Some notes on Bash Shell Script

#bash #shell #shellscript

#! Shebang

#! /bin/bash

Tell OS to use bash shell (interpreter) to run the script.

Declaring Variable

list_of_variables=("put" "like" "this")
list_of_variables+=("you" "can" "append" "more")
list_of_variables+=("even" "more" "list_of_variables+=")

If statement with environment variable

if [ "$ENABLE_DD_PROFILE" = "true" ]; then
  echo "something"
fi

Print variable

The java_args[@] syntax is used to access the elements of the java_args array. The @ symbol tells Bash to expand the array, so that the individual elements of the array are passed to the Java program as arguments.

# method 1 to print all the arguments
for arg in ${java_args[@]}; do
  echo $arg
done

# method 2 to print all the arguments
printf "%s\n" "${java_args[@]}"

Example of a non working java command bash shell script

#! /bin/bash

java_args=(
    "-Xmx1024m" # max heap size
    "-Xms512m" # min heap size
    "-XX:+HeapDumpOnOutOfMemoryError"
    "-XX:HeapDumpPath=./temp/heapdump"
    )

if [ "$ENABLE_DD_PROFILE" = "true" ]; then
java_args+=(
    "-javaagent:dd-java-agent.jar"
    "-Ddd.service=<YOUR_SERVICE>"
    "-Ddd.env=<YOUR_ENVIRONMENT>"
    "-Ddd.version=<YOUR_VERSION>"
    "-Ddd.profiling.enabled=true"
    "-XX:FlightRecorderOptions=stackdepth=256"
    )
fi

# if -cp and paths are placed before the javaagent, my java would not run correctly somehow. 
java_args+=(
    "-cp"
    "lib/*"
    "lib2/*"
    "-jar <YOUR_SERVICE>.jar"
    )

# print all the arguments
printf "%s\n" "${java_args[@]}"

# run java command
java ${java_args[@]}

Run command

You need to chmod a shell script so that it can be executed by the user. By default, a newly created file has no execute permissions, which means that it cannot be run. The chmod command is used to change the permissions of a file.

chmod +x make the script executable.

$ export ENABLE_DD_PROFILE=true
$ chmod +x java_args.sh && ./java_args.sh
-Xmx1024m
-Xms512m
-XX:+HeapDumpOnOutOfMemoryError
-XX:HeapDumpPath=./temp/heapdump
-javaagent:dd-java-agent.jar
-Ddd.service=<YOUR_SERVICE>
-Ddd.env=<YOUR_ENVIRONMENT>
-Ddd.version=<YOUR_VERSION>
-Ddd.profiling.enabled=true
-XX:FlightRecorderOptions=stackdepth=256
-cp
lib/*
lib2/*
-jar <YOUR_SERVICE>.jar
Error opening zip file or JAR manifest missing : dd-java-agent.jar
Error occurred during initialization of VM
agent library failed to init: instrument

The Java error is intentional, as the main goal is to showcase my new knowledge of bash shell scripts.

Kthxbye.

Top comments (0)

I/O redirections on Node.js via Git Bash for Windows (Windows-users oriented)

Lukas Gaucas - Aug 2 '22

Building a Web server in Bash, part III - Login

Leandro Proença - Aug 1 '22

Building a Web server in Bash, part II - parsing HTTP