compiles a program in java and runs it only if there are no errors from the compiler. This is my attempt, which doesn't work, and runs the program every time even if there are errors:
#!/bin/bash
cd ~/Desktop/Foo/src
compilerOutput=$(javac Foo.java)
if ["$compilerOutput" = ""]; then
java Foo
fi
Like any decent Unix tool, javac should set its exit code to non-zero if there is an error.
javac Foo.java && java Foo
Behind the scenes, the shell examines the exit code (which is available as $? if you want to examine it explicitly) and takes the next action only if it is zero.
If you want more complex logic, you can say
if javac Foo.java; then
date +"%c success" >>logfile.log
java Foo
else
date +"%c fail (exit code $?)" >>logfile.log
echo "Oh dear" | wall
fi
The root cause of problem is
compilerOutput=$(javac Foo.java)
When you use command substitution, only the standard output get assigned to compilerOutput
For javac there is no standard output, that is the standard output is always empty, What you see on the screen in standard error
There for even when there is no errors in the program or not
if ["$compilerOutput" = ""]; then
will always be true as the standard output is always empty
Solution
The proper way of doing this is to get the exit status of the previous command through the shell variable $?
Code
#!/bin/bash
cd ~/Desktop/Foo/src
javac Foo.java 2> errors
if [ $? = "0" ]; then
java Foo
fi
Changes made
javac Foo.java 2> errors
This statement redirects bstandard error to errors file
if [ $? = "0" ]; then
Checks if the exit status of previous command, javac Foo.java is 0 (Successfull compilation)
if command; then is more idiomatic and shorter than command; if [ $? = 0 ]; then; there really is no reason to use the latter, ever.