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Get the Java version in specific format using bash

My Java version is:
openjdk version "1.8.0_312"
OpenJDK Runtime Environment (build 1.8.0_312-b07)
OpenJDK 64-Bit Server VM (build 25.312-b07, mixed mode)
How can I get the Java version in below format using bash script:
1.8.0
I tried multiple options like java -version 2>&1 | head -n 1 | cut -d'"' -f2, but I'm unable to get the desired output.

To check for a string (your expected version), you can use grep:
if java -version 2>&1 | head -n 1 | grep --fixed-strings '"1.8.0'
then echo expected version
else echo unexpected version
fi
Note the --fixed-strings (short -F), otherwise grep treats . as a RegEx and it will match any character. Thx Gorodon for pointing that out!
I also added the leading " quote so it will not match 11.8.0.

$ IFS='"_' read -r _ ver _ < <(java -version 2>&1)
$ echo "$ver"
1.8.0

You can try this one
java --version | head -n1 |cut -d " " -f1,2
output
openjdk 8.0

You can use grep (or egrep) to print matching patterns:
java --version | head -n 1 | grep -Po '[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}'
java --version | head -n 1 | egrep -o '[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}'

How to access Java property in terminal

I know I can get all the Java System properties from the terminal using
java -XshowSettings:properties -version
How do I access just one specific java system property?
For example, like "user.name"?
I want to do this in the terminal, not with Java.

Solution as a one liner script. Just change the val variable to the key you want to print:
val='java.library.path'; java -XshowSettings:properties -version 2>&1 | sed -re 's/^ +[^=]+ =/_&/' | gawk -v key=$val 'BEGIN{ RS="_"; IFS=" = "} { if($1 ~ key){ print $0 }}'
Details
Some property values like java.library.path contain new lines so we need to mark records before filtering and printing them.
sed allows us to do that, then awk can be used to filter and print.
java -XshowSettings:properties -version 2>&1 |\
sed -re 's/^ +[^=]+ =/_&/' |\
gawk -v key=java.library.path 'BEGIN{ RS="_"; IFS=" = "} { if($1 ~ key){ print $0 }}'
Result:
java.library.path = /usr/java/packages/lib/amd64
/usr/lib64
/lib64
/lib
/usr/lib
Pipeline parts explained:
2>&1: properties are printed to stderr so we need to redirect them to stdin.
sed -re 's/^ +[^=]+ =/_&/' : add an underscore in front of interesting lines, those starting with 4 spaces and containing =.
gawk -v key=java.library.path: set keyawk variable to the selected property key.
'BEGIN{ RS="_"; IFS=" = "}: set record separator to '_' and input field separator IFS to =.

If you need the current logged in user in bash just use whoami command. If you want to get the java property from terminal you can use the following command
java -XshowSettings:properties -version 2>&1 | grep user.name
which will print
$java -XshowSettings:properties -version 2>&1 | grep user.name
user.name = user
If you just need the user name only
java -XshowSettings:properties -version 2>&1 | grep user.name | cut -c 16-100
which will print
$java -XshowSettings:properties -version 2>&1 | grep user.name | cut -c 16-100
user

You can't.
What you can do is create a java file to get the information and run with java, here the documentation.
Since you already said that you don't want this, you can grep (filter) the output of
java -XshowSettings:properties -version 2>&1 | grep java.home
java.home = /usr/java/jdk1.8.0_112/jre
If you want to know the system properties of a running jvm, use the jcmd tool
jcmd PID VM.system_properties

Does java daemonize itself?

I have this code in a java wrapper shell script, written by a third party:
TEMPFILE=$(mktemp java-wrapper-XXXXX)
"$#" | awk -v t=${TEMPFILE} '/unable to fund java heap account/ {print 1 > t} {print}'
RC=$?
if [ 0 -eq ${RC} -o ! -s ${TEMPFILE} ]; then
exit
fi
$# is supposed to hold the java command line, along with the arguments. The logic here is to:
rerun the java command if it fails with heap issues, after invoking another script to defragment memory
do nothing and let the java process run in the background, otherwise
In "$#" | awk ..., java is being called in the foreground. Does java daemonize itself if it bootstraps successfully? That doesn't seem to be making sense to me. Unless there is a memory issue, this code would lead to java process running normally with its output being piped to awk, isn't it?
Please help me understand this. I welcome any suggestions to improve the logic. Please ignore the uppercase variables and other issues that can be found through shellcheck.
Here is the complete script:
#!/bin/bash
# Java Wrapper takes java command as input
# and runs java. If java fails due to zing memory
# it attempts to run get2Mpages.sh (az_fragger binary) for
# the given Xmx and them runs java again
set -o pipefail
BASE=$(cd -P "$(dirname "${BASH_SOURCE[0]}")" && pwd)
NAME=$(basename "${BASH_SOURCE[0]}")
GET_2MPAGES=${BASE}/get2Mpages.sh
fail() {
echo "ERROR: $#" >&2
echo "Usage: $NAME java [<args>...] -XmxNNN [<args>...]" >&2
exit 1
}
[ $# -gt 0 ] || fail "No command specified"
# extract the Xmx value
XMX=$(echo "$#" | sed -n 's/.*-Xmx\([0-9]*.\).*/\1/p')
[ -n "${XMX}" ] || fail "Unable to extract Xmx argument from the command-line"
trap on_exit SIGTERM SIGQUIT EXIT
on_exit() {
rm -f "${TEMPFILE}"
exit ${RC}
}
TEMPFILE=$(mktemp java-wrapper-XXXXX)
"$#" | awk -v t=${TEMPFILE} '/unable to fund java heap account/ {print 1 > t} {print}'
RC=$?
if [ 0 -eq ${RC} -o ! -s ${TEMPFILE} ]; then
exit
fi
# OOM Detected
cat << EOF >&2
Info: Failed to run JAVA due to insufficient 2MB pages
Info: Now running $GET_2MPAGES ${XMX}
EOF
${GET_2MPAGES} ${XMX} && {
echo "Info: attempting to run JAVA again" >&2
echo
"$#"
}
RC=$?

