Bash

Automate your Work

Thomas Förster

September 17, 2025

Bash Basics

Why bother to use a shell?

The shell is like a chat with your computer. It helps you to interact with your computer using text commands.

The shell is a very productive programming environments. Once mastered, you can automate your work.

CLI Syntax

Most commands follow a similar convention.

$> COMMAND FLAGs

We can modify commands by setting various flags (also known as options, parameters, or, most frequently, arguments)

Getting Around Your Shell

• Print working directory pwd

  $> pwd

• list directory’s content ls

  $> ls

• list directory content in more details and in list form ls -l

  $> ls -l

• Let’s make the output more human readable

  $> ls -l -h
  $> ls -lh
  $> ls -lh --color=auto -g

• To make life easier we can define an alias.

  $> alias ll='ls -l --color=auto -g'
  $> ll

• Let’s dive into a directory via change directory cd

  $> cd Desktop
  $> pwd

• Let’s test error handling of cd

  $> cd "This is a great string."

• Let’s move around with via cd

  $> ls -l -a
  ..
  .
  $> cd ..
  $> cd -
  $> cd

How to Get Help

• using the --help option

• using man COMMAND

  $> ls --help
  $> man ls

Basic File/Folder Manipulation

• Let’s create a new folder with make directory

  $> mkdir sandbox
  $> cd sandbox

• touch: create a file.

  $> touch myfile01.md

• cp: copy the file.

  $> cp myfile01.md myfile02.md

• mv: rename/move a file.

  $> mv myfile02.md myfile03.md

• rm: remove a file

  $> touch test.md
  $> rm test.md

• rm: remove a folder

  $> mkdir testfolder
  $> rm -r testfolder

• create several folders and files

  $> mkdir aaa bbb ccc
  $> mkdir --parents one/two/three
  $> touch 01.log 02.log 03.log 04.log

Wildcards

• The ? wildcard matches one character. The * wildcard matches zero or more characters.

  $> rm -rf *

Succession of Commands

• going from command to command (next) ;

  $> mkdir one/two/three; cd one; touch 01 02 03; mv 0* two/

  • What went wrong?

• going from command to command if successful via &&

  $> mkdir -p one/two/three && cd one && touch 01 02 03 && mv 0* two/

Using the History

• You’ve got a bash history

  $ history

• You can access it via !NUM

  $ !656

• You can access your last command via

  $ !-1
  $ !!

Using Redirection

> or >> is used for redirecting either to a command or a file.

• Let’s print something to the shell

  $> echo "Hallo world!"

• Two options overwrite (create) > or append >>.

  $ echo "Hallo world!" > myfile.md
  $ echo "Hallo world!" >> myfile.md

• How to view the content of files

• cat: concatenate, it is not an editor => go for vim or nano

  $> cat file1.md
  $> cat file1.md file2.md > combi.md

find, xargs and grep

• Let’s find all files with find

  $> find . -type f

• Find all markdown files

  $> mkdir folder.md
  $> touch file.md
  $> find . -name "*.md"
  $> find . -name "*.md" -type f
  $> find . -name "*.md" -type d

• Let’s get some more information via ls -l using xargs

  $> find . -name "*.md" -type f | xargs ll

• Let’s add a file with spaces into the mix,

  $> touch 'a file with spaces.md'
  $> find . -name "*.md" -type f | xargs ls -l

• What happend?

  $> find . -name "*.md" -type f -print0 | xargs -0 ls -l

• Let’s find all *.md files which contain the word “Hallo”, using grep -l.

  $> find . -type f -name '*.md' -print0 | xargs -0 grep -l 'Hallo'

• Let’s get more information about those file via ls -l

  $> find . -type f -name '*.md' -print0 | xargs -0 grep -l 'Hallo' | xargs ls -l

Summary

We have learned:

• Getting around the shell
• Basic file/folder manipulation
• Getting information and help

Bash Scripting

<<< Everything is considered to be a String >>>

Variables

Strings and Integers

$> var=1
$> echo $var
$> var="Hallo world!"
$> echo "$var"
$> echo '$var'
$> var1="Hallo"
$> var2="world!"
$> varN="$var1 $var2"
$> echo $varN

Arrays

$> array=( 'two' 'three' 'four' )
$> echo "$array"

• Only the first element is accessed!

• Access all elements

  $> echo "${array[@]}" 

• Number of elements via #

  $> echo "${#array[@]}"

• You can access elements with integers (stating with 0)

  $> echo "${array[2]}"
  $> echo "${array[0]}"
  $> echo "${array[-1]}"

Integer Arithmetics

$((...)) is called arithmetic expansion. It can be used for calculations incl. arithmetical conditionals. It is derived from let (Low Execution Time integer).

• calculations

  $> let 'new1=3*12'; echo "$new1"
  $> new2=$((3*12)); echo "$new2"

• conditionals

  $> i=0
  $> let 'i>2'; echo $? %false 1
  1
  $> let 'i<2'; echo $? %true 0
  0

[[..]] Compound Command + if

• General-purpose compound command ([[...]])

