Question

This will be a really simple freebie for a bash guru:

Question

Using bash, how do you make a classpath out of all files in a directory?

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Details

Given a directory:

LIB=/path/to/project/dir/lib

that contains nothing but *.jar files such as:

junit-4.8.1.jar
jurt-3.2.1.jar
log4j-1.2.16.jar
mockito-all-1.8.5.jar

I need to create a colon-separated classpath variable in the form:

CLASSPATH=/path/to/project/dir/lib/junit-4.8.1.jar:/path/to/project/dir/lib/jurt-3.2.1.jar:/path/to/project/dir/lib/log4j-1.2.16.jar:/path/to/project/dir/lib/mockito-all-1.8.5.jar

Some seudo-code that nearly expresses the logic I’m looking for would be along the lines of:

for( each file in directory ) {
   classpath = classpath + ":" + LIB + file.name
}

What is a simple way to accomplish this via bash script?

Answer 1

New Answer
(October 2012)

There’s no need to manually build the classpath list. Java supports a convenient wildcard syntax for directories containing jar files.

java -cp "$LIB/*"

(Notice that the * is inside the quotes.)

Explanation from man java:

As a special convenience, a class path element containing a basename of * is considered equivalent to specifying a list of all the files in the directory with the extension .jar or .JAR (a java program cannot tell the difference between the two invocations).

For example, if directory foo contains a.jar and b.JAR, then the class path element foo/* is expanded to a A.jar:b.JAR, except that the order of jar files is unspecified. All jar files in the specified directory, even hidden ones, are included in the list. A classpath entry consisting simply of * expands to a list of all the jar files in the current directory. The CLASSPATH environment variable, where defined, will be similarly expanded. Any classpath wildcard expansion occurs before the Java virtual machine is started — no Java program will ever see unexpanded wildcards except by querying the environment.

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Old Answer

Good

Simple but not perfect solution:

CLASSPATH=$(echo "$LIB"/*.jar | tr ' ' ':')

There’s a slight flaw in that this will not handle file names with spaces correctly. If that matters try this slightly more complicated version:

Better

CLASSPATH=$(find "$LIB" -name '*.jar' -printf '%p:' | sed 's/:$//')

This only works if your find command supports -printf (as GNU find does).

If you don’t have GNU find, as on Mac OS X, you can use xargs instead:

CLASSPATH=$(find "." -name '*.jar' | xargs echo | tr ' ' ':')

Best?

Another (weirder) way to do it is to change the field separator variable $IFS. This is very strange-looking but will behave well with all file names and uses only shell built-ins.

CLASSPATH=$(JARS=("$LIB"/*.jar); IFS=:; echo "${JARS[*]}")

Explanation:

1. JARS is set to an array of file names.
2. IFS is changed to :.
3. The array is echoed, and $IFS is used as the separator between array entries. Meaning the file names are printed with colons between them.

All of this is done in a sub-shell so the change to $IFS isn’t permanent (which would be baaaad).