minimal typing command line calculator - tcsh vs bash

Question

I like to have a command-line calculator handy. The requirements are:

• Support all the basic arithmetic operators: +, -, /, *, ^ for exponentiation, plus parentheses for grouping.
• Require minimal typing, I don't want to have to call a program interact with it then asking it to exit.
• Ideally only one character and a space in addition to the expression itself should be entered into the command line.
• It should know how to ignore commas and dollar (or other currency symbols) in numbers to allow me to copy/paste from the web without worrying about having to clean every number before pasting it into the calculator
• Be white-space tolerant, presence or lack of spaces shouldn't cause errors
• No need for quoting anything in the expression to protect it from the shell - again for the benefit of minimal typing

Since tcsh supports alias positional arguments, and since alias expansion precedes all other expansions except history-expansion, it was straight forward to implement something close to my ideal in tcsh.

I used this:

alias C 'echo '\''\!*'\'' |tr -d '\'',\042-\047'\'' |bc -l'

Now I can do stuff like the following with minimal typing:

# the basic stuff:
tcsh>  C 1+2
3

# dollar signs, multiplication, exponentiation:
tcsh>  C $8 * 1.07^10
15.73721085831652257992

# parentheses, mixed spacing, zero power:
tcsh>  C ( 2+5 ) / 8 * 2^0
.87500000000000000000

# commas in numbers, no problem here either:
tcsh>  C 1,250.21 * 1.5
1875.315

As you can see there's no need to quote anything to make all these work.

Now comes the problem. Trying to do the same in bash, where parameter aliases aren't supported forces me to implement the calculator as a shell function and pass the parameters using "$@"

function C () { echo  "$@" | tr -d ', \042-\047' | bc -l; }

This breaks in various ways e.g:

# works:
bash$  C 1+2
3

# works:
bash$  C 1*2
2

# Spaces around '*' lead to file expansion with everything falling apart:
bash$  C  1 * 2
(standard_in) 1: syntax error
(standard_in) 1: illegal character: P
(standard_in) 1: illegal character: S
(standard_in) 1: syntax error
...

# Non-leading parentheses seem to work:
bash$  C  2*(2+1)
6

# but leading-parentheses don't:
bash$  C  (2+1)*2
bash: syntax error near unexpected token `2+1'

Of course, adding quotes around the expression solves these issues, but is against the original requirements.

I understand why things break in bash. I'm not looking for explanations. Rather, I'm looking for a solution which doesn't require manually quoting the arguments. My question to bash wizards is is there any way to make bash support the handy minimal typing calculator alias. Not requiring quoting, like tcsh does? Is this impossible? Thanks!

Answer 1

At least preventing the expansion of * is possible using 'set -f' (following someone's blog post:

alias C='set -f -B; Cf '
function Cf () { echo  "$@" | tr -d ', \042-\047' | bc -l; set +f; };  

Turning it off in the alias, before the calculation, and back on afterwards

$ C 2 * 3
6

---

I downloaded the bash sources and looked very closely. It seems the parenthesis error occurs directly during the parsing of the command line, before any command is run or alias is expanded. And without any flag to turn it off. So it would be impossible to do it from a bash script.

This means, it is time to bring the heavy weapons. Before parsing the command line is read from stdin using readline. Therefore, if we intercept the call to readline, we can do whatever we want with the command line.

Unfortunately bash is statically linked against readline, so the call cannot be intercepted directly. But at least readline is a global symbol, so we can get the address of the function using dlsym, and with that address we can insert arbitrary instructions in readline.

Modifying readline directly is prune to errors, if readline is changed between the different bash version, so we modify the function calling readline, leading to following plan:

1. Locate readline with dlsym
2. Replace readline with our own function that uses the current stack to locate the function calling readline (yy_readline_get) on its first call and then restores the original readline
3. Modify yy_readline_get to call our wrapper function
4. Within the wrapper function: Replace the parentheses with non problematic symbols, if the input starts with "C "

Written in C for amd64, we get:

#include <string.h>
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#ifndef __USE_GNU
#define __USE_GNU
#endif
#ifndef  __USE_MISC
#define  __USE_MISC
#endif
#include <dlfcn.h>
#include <unistd.h>
#include <sys/mman.h>
#include <errno.h>

//-----------Assembler helpers----------

#if (defined(x86_64) || defined(__x86_64__))

    //assembler instructions to read rdp, which we need to read the stack
#define MOV_EBP_OUT "mov %%rbp, %0"
    //size of a call instruction 
#define RELATIVE_CALL_INSTRUCTION_SIZE 5

#define IS64BIT (1)

