20 Essential Shell Functions for File Management, System Monitoring, and Automation

Overview

This reference collects twenty reusable Bash functions that address common scripting tasks such as file and directory handling, system inspection, text formatting, and simple service control. Each function is self‑contained and can be sourced into a script or interactive shell.

File and Directory Checks

Check if a file exists

file_exists() {
  [ -f "$1" ] && echo "File exists." || echo "File does not exist."
}

Check if a directory exists

dir_exists() {
  [ -d "$1" ] && echo "Directory exists." || echo "Directory does not exist."
}

List files in a directory (non‑recursive)

list_files() {
  find "$1" -maxdepth 1 -type f -print
}

Count files in a directory

count_files() {
  find "$1" -type f | wc -l
}

Get the most recently modified file in the current directory

last_modified_file() {
  ls -t | head -n 1
}

Empty a file

empty_file() {
  > "$1"
}

File Information

Show file size in human‑readable format

get_file_size() {
  du -sh "$1" 2>/dev/null | cut -f1
}

System Utilities

Test whether a command is available

command_exists() {
  command -v "$1" >/dev/null 2>&1
}

Print the current date (YYYY‑MM‑DD)

current_date() {
  date +"%Y-%m-%d"
}

Print the current time (HH:MM:SS)

current_time() {
  date +"%H:%M:%S"
}

Calculate the difference between two dates (in days)

date_diff() {
  # Arguments: start_date end_date (format accepted by GNU date)
  start=$(date -d "$1" +%s)
  end=$(date -d "$2" +%s)
  echo $(( (end - start) / 86400 ))
}

Print coloured text

print_color() {
  local text=$1
  local color=$2
  case "$color" in
    red)    echo -e "\033[31m${text}\033[0m" ;;
    green)  echo -e "\033[32m${text}\033[0m" ;;
    yellow) echo -e "\033[33m${text}\033[0m" ;;
    blue)   echo -e "\033[34m${text}\033[0m" ;;
    *)      echo "$text" ;;
  esac
}

Delay execution for a given number of seconds

delay() {
  sleep "$1"
}

Check whether the script runs as root

is_root() {
  [ "$(id -u)" -eq 0 ] && echo "Root user" || echo "Not root user"
}

Retrieve the primary IP address of the host

get_ip() {
  hostname -I | awk '{print $1}'
}

Generate a random alphanumeric string (default length 10)

generate_random_string() {
  local length=${1:-10}
  tr -dc 'a-zA-Z0-9' </dev/urandom | fold -w "$length" | head -n 1
}

Test whether a TCP port on a host is open

is_port_open() {
  nc -zv "$1" "$2" 2>/dev/null | grep -q open
}

Service Management (systemd)

Restart a systemd service

restart_service() {
  systemctl restart "$1"
}

Check service status silently (returns 0 if active)

service_status() {
  systemctl is-active --quiet "$1"
}

Email Sending

Send a simple email using the local mail command

send_email() {
  # Arguments: subject body recipient
  echo "$2" | mail -s "$1" "$3"
}

Usage Notes

All functions assume a GNU/Linux environment with standard utilities ( find, date, nc, systemctl, etc.). Adjust paths or replace commands when running on non‑systemd distributions or minimal containers. The send_email function requires a correctly configured mail (or mailx) utility; otherwise it will fail silently.