Bash Scripts

Bash Scripting

Basics

• Bash is the Bourne Again Shell

• First line: #!/bin/bash or #!/usr/bin/env bash

• Run file by adding ./ at the beginning

• For example: ./say_hello.sh

Change a file to be executable

1. Look at the file:
   • ls -l <filename.sh>
2. If this is at the beginning of the file (-rw-r--r--), you need to change it to be executable
3. Change the file permissions:
   • chmod 755 <filename.sh>

The shorthand 755 is often used for scripts as it allows you the owner to write or modify the script and for everyone to execute the script.

1. A different way to make a file executatble
   • chmod +x <filename.sh>

A cron job is when you schedule a recurring task (when a script is run)

the $PATH variable is a set of folders where executable programs live.

Isolate the name of the directory you're in

DIRECTORY_NAME= pwd | xargs basename echo $DIRECTORY_NAME

${PWD##*/}

Regex, Find and Cut

• Regex for exact match: ^pattern_here$
• Find only directories or files: find dir_name -type d or find dir_name -type f
• Find directories and exclude paths: find dir_name -type d -not -path '*/\*'

Common Safguards in a Shell Script

Bash Best Practices

• Exit a script if part of it fails:
  • set -o errexit or set -e
• Exit a script if a variable is use which has not been initialized
  • set -o nounset or set -u
• Prevent piping from causing non-zero exit codes to be ignored
  • set -o pipefail

Comparison Operators

• == *
• !=
• =~
  • Used when a regular expression is involved. The right side is considered an extended regular expression. If the left side matches, the operator returns 0, and 1 otherwise.

Exit Codes

• success is 0
• failure is anything other than 0
• Show the exit code of the last process:
  • echo $?
• Scripts should

Built-In Vairiables

• run ./install.sh 1 2 3
  • $#: the length of the input (3)
  • $@: the input (1 2 3)
  • $?: was the last command successful? If yes, the result will be 0

OMG, why isn't this working?

• try wrapping your command in $( )
  • it means 'evaluate this and use the result'
• Example:

exitcode=$(curl -s -o /dev/null -w "%{http_code}" https://gitlab.com/stable-orbit/$REPO_NAME)

echo ${exitcode}

Stuff EJ told me

function clone_and_clean_repo {
    mkdir temp || true #prevents exit if the dir already exists
    cd temp
    git clone git@github.com:reactiveops/$REPO_NAME
    cd $REPO_NAME
    rm -rf ./* # rm -rf in a script can be dangerous. $(PWD) was removing the entire direcotry, `./*` removes all non-hidden files in the current directory leaving .git which is required for the next steps
    echo "# [deprecated] $REPO_NAME\nIf you would like this code, please go to https://gitlab.com/stable-orbit/$REPO_NAME)" > README.md
    push_changes
}