    /*
      To replace a function with a new one, we use the push-ret trick, pushing the destination address on the stack and let ret jump "back" to it
      This has the advantage that we can set an additional return address in the same way, if the jump goes to a function

    This struct corresponds to the following assembler fragment:          
     68       ????  push                   <low_dword  (address)>
     C7442404 ????  mov DWORD PTR [rsp+4], <high_dword (address) )
     C3             ret
    */
typedef struct __attribute__((__packed__)) LongJump { 
  char push; unsigned int destinationLow;
  unsigned int mov_dword_ptr_rsp4; unsigned int destinationHigh;
  char ret;
//  char nopFiller[16];
} LongJump;

void makeLongJump(void* destination, LongJump* res) {
  res->push = 0x68;
  res->destinationLow = (uintptr_t)destination & 0xFFFFFFFF;
  res->mov_dword_ptr_rsp4 = 0x042444C7;
  res->destinationHigh = ((uintptr_t)(destination) >> 32) & 0xFFFFFFFF;
  res->ret = 0xC3;
}

//Macros to save and restore the rdi register, which is used to pass an address to readline (standard amd64 calling convention)
typedef unsigned long SavedParameter;
#define SAVE_PARAMETERS SavedParameter savedParameters;  __asm__("mov %%rdi, %0": "=r"(savedParameters)); 
#define RESTORE_PARAMETERS __asm__("mov %0, %%rdi": : "r"(savedParameters)); 

#else
#error only implmented for amd64...
#endif

//Simulates the effect of the POP instructions, popping from a passed "stack pointer" and returning the popped value
static void * pop(void** stack){
  void* temp = *(void**)(*stack);
  *stack += sizeof(void*); 
  return temp;
}

//Disables the write protection of an address, so we can override it
static int unprotect(void * POINTER){
  const int PAGESIZE = sysconf(_SC_PAGE_SIZE);;
  if (mprotect((void*)(((uintptr_t)POINTER & ~(PAGESIZE-1))), PAGESIZE, PROT_READ | PROT_WRITE | PROT_EXEC)) {
    fprintf(stderr, "Failed to set permission on %p\n", POINTER);
    return 1;
  }
  return 0;
}

//Debug stuff
static void fprintfhex(FILE* f, void * hash, int len) {
  for (int i=0;i<len;i++) {
    if ((uintptr_t)hash % 8 == 0 && (uintptr_t)i % 8 == 0 && i ) fprintf(f, " ");
    fprintf(f, "%.2x", ((unsigned char*)(hash))[i]);
  }
  fprintf(f, "\n");
}

//---------------------------------------


//Address of the original readline function
static char* (*real_readline)(const char*)=0; 

//The wrapper around readline we want to inject.
//It replaces () with [], if the command line starts with "C "
static char* readline_wrapper(const char* prompt){
  if (!real_readline) return 0;
  char* result = real_readline(prompt);
  char* temp = result; while (*temp == ' ') temp++;
  if (temp[0] == 'C' && temp[1] == ' ') 
    for (int len = strlen(temp), i=0;i<len;i++) 
      if (temp[i] == '(') temp[i] = '[';
      else if (temp[i] == ')') temp[i] = ']';
  return result;
}


//Backup of the changed readline part
static unsigned char oldreadline[2*sizeof(LongJump)] = {0x90};
//A wrapper around the readline wrapper, needed on amd64 (see below)
static LongJump* readline_wrapper_wrapper = 0;



static void readline_initwrapper(){
  SAVE_PARAMETERS
  if (readline_wrapper_wrapper) { fprintf(stderr, "ERROR!\n"); return; }

  //restore readline
  memcpy(real_readline, oldreadline, 2*sizeof(LongJump)); 

  //find call in yy_readline_get
  void * frame;
  __asm__(MOV_EBP_OUT: "=r"(frame)); //current stackframe
  pop(&frame); //pop current stackframe (??)
  void * returnToFrame = frame;
  if (pop(&frame) != real_readline) {  
    //now points to current return address
    fprintf(stderr, "Got %p instead of %p=readline, when searching caller\n", frame, real_readline); 
    return; 
  }
  void * caller = pop(&frame); //now points to the instruction following the call to readline
  caller -= RELATIVE_CALL_INSTRUCTION_SIZE; //now points to the call instruction
  //fprintf(stderr, "CALLER: %p\n", caller);
  //caller should point to 0x00000000004229e1 <+145>:   e8 4a e3 06 00  call   0x490d30 <readline>
  if (*(unsigned char*)caller != 0xE8) { fprintf(stderr, "Expected CALL, got: "); fprintfhex(stderr, caller, 16); return; }

  if (unprotect(caller)) return;

