Bash Scripting: Automating Repetitive Tasks

Your First Bash Script: Shebang and Execution

A Bash script is a text file containing commands that the shell executes automatically. Create check_system.sh:

#!/bin/bash
set -euo pipefail
echo "=== System Health ==="
echo "Date: $(date)"
echo "Host: $(hostname)"
echo "Uptime: $(uptime -p)"
df -h / | tail -1

The #!/bin/bash shebang tells Linux which interpreter to use. set -euo pipefail exits on errors, treats unset variables as errors, and fails pipelines properly.

chmod +x check_system.sh && ./check_system.sh

Variables and Command-Line Arguments

SENSOR_DIR="/opt/scada/data"
MAX_TEMP=95
SERVER=$(hostname)
echo "Collecting on $SERVER from ${SENSOR_DIR}"

No spaces around =. Use ${VAR} adjacent to text. Arguments come from $1, $2, etc.

Conditions: if, test, and [[ ]]

TEMP=97
if [[ $TEMP -gt 95 ]]; then
    echo "WARNING: Temperature exceeds threshold!"
elif [[ $TEMP -gt 80 ]]; then
    echo "NORMAL: Within range."
else
    echo "LOW: Below expected."
fi

Numeric: -eq, -ne, -gt, -lt, -ge, -le. Strings: ==, !=.

[[ -f "$FILE" ]]    # File exists
[[ -d "$DIR" ]]     # Directory exists
[[ -s "$FILE" ]]    # File not empty
[[ -x "$FILE" ]]    # File executable

Loops: for, while, and until

for sensor in sensor_01 sensor_02 sensor_03; do
    ping -c 1 "${sensor}.factory.local" > /dev/null 2>&1 \
      && echo "$sensor: OK" || echo "$sensor: DOWN"
done

for file in /opt/scada/data/*.csv; do
    echo "$(wc -l < "$file") lines in $file"
done

while IFS=',' read -r ts sensor val status; do
    [[ "$status" == "ALARM" ]] && echo "ALARM: $sensor=$val at $ts"
done < /var/log/sensors/today.csv

until curl -s http://localhost:8080/health > /dev/null 2>&1; do
    echo "Waiting for SCADA server..."; sleep 2
done

Functions: Reusing Code

#!/bin/bash
set -euo pipefail

log_msg() {
    local level="$1" msg="$2"
    echo "$(date '+%Y-%m-%d %H:%M:%S') [$level] $msg"
}

check_sensor() {
    local ip="$1" name="$2"
    if ping -c 1 -W 2 "$ip" > /dev/null 2>&1; then
        log_msg "INFO" "$name ($ip) online"; return 0
    else
        log_msg "ERROR" "$name ($ip) UNREACHABLE"; return 1
    fi
}

check_sensor "192.168.1.101" "Temp Sensor A"
check_sensor "192.168.1.102" "Pressure Sensor B"

Use local for function-scoped variables. return 0 = success; non-zero = failure.

Practical Example: A Script That Collects Sensor Data Every 5 Minutes and Sends a Report

#!/bin/bash
set -euo pipefail
SENSOR_DIR="/opt/scada/data"
REPORT="/var/log/sensors/report_$(date +%Y%m%d).csv"

collect() {
    local sensor="$1"
    local val=$(cat "${SENSOR_DIR}/${sensor}/current_value" 2>/dev/null || echo "N/A")
    echo "$(date '+%Y-%m-%dT%H:%M:%S'),$sensor,$val"
}

while true; do
    ALERTS=0
    for dir in "${SENSOR_DIR}"/sensor_*; do
        sensor=$(basename "$dir")
        reading=$(collect "$sensor")
        echo "$reading" >> "$REPORT"
        val=$(echo "$reading" | cut -d',' -f3)
        if [[ "$val" != "N/A" ]] && [[ "$val" -gt 95 ]]; then
            echo "ALERT: $sensor = $val"
            ALERTS=$((ALERTS + 1))
        fi
    done
    [[ $ALERTS -gt 0 ]] && tail -20 "$REPORT" | \
      mail -s "Sensor Alert" engineer@factory.local 2>/dev/null || true
    sleep 300
done

This reads sensors in a loop, logs to CSV, checks thresholds, and emails alerts.

Summary

In this lesson you learned Bash scripting foundations:

• Scripts use #!/bin/bash and set -euo pipefail for safety.
• Variables store values; $1, $2 provide command-line arguments.
• if [[ ]] handles numeric, string, and file conditions.
• for, while, and until loops automate repetitive tasks.
• Functions organize code with local variables and return codes.
• Industrial scripts combine these to collect data, check thresholds, and send alerts.

In the next lesson, you will learn networking commands for diagnosing PLC and SCADA communication problems.