diff --git a/cmd/convergents.go b/cmd/convergents.go
index 2f512ff..71026bb 100644
--- a/cmd/convergents.go
+++ b/cmd/convergents.go
@@ -18,7 +18,6 @@ package cmd
 
 import (
 	"fmt"
-	"log"
 	"math/big"
 
 	"github.com/spf13/cobra"
@@ -41,14 +40,14 @@ func convergents(cmd *cobra.Command, args []string) {
 
 	a0, ok := new(big.Int).SetString(args[0], 10)
 	if !ok {
-		log.Fatal("invalid input " + args[0])
+		cobra.CheckErr("invalid input " + args[0])
 	}
 
 	denoms := make([]big.Int, len(args[1:]))
 	for i, d := range args[1:] {
 		_, ok = denoms[i].SetString(d, 10)
 		if !ok {
-			log.Fatal("invalid input " + d)
+			cobra.CheckErr("invalid input " + d)
 		}
 	}
 
diff --git a/cmd/crt.go b/cmd/crt.go
index b3fa80a..d024aae 100644
--- a/cmd/crt.go
+++ b/cmd/crt.go
@@ -18,7 +18,6 @@ package cmd
 
 import (
 	"fmt"
-	"log"
 	"math/big"
 
 	"github.com/spf13/cobra"
@@ -30,7 +29,7 @@ var moduli []string
 
 func crt(cmd *cobra.Command, args []string) {
 	if len(remainders) != len(moduli) {
-		log.Fatal("number of remainders and moduli do not match")
+		cobra.CheckErr("number of remainders and moduli do not match")
 	}
 
 	ns := make([]*big.Int, len(moduli))
@@ -39,18 +38,19 @@ func crt(cmd *cobra.Command, args []string) {
 		var ok bool
 		ns[i], ok = new(big.Int).SetString(moduli[i], 10)
 		if !ok {
-			log.Fatal("invalid input " + moduli[i])
+			cobra.CheckErr("invalid input " + moduli[i])
 		}
 
 		rs[i], ok = new(big.Int).SetString(remainders[i], 10)
 		if !ok {
-			log.Fatal("invalid input " + remainders[i])
+			cobra.CheckErr("invalid input " + remainders[i])
 		}
 	}
 
 	// TODO: support non-pairwise coprime moduli
 	if !lib.ArePairwiseCoprime(ns) {
-		log.Fatalf("moduli %v are not pairwise coprime", moduli)
+		err := fmt.Errorf("moduli %v are not pairwise coprime", moduli)
+		cobra.CheckErr(err)
 	}
 
 	x, N := lib.CRTSolution(rs, ns)
diff --git a/cmd/gcd.go b/cmd/gcd.go
index cf95851..597466a 100644
--- a/cmd/gcd.go
+++ b/cmd/gcd.go
@@ -18,7 +18,6 @@ package cmd
 
 import (
 	"fmt"
-	"log"
 	"math/big"
 
 	"github.com/spf13/cobra"
@@ -47,7 +46,7 @@ func gcd(cmd *cobra.Command, args []string) {
 	for i, s := range args {
 		_, ok := w.SetString(s, 10)
 		if !ok {
-			log.Fatal("invalid input " + s)
+			cobra.CheckErr("invalid input " + s)
 		}
 
 		z.GCD(x, y, z, w)
diff --git a/cmd/jacobi.go b/cmd/jacobi.go
index 7978bd2..8167bb9 100644
--- a/cmd/jacobi.go
+++ b/cmd/jacobi.go
@@ -18,7 +18,6 @@ package cmd
 
 import (
 	"fmt"
-	"log"
 	"math/big"
 
 	"github.com/spf13/cobra"
@@ -30,18 +29,18 @@ var n string
 func jacobi(cmd *cobra.Command, args []string) {
 	x, ok := new(big.Int).SetString(a, 10)
 	if !ok {
-		log.Fatal("invalid input " + a)
+		cobra.CheckErr("invalid input " + a)
 	}
 
 	y, ok := new(big.Int).SetString(n, 10)
 	if !ok {
-		log.Fatal("invalid input " + n)
+		cobra.CheckErr("invalid input " + n)
 	}
 
 	mod := big.NewInt(2)
 	mod.Mod(y, mod)
 	if mod.Cmp(big.NewInt(0)) == 0 {
-		log.Fatal("modulus must be odd")
+		cobra.CheckErr("modulus must be odd")
 	}
 
 	z := big.Jacobi(x, y)
diff --git a/cmd/primitiveRoot.go b/cmd/primitiveRoot.go
index 80a7b48..a5c3108 100644
--- a/cmd/primitiveRoot.go
+++ b/cmd/primitiveRoot.go
@@ -18,7 +18,6 @@ package cmd
 
 import (
 	"fmt"
-	"log"
 	"math/big"
 
 	"github.com/spf13/cobra"
@@ -45,7 +44,7 @@ func count(tpf []string) map[string]*big.Int {
 func primitiveRoot(cmd *cobra.Command, args []string) {
 	m, ok := new(big.Int).SetString(modulus, 10)
 	if !ok {
-		log.Fatal("invalid input " + modulus)
+		cobra.CheckErr("invalid input " + modulus)
 	}
 
 	factors := count(tpf)
@@ -55,12 +54,12 @@ func primitiveRoot(cmd *cobra.Command, args []string) {
 	if len(factors) == 0 {
 		root, err = lib.PrimitiveRoot(m)
 		if err != nil {
-			log.Fatal(err)
+			cobra.CheckErr(err)
 		}
 	} else {
 		root, err = lib.PrimitiveRootFast(m, factors)
 		if err != nil {
-			log.Fatal(err)
+			cobra.CheckErr(err)
 		}
 	}