The scripting language of the terminal, automates the commands you'd otherwise type by hand. bash is the most common shell on Linux and (via WSL/Git Bash) widely used on Windows too; macOS ships zsh by default but is largely bash-compatible for scripting purposes.

Running a script

#!/usr/bin/env bash
set -euo pipefail   # exit on error, undefined var, or failed pipe, put this in almost every script

echo "Hello, world"

chmod +x script.sh
./script.sh

set -euo pipefail is close to a universal best practice: without it, a script silently continues past a failed command, which is rarely what you want.

Variables

name="Ada"
count=0

echo "Hello, $name! Count: $count"
echo "Hello, ${name}!"   # braces avoid ambiguity when concatenating

# command substitution
current_dir=$(pwd)
files=$(ls | wc -l)

No spaces around = in assignment, name = "Ada" is a syntax error (Bash parses it as running a command called name with arguments).

Conditionals

if [ -f "config.yml" ]; then
    echo "Config found"
elif [ -d "config" ]; then
    echo "Config directory found"
else
    echo "No config"
fi

# [[ ]] is the more modern, more forgiving test syntax
if [[ "$name" == "Ada" && -n "$count" ]]; then
    echo "Match"
fi

Loops

for file in *.txt; do
    echo "Processing $file"
done

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

while read -r line; do
    echo "Line: $line"
done < input.txt

Functions

greet() {
    local name="$1"   # local scopes the variable to the function
    echo "Hello, $name!"
}

greet "Ada"

Pipes and redirection

cat access.log | grep "ERROR" | wc -l     # pipe output between commands
echo "log line" >> output.log             # append
echo "log line" > output.log              # overwrite
command 2> errors.log                     # redirect stderr only
command > all.log 2>&1                    # redirect both stdout and stderr

Argument handling

#!/usr/bin/env bash
echo "Script name: $0"
echo "First arg: $1"
echo "All args: $@"
echo "Arg count: $#"

Common gotchas

• Unquoted variables ($name instead of "$name") break on spaces/glob characters in the value, quote variables almost everywhere, including in [ ] tests.
• rm -rf $dir with an empty or unset $dir can expand to rm -rf / in the worst case, always quote, and consider set -u to fail loudly on unset variables instead.
• Exit codes matter for scripting logic (0 = success, non-zero = failure), check $? or use if command; then directly rather than parsing command output to detect failure.