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

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+---
+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]]
+- [[managing-linux-services-systemd-ansible]]