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Bash Basics

1. What is a Bash script?

A Bash script is a text file with terminal command executed in order.

Example:

#!/usr/bin/env bash

printf "Hello, World\n"

The first line is called the shebang:

#!/usr/bin/env bash

It tell Linux to run the file using Bash.

To execute the script:

chmod +x script.sh
./script.sh

2. Basic script structure

A clean Bash script can look like this:

#!/usr/bin/env bash

set -euo pipefail

main() {
    printf 'Running script...\n'
}

main "$@"

Explanation:

set -euo pipefail
Option Meaning
-e Exit immediately if a command fails
-u Treat undefined variables as errors
-o pipefail A pipeline fails if any command inside it fails

3. Variables

Declare variables without spaces around =:

name="Ciro"
project="uvm_testbench"

printf "Name: %s\n" "$name"
printf "Project: %s\n" "$project"

Important:

name = "Ciro" # Wrong
name="Ciro"   # Correct

Warning

Use quotes when expanding variables. This avoids problems with spaces or empty values.

4. Script arguments

Arguments are values passed when launching the script:

Example:

./script.sh run_dir coverage.vdb

Inside the script:

#!/usr/bin/env bash
set -euo pipefail

RUN_DIR="$1"
COV_DB="$2"

printf 'Run directory: %s\n' "$RUN_DIR"
printf 'Coverage DB: %s\n' "$COV_DB"

Special argument variables:

Variable Meaning
$0 Script name
$1, $2, ... First, second, etc. argument
$# Number of arguments
$@ All arguments, safely separated

5. Required arguments with useful errors

This pattern is very useful:

RUN_DIR="${1:?missing RUN_DIR}"

Meaning:

  • Use argument $1
  • If $1 is missing or empty, stop the script
  • Print the error missing RUN_DIR

Example:

#!/usr/bin/env bash
set -euo pipefail

RUN_DIR="${1:?missing RUN_DIR}"

printf 'RUN_DIR = %s\n' "$RUN_DIR"

If you run:

./script.sh

Bash will stop with an error.

missing RUN_DIR

6. Default values

Use this syntax to provide a default value:

MODE="${MODE:-debug}"

Meaning:

  • If MODE is defined and not empty, use it
  • Otherwise, use debug

Example:

#!/usr/bin/env bash
set -euo pipefail

MODE="${MODE:-debug}"

printf 'Mode: %s\n' "$MODE"

Run normally:

./script.sh

Output:

Mode: debug

Run with a custom value:

MODE=regression ./script.sh

Output:

Mode: regression

7. Checking files and directories

Bash has useful test operators.

if [[ -d "$RUN_DIR" ]]; then
    printf 'Directory exists: %s\n' "$RUN_DIR"
fi

Common checks:

Check Meaning
-d "$path" Path exists and is a directory
-f "$path" Path exists and is a regular file
-e "$path" Path exists
-z "$var" String is empty
-n "$var" String is not empty

Negation uses !:

if [[ ! -d "$RUN_DIR" ]]; then
    printf '[FAIL] Directory does not exist: %s\n' "$RUN_DIR"
    exit 1
fi

Meaning:

! -d "$RUN_DIR"

means:

If RUN_DIR is not a directory

8. Functions

Functions help organize your script.

print_error() {
    local msg="$1"
    printf '[FAIL] %s\n' "$msg"
}

print_error "Something went wrong"

Use local for variables inside functions:

check_dir() {
    local dir="${1:?missing dir}"

    if [[ ! -d "$dir" ]]; then
        printf '[FAIL] Directory does not exist: %s\n' "$dir"
        return 1
    fi
}

Then call it:

check_dir "$RUN_DIR"

9. Reading command output into a variable

The $() syntax in Bash is used for command substitution. It allows you to execute a command and substitute its output in place of the $() expression. This is useful when you want to use the output of a command as an argument for another command or assign it to a variable.

Example:

latest_file="$(
    find "$RUN_DIR" -type f -name '*.log' |
    sort |
    tail -n 1
)"

10. Reading values from a file with source

Suppose you have a file called coverage.mk

RUN_COV_DB=/path/to/run/simv.vdb
BUILD_COV_DB=/path/to/build/simv.vdb

You can load it is Bash with:

source $$RUN_MANIFEST_FILE"

After that, the variables are available:

printf 'RUN_COV_DB = %s\n' "$RUN_COV_DB"

Example:

#!/usr/bin/env bash
set -euo pipefail

RUN_MANIFEST_FILE="${1:?missing RUN_MANIFEST_FILE}"

if [[ ! -f "$RUN_MANIFEST_FILE" ]]; then
    printf '[FAIL] Manifest file does not exist: %s\n' "$RUN_MANIFEST_FILE"
    exit 1
fi

source "$RUN_MANIFEST_FILE"

if [[ -z "${RUN_COV_DB:-}" ]]; then
    printf '[FAIL] RUN_COV_DB is empty\n'
    exit 1
fi

printf 'RUN_COV_DB = %s\n' "$RUN_COV_DB"

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