Generate JNI header files for class files in JDK 10

An integral part of the Java Native Interface (JNI), is the bridging of JVM code and native code through C headers. The way to generate these header files used to be quite straight forward: simply call the command line utility javah on class files. This process would then generate prototypes for any method marked with the native modifier.
As of Java 10 however, the javah utility has been removed, and its suggested replacement is a new flag "-h" to javac. The replacement works fine if one has the Java source files available, however falls short in cases where only compiled class files are available. (The issue that sparked this question is that I'm trying to generate JNI bindings from Scala sources. My current approach has been to compile them first and then run javah over the resulting class files.)
In a situation where only compiled class files are available, is there a way to generate C header files, similar to the way javah used to?

You can always go via javap. I know, I know. It's ugly, has lots of assumptions, but in case you desperately need to generate headers for lots of files it might be the only option.
#!/bin/bash
# FIRST_ARG - full class name (with package)
# SECOND_ARG - class path
CLASS_NAME=`javap -cp $2 $1 | \
grep -v "Compiled from" | \
grep "public class" | \
cut -f3 -d" " | \
awk -F"." '{ print $NF }'`
PACKAGE_NAME=`javap -cp $2 $1 | \
grep -v "Compiled from" | \
grep "public class" | \
cut -f3 -d" " | \
sed s/\.${CLASS_NAME}$//`
DIR_NAME=`echo $PACKAGE_NAME | sed 's|\.|/|g'`
mkdir -p java_jni/${DIR_NAME}
JAVA_FILE_NAME="java_jni/${DIR_NAME}/${CLASS_NAME}.java"
echo "package ${PACKAGE_NAME};" > ${JAVA_FILE_NAME}
echo "public class ${CLASS_NAME} {" >> ${JAVA_FILE_NAME}
javap -cp $2 $1 | grep "native" | while read line; do
param=0
comma=`echo $line | grep "," | wc -l`
while [ $comma -gt 0 ]; do
line=`echo $line | sed "s/,/ param_${param}|/"`
let param=param+1
comma=`echo $line | grep "," | wc -l`
done
line=`echo $line | sed "s/)/ param_${param})/" | sed 's/|/,/g'`
echo " $line" >> ${JAVA_FILE_NAME}
done
echo "}" >> ${JAVA_FILE_NAME}
mkdir -p c_header
javac -h c_header ${JAVA_FILE_NAME}
I bet it can be made way more beautiful.
For me, now, when I slowly start to think about inevitable move towards Java 10, and all these cases, where I might be surprised by non existing Java source code, I think it's not a bad idea to have some tool at my disposal. Just in case.

We can use gjavah to generate JNI header files.

The best solution is just install a jdk8, i think.
And no need to uninstall jdk10, just modify the environment variable.

Running Different Bash Commands Based on Java Version

I'm trying to develop a bash build script for a Java project that will be run on Ubuntu and Fedora. Ubuntu uses the gcj compiler while Fedora uses IcedTea.
Both report their errors and warning in slightly different ways, and I want to ignore the warnings (I know, not generally a good idea, but some of the warnings are simply idiotic).
For gcj, I want to run:
javac *.java 2>&1 | grep -A 4 "error:"
but for IcedTea, I want to run:
javac *.java 2>&1 | grep -A 4 "error:\|errors\|.java:"
I'm still new to bash, so how would I write an if statement that would run one versus the other based upon the javac version?

Assuming your java and javac binaries match, and that icedtea is the special case.
#!/bin/bash
ERROR="error:"
java -version 2>&1 | grep -i icedtea > /dev/null
if [ $? -eq 0 ]; then
ERROR="error:\|errors\|.java:"
fi
javac *.java 2>&1 | grep -A 4 $ERROR
On my system, icedtea and sun have the same output for "javac -version", but not for "java -version".

Writing Java build scripts in bash (or any other shell language) has a number of problems:
scripts tend to be non-portable due to shell differences, different command locations, incompatible command options and so on ... even if you try to make the portable.
scripts cannot cope with dependencies (or at least not easily)
scripts cannot cope with recompiling only stuff that has changed
Instead, I suggest that you write a "build.xml" file and use the Ant build tool. Ant has the advantage of running on any build platform that runs Java, and of taking care of the vast majority of platform differences. It is sort of like a better "Make" designed specifically for building Java.

#!/bin/sh
JAVAC_VERSION="`java -version 2>&1 /dev/null | awk '/IcedTea/ {print $4}' | sed -e 's/[\(0-9]//g'`"
ICEDTEA="IcedTea"
if [ ${JAVAC_VERSION} = ${ICEDTEA} ]; then
javac *.java 2>&1 | grep -A 4 "error:\|errors\|.java:"
else
javac *.java 2>&1 | grep -A 4 "error:"
fi
exit 0
That should do it - if i understood your question correctly. How you get the version - im not quite sure of, but if my javac -version is incorrect just change it accordingly to your needs.