diff --git a/cmd/agg/doc.go b/cmd/agg/doc.go
new file mode 100644
index 0000000..3a9ec2d
--- /dev/null
+++ b/cmd/agg/doc.go
@@ -0,0 +1,73 @@
+/*
+
+Agg computes aggregate values over tabular text.
+It behaves somewhat like the SQL “GROUP BY” clause.
+
+Usage:
+
+	agg [function...]
+
+It reads input from stdin as a sequence of records, one per line.
+It treats each line as a set of fields separated by white space.
+One field (the first, by default) is designated as the key.
+Successive lines with equal keys are grouped into a group,
+and agg produces one line of output for each group.
+(Note that only contiguous input lines can form a group.
+If you need to make sure that all records for a given key
+are grouped together, sort the input first.)
+
+For each remaining field,
+agg applies a function to all the values in the group,
+producing a single output value.
+The command line arguments specify which functions to use,
+one per field in the input table.
+
+Functions
+
+The available functions are:
+
+    key        group by this field (default for field 1)
+    first      value from first line of group (default for rest)
+    last       value from last line of group
+    sample     value from any line of group, uniformly at random
+    prefix     longest common string prefix
+    join:sep   concatenate strings with given sep
+    smin       lexically least string
+    smax       lexically greatest string
+    min        numerically least value
+    max        numerically greatest value
+    sum        numeric sum
+    mean       arithmetic mean
+    count      number of records (ignores input value)
+    const:val  print val, ignoring input
+    drop       omit the column entirely
+
+The numeric functions skip items that don't parse as numbers.
+
+Examples
+
+Using the following input:
+
+    $ cat >input
+    -rwx   alice      100   /home/alice/bin/crdt
+    -rw-   alice   210002   /home/alice/thesis.tex
+    -rw-   bob      10051   /home/bob/expenses.tab
+    -rwx   kr      862060   /home/kr/bin/blog
+    -rwx   kr      304608   /home/kr/bin/agg
+
+Disk usage for each user, plus where that disk usage occurs
+(longest common prefix of filesystem paths):
+
+    $ agg <input drop key sum prefix
+    alice	210153	/home/alice/
+    bob	10051	/home/bob/expenses.tab
+    kr	1166668	/home/kr/
+
+Disk usage for executable vs non-executable files:
+
+    $ sort input | agg key drop sum join:,
+    -rw-	220053	/home/alice/thesis.tex,/home/bob/expenses.tab
+    -rwx	1166768	/home/alice/bin/crdt,/home/kr/bin/agg,/home/kr/bin/blog
+
+*/
+package main
diff --git a/cmd/agg/main.go b/cmd/agg/main.go
new file mode 100644
index 0000000..4239aa4
--- /dev/null
+++ b/cmd/agg/main.go
@@ -0,0 +1,112 @@
+package main
+
+// TODO(kr): tests
+
+import (
+	"bufio"
+	"fmt"
+	"log"
+	"math/rand"
+	"os"
+	"strings"
+	"time"
+)
+
+type agg interface {
+	merge(string)
+	String() string
+}
+
+var (
+	key     = 0
+	funcmap = make(map[int]func(init, arg string) agg)
+	argmap  = make(map[int]string)
+	symtab  = map[string]func(init, arg string) agg{
+		"first":  first,
+		"last":   last,
+		"prefix": prefix,
+		"sample": sample,
+		"join":   join,
+		"smin":   smin,
+		"smax":   smax,
+		"min":    min,
+		"max":    max,
+		"sum":    sum,
+		"mean":   mean,
+		"count":  count,
+		"const":  constf,
+		"drop":   nil,
+	}
+)
+
+func main() {
+	log.SetPrefix("agg: ")
+	log.SetFlags(0)
+	rand.Seed(time.Now().UnixNano())
+	for i, sym := range os.Args[1:] {
+		if p := strings.IndexByte(sym, ':'); p >= 0 {
+			sym, argmap[i] = sym[:p], sym[p+1:]
+		}
+		if sym == "key" {
+			key, sym = i, "first"
+		}
+		f, ok := symtab[sym]
+		if !ok {
+			log.Fatalf("bad function: %q", sym)
+		}
+		funcmap[i] = f
+	}
+
+	sc := bufio.NewScanner(os.Stdin)
+	var g *group
+	for sc.Scan() {
+		ss := strings.Fields(sc.Text())
+		if !matches(g, ss) {
+			emit(g)
+			g = &group{key: ss[key]}
+		}
+		mergeLine(g, ss)
+	}
+	emit(g)
+}
+
+type group struct {
+	key string
+	agg []agg
+}
+
+func matches(g *group, ss []string) bool {
+	return g != nil && g.key == ss[key]
+}
+
+func emit(g *group) {
+	if g == nil {
+		return
+	}
+	rest := false
+	for i, a := range g.agg {
