File, String, and Numeric Condition Tests

1. File tests answer specific filesystem questions

config="/etc/hosts"

if [[ -f $config && -r $config && -s $config ]]; then
  printf 'config is a readable non-empty regular file\n'
fi

2. Existence and symlink checks have edge cases

A dangling symbolic link can exist as a directory entry even though its target does not exist. A test focused on the target and a test focused on the link answer different questions.

target="missing-target"
link="demo-link"

ln -s "$target" "$link"

[[ -L $link ]] && printf 'symbolic link itself exists\n'
[[ -e $link ]] || printf 'resolved target does not exist\n'

rm -f "$link"

When automation manages links, decide whether you care about the link object, the resolved target, or both.

3. Permission predicates are observations, not guarantees

-r, -w, and -x test access according to current process credentials and filesystem semantics. Passing a test does not reserve the resource or guarantee the subsequent operation will succeed.

# Instead of only checking -w and assuming success:
if printf '%s\n' "new content" > "$target"; then
  printf 'write succeeded\n'
else
  printf 'write failed\n' >&2
  exit 1
fi

4. Bash can compare file modification times

Within conditional expressions, -nt means “newer than” and -ot means “older than.”

source_file="source.yaml"
generated_file="rendered.conf"

if [[ $source_file -nt $generated_file ]]; then
  printf 'regeneration is needed\n'
fi

Timestamp comparisons are convenient for local build-like workflows, but distributed systems and generated artifacts may need stronger change detection such as hashes or explicit version metadata.

5. -ef tests whether two paths refer to the same file

The -ef operator is useful when hard links or alternate path spellings could refer to the same underlying file.

if [[ path-a -ef path-b ]]; then
  printf 'both paths identify the same file\n'
fi

This is about file identity, not merely matching pathname text.

6. String tests should make emptiness explicit

token=${API_TOKEN:-}

if [[ -z $token ]]; then
  printf 'API_TOKEN is empty or unset\n' >&2
fi

7. Numeric comparison is not string comparison

Classic test syntax uses integer comparison operators such as -eq, -ne, -lt, -le, -gt, and -ge. In Bash, arithmetic context (( ... )) is often clearer for numeric expressions.

retries=5
limit=3

if [[ $retries -gt $limit ]]; then
  printf 'too many retries\n'
fi

if (( retries > limit )); then
  printf 'same numeric conclusion via arithmetic context\n'
fi

8. Validate numeric ranges explicitly

Configuration checks often need both syntax and range validation.

port=${1:-}

if [[ ! $port =~ ^[0-9]+$ ]]; then
  printf 'port must contain decimal digits only\n' >&2
  exit 64
fi

port=$((10#$port))

if (( port < 1 || port > 65535 )); then
  printf 'port out of range: %d\n' "$port" >&2
  exit 65
fi

printf 'valid port=%d\n' "$port"

9. Executable discovery is usually better with command -v

Checking a literal path such as [[ -x /usr/bin/jq ]] assumes where the executable is installed. If your real requirement is “can this shell resolve the command?”, use command -v.

if command -v jq >/dev/null 2>&1; then
  printf 'jq is available through command resolution\n'
else
  printf 'jq is unavailable\n' >&2
fi

10. Creating required directories can be more robust than pre-checking

If the desired state is simply “this directory exists,” mkdir -p is naturally idempotent and often clearer than an existence check followed by creation.

workspace="$HOME/devops-academy/work"

if mkdir -p -- "$workspace"; then
  printf 'workspace ready: %s\n' "$workspace"
else
  printf 'could not prepare workspace\n' >&2
  exit 1
fi

This reduces the race window between “does it exist?” and “create it.”

11. Hands-on lab: validate a deployment configuration

Build a validator for a configuration path, replica count, port, and environment name.

mkdir -p "$HOME/devops-academy/bash/chapter04/lesson03"
cd "$HOME/devops-academy/bash/chapter04/lesson03"

cat > validate-deploy.sh <<'EOF'
#!/usr/bin/env bash

config=${1:-}
replicas=${2:-}
port=${3:-}
environment=${4:-}

if [[ ! -f $config || ! -r $config || ! -s $config ]]; then
  printf 'invalid config file: %s\n' "$config" >&2
  exit 2
fi

if [[ ! $replicas =~ ^[0-9]+$ || ! $port =~ ^[0-9]+$ ]]; then
  printf 'replicas and port must be decimal integers\n' >&2
  exit 3
fi

replicas=$((10#$replicas))
port=$((10#$port))

if (( replicas < 1 || replicas > 100 )); then
  printf 'replicas out of range\n' >&2
  exit 4
fi

if (( port < 1 || port > 65535 )); then
  printf 'port out of range\n' >&2
  exit 5
fi

if [[ $environment != dev && $environment != staging && $environment != prod ]]; then
  printf 'unsupported environment\n' >&2
  exit 6
fi

printf 'configuration accepted\n'
EOF

printf 'service=api\n' > app.conf
bash validate-deploy.sh app.conf 3 8080 staging
bash validate-deploy.sh app.conf 0 8080 staging || true
bash validate-deploy.sh missing.conf 3 8080 staging || true

12. Knowledge check

13. Summary

Bash offers focused predicates for file type, access, size, timestamps, strings, and integers. Use the predicate that answers the exact question you care about, validate external numeric syntax before arithmetic, and remember that pre-checks do not make later operations atomic. When possible, attempt idempotent operations directly and handle failure.

14. Further reading

• GNU Bash Reference Manual — Bash Conditional Expressions.
• POSIX test utility specification.
• Linux stat(2), access(2), and symlink(7) manual pages.
• ShellCheck guidance on file and numeric tests.