echo: print newline even with no args, and add some tests

Makefile

@@ -5,8 +5,7 @@ LD = $(TARGET)-ld
 .SUFFIXES:
 
 .PHONY: all
-all: true false cat env hexdump
-#all: echo
+all: true false cat env hexdump echo
 
 %: %.o
 	$(LD) -o $@ $(filter %.o, $^)
@@ -20,3 +19,4 @@ sorttest: lib/sort.o
 .PHONY: test
 test:
 	@BIN=. test/true.sh
+	@BIN=. test/echo.sh

echo.s

@@ -3,15 +3,14 @@
 .option norelax
 
 .section .rodata
-space_or_newline:
-    .ascii " ", "\n"
+space:   .ascii " "
+newline: .ascii "\n"
 
 .text
 _start:
     # arguments are passed on the stack:
     # --bottom of stack--
     # dd argc
-    # dd argv
     # dd argv[0]
     # dd ...
     # dd 0
@@ -19,7 +18,6 @@ _start:
     # dd ...
     # dd 0
     ld s0, 0(sp) # get argc
-    #ld s1, 8(sp) # get argv
     add s1, sp, 8
 
     # TODO: iovec
@@ -38,6 +36,7 @@ _start:
     # write each argument
     li s11, 0
     mv s2, s0
+    la s8, space
 loop:
     # get argv[i]
     slli t0, s11, 3
@@ -61,21 +60,20 @@ loop:
     ecall
     bltz a0, write_error
 
-    # if not the last argument, print a space
-    # otherwise print a newline
-    #
-    # a1 = space_or_newline + (argc-s11 < 2)
-    la a1, space_or_newline
-    sub t0, s0, s11
-    sltiu t0, t0, 2
-    add a1, a1, t0
+    # increment index
+    # if this was the last argument, break
+    addi s11, s11, 1
+    bge s11, s0, end
 
+    # not the last argument, so print a space and loop
     li a7, 64 # sys_write
     li a0, 1  # stdout
-    #la a1, newline or space
+    mv a1, s8 # space
     li a2, 1 # size
     ecall
+
     bltz a0, write_error
+    j loop
 
 step:
     # increment index and loop
@@ -83,6 +81,15 @@ step:
     blt s11, s0, loop
 
 end:
+    # print a newline at the end,
+    # regardless of whether there were any arguments or not
+    li a7, 64 # sys_write
+    li a0, 1  # stdout
+    la a1, newline
+    li a2, 1 # size
+    ecall
+    bltz a0, write_error
+
     li a7, 93 # exit
     li a0, 0
     ecall

test/echo.sh

@@ -0,0 +1,56 @@
+#!/bin/bash
+set -eu
+set -o pipefail
+
+: ${BIN:=..}
+: ${EMU:=qemu-riscv64}
+
+cmd=$BIN/echo
+name=echo
+
+fail=0
+err() {
+    echo "FAIL $name: $*"
+    fail=1
+}
+
+# the shell strips the trailing newline when capturing output,
+# which makes writing tests for echo a bit difficult
+
+out=$($EMU $cmd)
+len=$($EMU $cmd | wc -c)
+stat=$?
+if [ "$out" != "" ]; then
+    err "expected a newline"
+fi
+if [ "$len" -ne 1 ]; then
+    err "expected len 1, got len $len"
+fi
+if [ "$stat" -ne 0 ]; then
+    err "exited with status code $stat, expected 0"
+fi
+
+
+out=$($EMU $cmd a b c)
+stat=$?
+if [ "$out" != "a b c" ]; then
+    err "expected 'a b c', got '$out'"
+fi
+if [ "$stat" -ne 0 ]; then
+    err "exited with status code $stat, expected 0"
+fi
+
+
+out=$($EMU $cmd 'a    b')
+stat=$?
+if [ "$out" != "a    b" ]; then
+    err "expected 'a    b', got '$out'"
+fi
+if [ "$stat" -ne 0 ]; then
+    err "exited with status code $stat, expected 0"
+fi
+
+
+if [ "$fail" -eq 0 ]; then
+    echo PASS $name
+fi

test/true.sh

@@ -2,7 +2,7 @@
 set -eu
 set -o pipefail
 
-: ${BIN:-..}
+: ${BIN:=..}
 : ${EMU:=qemu-riscv64}
 
 cmd=$BIN/true