+		if f, ok := funcmap[i]; ok && f == nil {
+			continue
+		}
+		if rest {
+			fmt.Print("\t")
+		}
+		rest = true
+		fmt.Print(a)
+	}
+	fmt.Println()
+}
+
+func mergeLine(g *group, ss []string) {
+	for i, s := range ss {
+		if i >= len(g.agg) {
+			f := funcmap[i]
+			if f == nil {
+				f = first
+			}
+			g.agg = append(g.agg, f(s, argmap[i]))
+		} else {
+			g.agg[i].merge(s)
+		}
+	}
+}
diff --git a/cmd/agg/num.go b/cmd/agg/num.go
new file mode 100644
index 0000000..93ac3fe
--- /dev/null
+++ b/cmd/agg/num.go
@@ -0,0 +1,99 @@
+package main
+
+import (
+	"math/big"
+	"strconv"
+)
+
+func min(s, arg string) agg { return newBinop(s, opmin) }
+func max(s, arg string) agg { return newBinop(s, opmax) }
+func sum(s, arg string) agg { return newBinop(s, opsum) }
+
+type binop struct {
+	v *big.Float
+	f func(a, b *big.Float) *big.Float
+}
+
+func newBinop(s string, f func(a, b *big.Float) *big.Float) *binop {
+	v, _ := parseFloat(s)
+	return &binop{v, f}
+}
+
+func (o *binop) String() string {
+	if o.v == nil {
+		return "NaN"
+	}
+	return o.v.Text('f', -1)
+}
+
+func (o *binop) merge(s string) {
+	v, ok := parseFloat(s)
+	if !ok {
+		return
+	}
+	o.v = o.f(o.v, v)
+}
+
+func opmin(a, b *big.Float) *big.Float {
+	if a != nil && (b == nil || a.Cmp(b) <= 0) {
+		return a
+	}
+	return b
+}
+
+func opmax(a, b *big.Float) *big.Float {
+	if a != nil && (b == nil || a.Cmp(b) >= 0) {
+		return a
+	}
+	return b
+}
+
+func opsum(a, b *big.Float) *big.Float {
+	if a == nil {
+		return b
+	} else if b == nil {
+		return a
+	}
+	return a.Add(a, b)
+}
+
+type meanagg struct {
+	v *big.Float
+	d float64 // actually an integer
+}
+
+func mean(s, arg string) agg {
+	v, ok := parseFloat(s)
+	if !ok {
+		return &meanagg{new(big.Float), 0}
+	}
+	return &meanagg{v, 1}
+}
+
+func (m *meanagg) String() string {
+	if m.d == 0 {
+		return "NaN"
+	}
+	v := new(big.Float).Quo(m.v, big.NewFloat(m.d))
+	return v.Text('f', -1)
+}
+
+func (m *meanagg) merge(s string) {
+	v, ok := parseFloat(s)
+	if !ok {
+		return
+	}
+	m.v.Add(m.v, v)
+	m.d++
+}
+
+func parseFloat(s string) (*big.Float, bool) {
+	v, _, err := big.ParseFloat(s, 0, 1000, big.ToNearestEven)
+	return v, err == nil
+}
+
+type counter int
+
+func count(init, arg string) agg  { return new(counter) }
+func (c *counter) String() string { return strconv.Itoa(int(*c) + 1) }
+func (c *counter) merge(string)   { *c++ }
diff --git a/cmd/agg/string.go b/cmd/agg/string.go
new file mode 100644
index 0000000..9a8cf78
--- /dev/null
+++ b/cmd/agg/string.go
@@ -0,0 +1,74 @@
+package main
+
+import (
+	"math/rand"
+	"strings"
+)
+
+func first(s, arg string) agg  { return &sbinop{s, opfirst} }
+func last(s, arg string) agg   { return &sbinop{s, oplast} }
+func prefix(s, arg string) agg { return &sbinop{s, opprefix} }
+func join(s, arg string) agg   { return &sbinop{s, opjoin(arg)} }
+func smin(s, arg string) agg   { return &sbinop{s, opsmin} }
+func smax(s, arg string) agg   { return &sbinop{s, opsmax} }
+
+type sbinop struct {
+	s string
+	f func(a, b string) string
+}
+
+func (o *sbinop) String() string { return o.s }
+
+func (o *sbinop) merge(s string) { o.s = o.f(o.s, s) }
+
+func opfirst(a, b string) string { return a }
+func oplast(a, b string) string  { return b }
+
+func opprefix(a, b string) string {
+	for i := range a {
+		if i >= len(b) || a[i] != b[i] {
+			return a[:i]
+		}
+	}
+	return a
+}
+
+func opjoin(sep string) func(a, b string) string {
+	return func(a, b string) string {
+		return a + sep + b // TODO(kr): too slow? maybe strings.Join?
+	}
+}
+
+func opsmin(a, b string) string {
+	if strings.Compare(a, b) <= 0 {
+		return a
+	}
+	return b
+}
+
+func opsmax(a, b string) string {
+	if strings.Compare(a, b) >= 0 {
+		return a
+	}
+	return b
+}
+
+type sampler struct {
+	n int
+	s string
+}
+
+func sample(s, arg string) agg    { return &sampler{1, s} }
+func (p *sampler) String() string { return p.s }
+func (p *sampler) merge(s string) {
+	p.n++
+	if rand.Intn(p.n) == 0 {
+		p.s = s
+	}
+}
+
+type constant string
+
+func constf(init, arg string) agg { return constant(arg) }
+func (c constant) String() string { return string(c) }
+func (c constant) merge(string)   {}