Linux command to return number of bits (32 or 64)?

• You can see whether the CPU is 64-bit, 32-bit, or capable of both by checking the flags line in /proc/cpuinfo. You have to know the possible flags on your architecture family. For example, on i386/amd64 platforms, the lm flag identifies amd64-capable CPUs (CPUs that don't have that flag are i386-only).

  grep -q '^flags\s*:.*\blm\b' /proc/cpuinfo    # Assuming a PC
  

• You can see whether the kernel is 32-bit or 64-bit by querying the architecture with uname -m. For example, i[3456]86 are 32-bit while x86_64 is 64-bit. Note that on several architectures, a 64-bit kernel can run 32-bit userland programs, so even if the uname -m shows a 64-bit kernel, there is no guarantee that 64-bit libraries will be available.

  [ "$(uname -m)" = "x86_64" ]    # Assuming a PC
  

  Note also that uname -m may return a “virtualized” value. For example, under Linux, if you run setarch i386 bash on an amd64 system, and you run uname -m from that bash, you'll see uname -m reporting i386. This effectively lets you pretend that you're on a “32-bit system” even though the kernel is a 64-bit one, for example to compile 32-bit programs without setting up cross-compilation.

• You can see what is available in userland by querying the LSB support with the lsb_release command. More precisely, lsb_release -s prints a :-separated list of supported LSB features. Each feature has the form module-version-architecture. For example, availability of an ix86 C library is indicated by core-2.0-ia32, while core-2.0-amd64 is the analog for amd64. Not every distribution declares all the available LSB modules though, so more may be available than is detectable in this way.

• You can find out the preferred word size for development (assuming a C compiler is available) by compiling a 5-line C program that prints sizeof(void*) or sizeof(size_t).

You can use uname -a and look for x86_64 to see if you are running 64-bit. Anything else (As far as I know) and you are running 32-bit or you are on non-PC hardware such as alpha, sparc, or ppc64.

uname -m | sed 's/x86_//;s/i[3-6]86/32/'