  //We can now override caller to call an arbitrary function instead of readline.
  //However, the CALL instruction accepts only a 32 parameter, so the called function has to be in the same 32-bit address space
  //Solution: Allocate memory at an address close to that CALL instruction and put a long jump to our real function there
  void * hint = caller;
  readline_wrapper_wrapper = 0;
  do { 
    if (readline_wrapper_wrapper) munmap(readline_wrapper_wrapper, 2*sizeof(LongJump));
    readline_wrapper_wrapper = mmap(hint, 2*sizeof(LongJump), PROT_EXEC | PROT_READ | PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0); 
    if (readline_wrapper_wrapper == MAP_FAILED) { fprintf(stderr, "mmap failed: %i\n", errno);  return; }
    hint += 0x100000;
  } while ( IS64BIT && ( (uintptr_t)readline_wrapper_wrapper >= 0xFFFFFFFF + ((uintptr_t) caller) ) ); //repeat until we get an address really close to caller
  //fprintf(stderr, "X:%p\n", readline_wrapper_wrapper);
  makeLongJump(readline_wrapper, readline_wrapper_wrapper); //Write the long jump in the newly allocated space

   //fprintfhex(stderr, readline_wrapper_wrapper, 16);
   //fprintfhex(stderr, caller, 16);

  //patch caller to become call <readline_wrapper_wrapper>
  //called address is relative to address of CALL instruction
  *(uint32_t*)(caller+1) = (uint32_t) ((uintptr_t)readline_wrapper_wrapper - (uintptr_t)(caller + RELATIVE_CALL_INSTRUCTION_SIZE) ); 

   //fprintfhex(stderr, caller, 16);

   *(void**)(returnToFrame) = readline_wrapper_wrapper; //change stack to jump to wrapper instead real_readline (or it would not work on the first entered command)

   RESTORE_PARAMETERS
}




static void _calc_init(void) __attribute__ ((constructor));


static void _calc_init(void){
  if (!real_readline) {
    //Find readline
    real_readline = (char* (*)(const char*)) dlsym(RTLD_DEFAULT, "readline");
    if (!real_readline) return;
    //fprintf(stdout, "loaded %p\n", real_readline);
    //fprintf(stdout, "  => %x\n", * ((int*) real_readline));

    if (unprotect(real_readline)) { fprintf(stderr, "Failed to unprotect readline\n"); return; }
    memcpy(oldreadline, real_readline, 2*sizeof(LongJump)); //backup readline's instructions

    //Replace readline  with readline_initwrapper
    makeLongJump(real_readline, (LongJump*)real_readline); //add a push/ret long jump from readline to readline, to have readline's address on the stack in readline_initwrapper
    makeLongJump(readline_initwrapper, (LongJump*)((char*)real_readline + sizeof(LongJump) - 1)); //add a push/ret long jump from readline to readline_initwrapper, overriding the previous RET

  }
}

This can be compiled to an intercepting library with:

gcc -g -std=c99 -shared -fPIC  -o calc.so -ldl calc.c

and then loaded in bash with:

gdb --batch-silent -ex "attach $BASHPID" -ex 'print dlopen("calc.so", 0x101)' 

Now, when the previous alias extended with parenthesis replacement is loaded:

alias C='set -f -B; Cf '
function Cf () {  echo  "$@" | tr -d ', \042-\047' | tr [ '(' | tr ] ')' | bc -l; set +f; };  

We can write:

$  C  1 * 2
  2
$  C  2*(2+1)
  6
$  C  (2+1)*2
  6

---

Even better it becomes, if we switch from bc to qalculate:

 alias C='set -f -B; Cf '
 function Cf () {  echo  "$@" | tr -d ', \042-\047' | tr [ '(' | tr ] ')' | xargs qalc ; set +f; };

Then we can do:

$ C e ^ (i * pi)
  e^(i * pi) = -1

$ C 3 c 
  3 * speed_of_light = approx. 899.37737(km / ms)

Answer 2

If you're prepared to type C Enter instead of C Space, the sky's the limit. The C command can take input in whatever form you desire, unrelated to the shell syntax.

C () {
  local line
  read -p "Arithmetic: " -e line
  echo "$line" | tr -d \"-\', | bc -l
}

In zsh:

function C {
  local line=
  vared -p "Arithmetic: " line
  echo $line | tr -d \"-\', | bc -l
}

---

In zsh, you can turn off globbing for the arguments of a specific command with the noglob modifier. It is commonly hidden in an alias. This prevents *^() from begin interpreted literally, but not quotes or $.

quickie_arithmetic () {
  echo "$*" | tr -d \"-\', | bc -l
}
alias C='noglob quickie_arithmetic'