Standard Error, Logging, and Output Redirection

# Redirect echo to stderr
echo "Error: file not found" >&2

# Preferred: redirect entire block
{
    echo "Error: configuration is invalid"
    echo "Check: $config_file"
} >&2

# printf to stderr
printf "Error: expected %d args, got %d\n" 2 "$#" >&2

# Function pattern
function die {
    echo "$*" >&2
    exit 1
}

die "Fatal: database connection failed"

#!/usr/bin/env bash

# Log level: DEBUG=0, INFO=1, WARN=2, ERROR=3
LOG_LEVEL=${LOG_LEVEL:-1}
LOG_FILE=${LOG_FILE:-""}

function _log {
    local level="$1"
    local level_num="$2"
    shift 2
    local message="$*"
    local timestamp
    timestamp=$(date '+%Y-%m-%d %H:%M:%S')
    local line="[$timestamp] [$level] $message"

    if [[ $level_num -ge $LOG_LEVEL ]]; then
        if [[ $level_num -ge 2 ]]; then
            echo "$line" >&2     # WARN and ERROR → stderr
        else
            echo "$line"         # DEBUG and INFO → stdout
        fi
    fi

    if [[ -n "$LOG_FILE" ]]; then
        echo "$line" >> "$LOG_FILE"
    fi
}

function log_debug { _log "DEBUG" 0 "$@"; }
function log_info  { _log "INFO " 1 "$@"; }
function log_warn  { _log "WARN " 2 "$@"; }
function log_error { _log "ERROR" 3 "$@"; }

# Usage
log_info  "Script started"
log_warn  "Disk usage at 85%"
log_error "Failed to connect to DB"

# Capture stdout, discard stderr
output=$(cmd 2>/dev/null)

# Capture stdout, let stderr through to terminal
output=$(cmd)

# Capture both in separate variables (Bash 4+ trick)
exec 3>&1                          # save stdout to fd 3
output=$(cmd 2>&1 1>&3)            # stderr captured, stdout goes to fd3 (terminal)
exec 3>&-                          # close fd 3
# Now: $output has stderr, terminal saw stdout

# Simple: capture both in one variable
combined=$(cmd 2>&1)

# Capture stdout to file, stderr to terminal
cmd > output.txt

# Capture stderr to file, stdout to terminal
cmd 2> errors.txt