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BASH scripting

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#!/bin/bash

Variables

Defining a variable:

my_name=Jon

Referencing (using) a variable:

$my_name

&&

&& means "and if successful"

A Unix pipe |

||

>

;

then

if statement

Single [ ... ] vs double [[ ... ]] brackets Using double brackets [[ ... ]] in bash scripting provides several advantages over single brackets [ ... ]:

  1. Enhanced Syntax: Double brackets support more complex conditional expressions, such as pattern matching =~, logical operators &&, ||, and arithmetic comparisons without needing to escape special characters.
  2. Safer Variable Handling: Double brackets handle variables with spaces or special characters more safely, reducing the risk of syntax errors.
  3. No Word Splitting or Filename Expansion: Inside double brackets, word splitting and filename expansion are not performed, which can prevent unexpected behaviour. The result is a more robust and less prone to errors script.
if [[ -f "$file" ]]; then
    echo "$file"
fi

Parameter expansion

It allows you to manipulate the value of variables in various ways, for example: extracting substrings, setting default values, removing parts of strings based on patterns, etc.

${var}

Usefull when working file names, for example, getting the extenshion of file, file name without the extension, etc.

Remove the shortest match of a pattern from the beginning:

# example: file.gz.html -> gz.html
${filename##*.}

Remove the longest match of a pattern from the beginning:

# example: file.tar.gz -> gz
${filename##*.}

Remove the shortest match of a pattern from the end:

# example: file.html -> file
${file%.*}

Remove the longest match of a pattern from the end:

# example: file.html -> file
${file%%.*}

Additional info:

  • ## removes the longest matching pattern from the beginning of the variable's value
  • *. matches everything up to and including the last dot . in the filename

For loops

For each file in current directory with the extension of *.html

for file in *.html; do 
    # Using double quotes around $file is a good practice in bash scripting.
    # Using double quotes ensures that the script handles filenames with spaces or special characters correctly
    if [ -f "$file" ]; then 
        echo "$file"
    fi 
done

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