bash

History Expansion

Reuse parts of previous commands to speed up your workflow.

Brace Expansion

Generate arbitrary strings.

echo filename{,.bak}

Create directory structure.

mkdir -p project/{src,test,doc}/{html,pdf}

Generate ranges.

echo {1..10}

Generate letter ranges.

echo {a..z}

Parameter Expansion

Manipulate variables without external tools like sed or awk.

Arrays

Indexed Arrays

Declare an indexed array.

arr=(one two three)

Read the first element.

echo "${arr[0]}"

Read all elements.

echo "${arr[@]}"      # All elements

Read the array length.

echo "${#arr[@]}"     # Array length

Read all array indices.

echo "${!arr[@]}"     # All indices

Append an element.

arr+=(four)

Delete one element.

unset "arr[1]"

Associative Arrays (Bash 4.0+)

declare -A map
map[foo]="bar"
map[baz]="qux"

echo "${map[foo]}"

Special Variables

Commonly used special shell variables.

Conditional Expressions

File Test Operators

String & Integer Comparison

Advanced Tests ([[ ]])

The [[ ... ]] construct is safer and more powerful than [ ... ].

Regex matching example.

if [[ $str =~ ^[0-9]+$ ]]; then
  echo "Is a number"
fi

Pattern matching example.

if [[ $file == *.tar.gz ]]; then
  echo "Is a tarball"
fi

Logical operator example.

if [[ -f $file && ! -s $file ]]; then
  echo "File exists but is empty"
fi

Loops

For Loop

Iterate over a fixed list.

for item in one two three; do
  echo "$item"
done

Use a C-style loop.

for ((i=0; i<10; i++)); do
  echo "$i"
done

Iterate over a numeric range.

for i in {1..5}; do
  echo "$i"
done

While Loop

count=0
while [[ $count -lt 5 ]]; do
  echo "$count"
  ((count++))
done

Functions

Define reusable code blocks.

Use local to limit a variable to the function scope.

my_function() {
  local var="local variable"
  echo "Argument 1: $1"
  return 0
}

my_function "hello"

💡 Tip: Always use local for variables inside functions to avoid polluting the global scope.

Built-in Commands

Colon (:)

The colon command is a built-in "true" null command that does nothing and always returns success (exit status 0).

Clear a file content without deleting the file:

:> file.txt

Process Substitution

Compare output of two commands.

diff <(ls dir1) <(ls dir2)

Loop over command output.

while read -r line; do echo "$line"; done < <(grep "something" file.txt)

Redirection & Pipes

Advanced Redirection

Write a here-document to a file.

cat <<EOF > file.txt
Multi-line
content
EOF

Close file descriptor 3.

exec 3>&-

Open a file for reading on file descriptor 3.

exec 3< file.txt

Keyboard Shortcuts

Safety & Debugging

Strict Mode

Start your scripts with this to fail fast on errors.

set -euo pipefail
IFS=$'\n\t'

Traps

Execute code when the script exits or receives a signal.

cleanup() {
  rm -f /tmp/tempfile
}

trap cleanup EXIT

Job Control

Manage background processes.

Resources

• GNU Bash Manual
• ShellCheck - Static analysis tool for shell scripts.
• Bash Hackers Wiki