🖥️jar

➡️This is a command-line reference manual for commands and command combinations that you don’t use often enough to remember it. This cheatsheet explains the jar command with important options and switches using examples.

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# List all packages with at least a class defined in a JAR file
jar tf "$1" | grep '/.*\.class$' | xargs dirname | sort -u | tr / .
# Explanation: The jar command allows you to read or manipulate JAR (Java ARchive) files, which are ZIP files that usually contain classfiles (Java compiled bytecode files) and possibly manifests and configuration files. We specify that we want to list file contents (t) that we provide as an argument (f, otherwise the jar will be read from stdin). From the output, we get only the paths that contain a classfile (grep), then the path to the package that contains it (xargs dirname), we get the unique, sorted paths and translate /s to .s (to display their names as they would be shown in Java syntax).
# Limitations: Will only exhaustively list the packages with a defined class for languages that require packages to map to the directory structure (e.g.: Java does, Scala does not). If this convention is respected, the command will output an exhaustive list of packages nonetheless.

cat foo.zip | jar xv
# Extract a Zip File from STDOUT with the Jar Command
# Plain old unzip will not unzip output coming from STDOUT the ZIP file format includes a directory (index) at the end of the archive. This directory says where, within the archive each file is located and thus allows for quick, random access, without reading the entire archive. This would appear to pose a problem when attempting to read a ZIP archive through a pipe, in that the index is not accessed until the very end and so individual members cannot be correctly extracted until after the file has been entirely read and is no longer available. As such it appears unsurprising that most ZIP decompressors simply fail when the archive is supplied through a pipe. The directory at the end of the archive is not the only location where file meta information is stored in the archive. In addition, individual entries also include this information in a local file header, for redundancy purposes. From the `jar` manpage: > The jar command is a general-purpose archiving and compression tool, based on ZIP and the ZLIB compression format. JAR is smart enough to know how to handle these local file headers when the index is unavailable when reading through the pipe. (Most of the explanation in this description is taken from https://serverfault.com/a/589528/314226 , though they recommend using `bsdtar`, but that is not always available on systems) Show Sample Output:

        # ?? cat foo.zip | jar xv 
        # extracted: foo/bae.txt
        # extracted: foo/bar.txt
        # extracted: foo/baz.txt
        # extracted: foo/boo.txt

Cheatsheets are an excellent complement to other information sources like Linux man-pages, Linux help, or How-To’s and tutorials, as they provide compact and easily accessible information. While man-pages and detailed tutorials often contain comprehensive explanations and extensive guides, cheatsheets summarize the most important options forthe command jar in a clear format. This allows users to quickly access the needed information for jar without having to sift through lengthy texts. Especially in stressful situations or for recurring tasks, cheatsheets for jar are a valuable resource to work efficiently and purposefully.

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