• It is based on the derivative of the older test command ([...]).

• String comparison (ex. ==)

  $> var="Nice"
  $> if [[ "$var" == "Bad" ]]; then echo "Bad example!"; else echo "Good example"; fi

• Number testing (ex. -eq)

  $> var="10"
  $> if [[ "$var" -eq 10 ]]
  then 
  echo "Equal 10"
  else 
  echo "Not 10"
  fi

• File testing and boolean operation (-f, -r, &&)

  $> FILE="README.md"
  $> if [[ -f "$FILE" && -r "$FILE" ]]
  then 
    echo "File exists and is readable!"
  else 
    echo "File $FILE does not exist"
  fi

• Checking if the folder is empty.

  $> if  [[ -z "$(ls -A .)" ]]; then echo "Empty"; else echo "Not empty"; fi
  $> if  [[ ! -z "$(ls -A .)" ]]; then echo "Not empty"; else echo "Empty"; fi

Loops

for Loop

• Going through elements in an Array. Let’s give it a try with:

  $> array=( 'two' 'three' 'four' )
  $> for item in $array
  do
    echo "$item trees"
  done

• Not working, this is much better:

  $> for item in ${array[@]}
  do
    echo "$item trees"
  done

• Using a Counter

  $> for (( i=0; i<5; ++i ))
  do
    echo "${array[i]} trees"
  done

• Let’s make it better

  $> for (( i=0; i<${#array[@]}; ++i ))
  do
    echo "${array[i]} trees"
  done

Functions

• are like mini bash scripts

• Let’s create a function from this line

  $> function findfiles {
      local filetype="$1"
      find . -type f -name "*.$filetype" -print0 | xargs -0 ls -l
  }

• Let’s check if an argument is provided: -z

  if [[ -z "$filetype" ]]; then
      echo "Error: You must provide a file extension as an argument."
      return 1
  fi

Result:

$> function findfiles {
    local filetype="$1"
    
    if [[ -z $filetype ]]; then
        echo "Error: You must provide a file extension as an argument."
        return 1
    fi

    find . -type f -name "*.$filetype" -print0 | xargs -0 ls -l
}
$> findfiles md

Special Variables

• $0 .. currently executing command
• $1..$9 .. argument number 1 to 9
• $#.. number of arguemts
• $@ .. all arguments
• $? .. last exit status
• $$ .. current pid of the current shell

$> function myfunc {
echo "$0"
echo "$1"
echo "$#"
echo "$@"
echo "$?"
echo "$$"
}
$> myfunc "Check!"

Script Layout

#!/usr/bin/env bash
# SPDX-FileCopyrightText: Copyright (c) 2025 Max Musterman
# SPDX-License-Identifier: MIT

# Name        : projectA
# Version     : 0.1.0
# Description : Creates basic project layout
# Required    : -

#==========#
#== INIT ==#
#==========#

#================#
#== USER INPUT ==#
#================#

#===============#
#== FUNCTIONS ==#
#===============#


#---------------#

function testing {
  echo "Here are all my tests."
}

#---------------#

function main {
  echo "I am looking for content!"
}

#==========#
#== MAIN ==#
#==========#
testing
main

The Challenge

Task: Create a Project Template

$> bash ../myscript.sh
.
├── a_literature
├── b_geometry
├── c_setup
├── d_calculations
├── e_analysis
├── f_documentation
└── README.md

Hints

mydirs=(
"a_literature"
"b_geometry"
"c_setup"
"d_calculations"
"e_analysis"
"f_documentation"
)

myfiles=(
"README.md"
)

• Create two functions:

  function create_dirs {}
  function create_files {}

One Solution

#!/usr/bin/env bash
# SPDX-FileCopyrightText: Copyright (c) 2025 Max Musterman
# SPDX-License-Identifier: MIT

# Name        : foobar
# Version     : 0.1.0
# Description : Foo for Bar
# Required    : -
# Note        : -

#==========#
#== INIT ==#
#==========#
mydirs=(
"a_literature"
"b_geometry"
"c_setup"
"d_calculations"
"e_analysis"
"f_documentation"
)

myfiles=(
"README.md"
)

#================#
#== USER INPUT ==#
#================#

#===============#
#== FUNCTIONS ==#
#===============#

function create_dirs {
  local inputdirs=("$@")
  for dir in ${inputdirs[@]}
  do 
    mkdir $dir 
  done
}

#---------------#

function create_files {
  local inputfiles=("$@")
  for file in ${inputfiles[@]}
  do 
    touch $file
  done
}

#---------------#

function test_empty_dir {
  if  [[ ! -z "$(ls -A .)" ]]; then 
    echo "Folder not empty! Could not create project!"
    exit 1
  fi
}

#---------------#

function testing {
  test_empty_dir
}

#---------------#

function main {
  create_dirs ${mydirs[@]}
  create_files ${myfiles[@]}
  ls -l
}

#==========#
#== MAIN ==#
#==========#
testing
main

Script Management

…