Skip to content

Arrays and String Manipulation

Overview

Host lists, service maps, and path rewriting are array and string problems. Handle them without fragile IFS hacks.

This is Tutorial 8 in Module 8: Arrays & Strings of the REBASH Academy Shell Scripting for DevOps Engineers series — written for Linux administrators, DevOps engineers, SREs, and platform engineers who automate production hosts with Bash.

Prerequisites

  • Functions, Parameters, and Locals
  • Bash 4.2+ on Linux (WSL2/VM/cloud)

Learning Objectives

By the end of this tutorial, you will be able to:

  • Apply the core ideas of “Arrays and String Manipulation” in a real ops script
  • Use set -euo pipefail as the production default
  • Use quoted expansions and clear stderr diagnostics
  • Produce meaningful exit codes for automation consumers
  • Debug behaviour with bash -x when something fails
  • Relate this topic to day-to-day Linux admin and DevOps work

Architecture

Ops scripts sit between humans/automation and system tools. This topic’s control points are shown below.

Architecture diagram for Arrays and String Manipulation

Theory

What it is

Bash arrays store ordered lists (indexed) or key/value maps (associative, Bash 4+). Alongside arrays, parameter expansion offers built-in string surgery: strip prefixes and suffixes, replace substrings, and take slices — without spawning sed for every small edit. Pattern matching with [[ ]], case, and =~ validates names and selects behaviour. Together these features let ops scripts manage host inventories, port maps, and path rewriting cleanly.

Why it matters

DevOps work is full of lists: targets to patch, services to restart, files to archive. Storing them in a properly quoted array avoids the “split on spaces” trap that breaks paths and hostnames. Associative arrays replace brittle parallel lists (names[i] with ports[i]) with named lookups. Built-in string expansions keep simple transforms fast and dependency-free — useful in constrained Continuous Integration (CI) images — while patterns stop unsafe input before a destructive command runs.

How it works

Indexed arrays hold elements in order. Always expand with "${array[@]}" so spaces inside elements are preserved:

hosts=(web01 web02 web03)
hosts+=("web04")
printf '%s\n' "${hosts[@]}"
n=0
for _ in "${hosts[@]}"; do n=$((n + 1)); done
echo "count=$n"

Associative arrays need an explicit declaration:

declare -A ports=( [web]=80 [db]=5432 )
printf 'web port=%s\n' "${ports[web]}"

They require Bash 4+ and suit environment-to-value maps and small inventories. For strings, parameter expansions rewrite without external tools. Combine with [[ $name == *.log ]], case patterns, or =~ regex — and validate before any destructive use.

Key concepts

Expansion / form Use
"${array[@]}" All elements, safely quoted
declare -A Associative map (Bash 4+)
${var%%.*} / ${var##*/} Strip longest suffix / basename-like
${var%/*} Dirname-like
${var/old/new} Replace first match
${var:0:3} Substring slice
[[ ]] / case / =~ Pattern and regex tests

Common pitfalls

  • Expanding ${array[*]} or unquoted $array and merging elements incorrectly
  • Using associative arrays under Bash 3 (for example older macOS defaults)
  • Forgetting declare -A before assignments to a map
  • Applying string replacements to untrusted input without validation
  • Confusing ${var%} (shortest) with ${var%%} (longest) suffix removal

Hands-on Lab

Create a workspace for this tutorial.

mkdir -p ~/rebash-shell/lab08 && cd ~/rebash-shell/lab08

Focus: indexed host list; associative ports; string strip/replace; patterns

Step 1 – Arrays and strings

cat > arrays.sh << 'EOF'
#!/usr/bin/env bash
set -euo pipefail
hosts=(web01 web02 'web 03')
hosts+=("web04")
n=0
for _ in "${hosts[@]}"; do n=$((n + 1)); done
echo "count=$n"
printf '%s\n' "${hosts[@]}"
declare -A ports=( [web]=80 [db]=5432 )
echo "web=${ports[web]}"
path=/var/log/app.log
echo "base=${path##*/}"
[[ $path == *.log ]] && echo 'pattern ok'
EOF
chmod +x arrays.sh
./arrays.sh

Final step – Cleanup note

# Keep ~/rebash-shell/ for later tutorials; destroy disposable cloud resources from this lab

Validation

  • Lab commands run under ~/rebash-shell/lab08/
  • You can explain each Theory heading in your own words
  • Failure path exits non-zero and prints diagnostics to stderr (where applicable)
  • You can relate this topic to a real DevOps or Linux admin task

Code Walkthrough

Production Bash for Arrays and String Manipulation always combines:

  1. A clear shebang (#!/usr/bin/env bash)
  2. Strict mode near the top (set -euo pipefail) from Module 2 onward
  3. Quoted expansions and explicit tests
  4. Functions with local for reusable behaviour
  5. Documented exit codes and stderr logging

Keep scripts short enough to review in a single merge request. When logic grows (complex JSON APIs, heavy state), hand off to Python and keep Bash as the launcher.

Security Considerations

  • Treat all external input (args, files, env) as untrusted until validated
  • Never log secrets; prefer masked CI variables and secret stores
  • Prefer least privilege — do not require root for file-local tasks
  • Avoid eval and unquoted expansions in destructive commands
  • Validate paths stay under an allow-listed root before rm or overwrite

Common Mistakes

Skipping strict mode

Cron and CI hide failures that an interactive terminal would show. Fix: start with set -euo pipefail from Module 2 onward.

Unquoted path expansions

Spaces and globs rewrite your command line. Fix: always "$path" / "$@".

Assuming interactive PATH

Aliases and fancy PATH entries disappear under schedulers. Fix: set PATH or use absolute paths.

Best Practices

  • One purpose per script; compose with functions or small binaries
  • Log to stderr; reserve stdout for data or RESULT lines
  • Idempotent behaviour where scheduling may overlap
  • Pair every new script with a failing-path test you actually run
  • Run ShellCheck in CI before merging automation

Troubleshooting

Symptom Likely cause Fix
Works in terminal, fails in cron PATH / cwd / env Fingerprint env; set PATH
unbound variable set -u Provide defaults or export vars
Pipeline “succeeds” incorrectly Missing pipefail set -o pipefail
[[ unexpected operator Running under sh/dash Fix shebang to Bash

Summary

Arrays and String Manipulation is a core skill for Linux admins and DevOps engineers automating real hosts and pipelines. Practise the lab until the failure path is as familiar as the happy path, then continue the track.

Interview Questions

  1. How does this topic show up in production Linux administration or CI?
  2. What failure mode appears if you ignore quoting or strict mode here?
  3. How would you test this behaviour under a minimal cron-like environment?
  4. When would you move this logic out of Bash into Python or another tool?
  5. What exit code contract would you document for teammates?

Sample answer — question 2

Unquoted expansions and missing pipefail create silent or partial failures — especially under cron — that look healthy in monitoring until data is wrong.

References