MajorWiki/01-linux/shell-scripting/bash-scripting-patterns.md

---
title: "Bash Scripting Patterns for Sysadmins"
domain: linux
category: shell-scripting
tags: [bash, scripting, automation, linux, shell]
status: published
created: 2026-03-08
updated: 2026-03-08
---

# Bash Scripting Patterns for Sysadmins

These are the patterns I reach for when writing bash scripts for server automation and maintenance tasks. Not a tutorial from scratch — this assumes you know basic bash syntax and want to write scripts that don't embarrass you later.

## The Short Answer

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

Start every script with these two lines. `set -e` exits on error. `set -u` treats unset variables as errors. `set -o pipefail` catches errors in pipes. Together they prevent a lot of silent failures.

## Script Header

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

# ── Config ─────────────────────────────────────────────────────────────────────
SCRIPT_NAME="$(basename "$0")"
LOG_FILE="/var/log/myscript.log"
# ───────────────────────────────────────────────────────────────────────────────
```

## Logging

```bash
log() {
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" | tee -a "$LOG_FILE"
}

log "Script started"
log "Processing $1"
```

## Error Handling

```bash
# Exit with a message
die() {
    echo "ERROR: $*" >&2
    exit 1
}

# Check a condition
[ -f "$CONFIG_FILE" ] || die "Config file not found: $CONFIG_FILE"

# Trap to run cleanup on exit
cleanup() {
    log "Cleaning up temp files"
    rm -f "$TMPFILE"
}
trap cleanup EXIT
```

## Checking Dependencies

```bash
check_deps() {
    local deps=("curl" "jq" "rsync")
    for dep in "${deps[@]}"; do
        command -v "$dep" &>/dev/null || die "Required dependency not found: $dep"
    done
}

check_deps
```

## Argument Parsing

```bash
usage() {
    cat <<EOF
Usage: $SCRIPT_NAME [OPTIONS] <target>

Options:
  -v, --verbose    Enable verbose output
  -n, --dry-run    Show what would be done without doing it
  -h, --help       Show this help
EOF
    exit 0
}

VERBOSE=false
DRY_RUN=false

while [[ $# -gt 0 ]]; do
    case $1 in
        -v|--verbose) VERBOSE=true; shift ;;
        -n|--dry-run) DRY_RUN=true; shift ;;
        -h|--help)    usage ;;
        --)           shift; break ;;
        -*)           die "Unknown option: $1" ;;
        *)            TARGET="$1"; shift ;;
    esac
done

[[ -z "${TARGET:-}" ]] && die "Target is required"
```

## Running Commands

```bash
# Dry-run aware command execution
run() {
    if $DRY_RUN; then
        echo "DRY RUN: $*"
    else
        "$@"
    fi
}

run rsync -av /source/ /dest/
run systemctl restart myservice
```

## Working with Files and Directories

```bash
# Check existence before use
[[ -d "$DIR" ]] || mkdir -p "$DIR"
[[ -f "$FILE" ]] || die "Expected file not found: $FILE"

# Safe temp files
TMPFILE="$(mktemp)"
trap 'rm -f "$TMPFILE"' EXIT

# Loop over files
find /path/to/files -name "*.log" -mtime +30 | while read -r file; do
    log "Processing: $file"
    run gzip "$file"
done
```

## String Operations

```bash
# Extract filename without extension
filename="${filepath##*/}"         # basename
stem="${filename%.*}"              # strip extension

# Check if string contains substring
if [[ "$output" == *"error"* ]]; then
    die "Error detected in output"
fi

# Convert to lowercase
lower="${str,,}"

# Trim whitespace
trimmed="${str#"${str%%[![:space:]]*}"}"
```

## Common Patterns

**Backup with timestamp:**

```bash
backup() {
    local source="$1"
    local dest="${2:-/backup}"
    local timestamp
    timestamp="$(date '+%Y%m%d_%H%M%S')"
    local backup_path="${dest}/$(basename "$source")_${timestamp}.tar.gz"

    log "Backing up $source to $backup_path"
    run tar -czf "$backup_path" -C "$(dirname "$source")" "$(basename "$source")"
}
```

**Retry on failure:**

```bash
retry() {
    local max_attempts="${1:-3}"
    local delay="${2:-5}"
    shift 2
    local attempt=1

    until "$@"; do
        if ((attempt >= max_attempts)); then
            die "Command failed after $max_attempts attempts: $*"
        fi
        log "Attempt $attempt failed, retrying in ${delay}s..."
        sleep "$delay"
        ((attempt++))
    done
}

retry 3 10 curl -f https://example.com/health
```

## Gotchas & Notes

- **Always quote variables.** `"$var"` not `$var`. Unquoted variables break on spaces and glob characters.
- **Use `[[` not `[` for conditionals.** `[[` is a bash built-in with fewer edge cases.
- **`set -e` exits on the first error — including in pipes.** Add `set -o pipefail` or you'll miss failures in `cmd1 | cmd2`.
- **`$?` after `if` is almost always wrong.** Use `if command; then` not `command; if [[ $? -eq 0 ]]; then`.
- **Bash isn't great for complex data.** If your script needs real data structures or error handling beyond strings, consider Python.

## See Also

- [ansible-getting-started](ansible-getting-started.md)
- [managing-linux-services-systemd-ansible](../process-management/managing-linux-services-systemd-ansible.md)