Add pre-cat file existance check

src/file.S

@@ -2,9 +2,9 @@
 cat_buf:    .space 4096
 
 .section .text
-.globl cat_file, open_fd
+.globl cat_file, open_fd, file_exists
 
-# Fetch argument count
+# Cat out the given file to stdout
 # @param[in] a0: ptr to null-terminated file str.
 # @param[out] a0: result code.
 cat_file:
@@ -46,7 +46,7 @@ cat_file:
     ret
 
 
-# Fetch argument count
+# Open fd
 # @param[in] a0: ptr to null-terminated file str.
 # @param[out] a0: result code. (0-exists 1-doesn't exist)
 # @param[out] a1: fd.
@@ -67,3 +67,20 @@ open_fd:
 
     2:
     ret
+
+# Check if the given file exists without opening fd
+# @param[in] a0: ptr to null-terminated file str.
+# @param[out] a0: result code (0-exists 1-doesn't exist).
+file_exists:
+    mv a1, a0
+    li a0, -100 # relative/absolute path
+    li a2, 0
+    li a7, 48
+    ecall
+    beqz a0, 1f
+    li a0, 1
+    ret
+
+    1:
+    li a0, 0
+    ret

src/main.S

@@ -1,6 +1,6 @@
 .data
 argc:           .space 8
-file_err:       .asciz "\nfile not found: "
+file_err:       .asciz "File not found or cannot be accessed: "
 
 .equ FILE_ERR_LEN, . - file_err
 
@@ -18,8 +18,19 @@ _start:
     li t0, 1
     ble a1, t0, 1f # return 1 if no file given
     mv s0, a1 # store argc
-    li s1, 1 # i=1 (skip program name)
+    li s1, 1
 
+    5: #iterate through and check that all files exist first
+    mv a0, s1
+    call fetch_argv
+    bnez a0, 1f # error if failed
+    mv a0, a1
+    call file_exists
+    bnez a0, 4f
+    addi s1, s1, 1
+    blt s1, s0, 5b
+
+    li s1, 1 # reset counter now that we've verified all files exist, skip command
 
     3:
     mv a0, s1