How to know if a CPU is 32 or 64-bit

Nowadays almost all the computers have a 64-bit CPU, but sometimes we are logged in on a remote server and do not know what kind of CPU it has and we need to know it to install a package or… out of mere curiosity.

In those cases we can run the following command:

$ grep --color lm /proc/cpuinfo
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts
acpi mmx fxsr sse sse2 ss syscall nx rdtscp lm constant_tsc up ida nonstop_tsc arat pni
ssse3 cx16 sse4_1 sse4_2 popcnt lahf_lm 

If the lm flag is present (i.e. if there is any output to the grep command), then the CPU is 64-bit. lm means long mode (64 bit extension).

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