Somethings that trip me up from time to time are bash bultins and bash keywords that shadow normal commands. The list isn’t long and you don’t hit the problem that often but when you do… it’s kind of a wild ride of questioning your sanity until you remember to check type $COMMAND, the command which obviously doesn’t support bash builtin lookup.

You can enforce to use the command instead of the shell keyword by prefixing \ before the command or using command $COMMAND builtin (note that shell builtins != shell keywords).

The list of bash builtin and keywords that shadow normal commands is as follows:

• [  
• echo
• false
• kill
• printf
• pwd
• test
• time
• true

I created it using for b in `ls $P` ; do type $b ; done | grep -v $P and setting a P variable to /bin/ and /usr/bin

Some differences in relation to the GNU implementations (not counting the obvious ones like the builtins not spawning a process etc).

All

Bultin commands don’t recognize the --version and --help flags. This won’t b repeated in the list below

[/test

/usr/bin/[ doesn’t support:

• -N file
• -o optname
• -v varname

echo

Using version 4.2.37 that comes with my debian this properly supports -e and -E flags, but some of the previous versions had problems with them.

kill

The builtin doesn’t support the following options:

• -L/--table
• -s/--signal options (though they’re exactly the same as the - option)

The builtin has different output for the -l option and doesn’t support the long option --list.

printf

The builtin doesn’t recognize the \c option.

pwd

The builtin doesn’t support the long options --logical and --physical.

time

The builtin has different output format compare:

$ time sleep 0

real	0m0.001s
user	0m0.004s
sys	0m0.000s
$ /usr/bin/time sleep 0
0.00user 0.00system 0:00.00elapsed ?%CPU (0avgtext+0avgdata 604maxresident)k
0inputs+0outputs (0major+197minor)pagefaults 0swaps

The builting doesn’t support the following options:

• -o/--output
• -a/--append
• -f/--format
• -v/--verbose
• --portability - though the short option p is supported
• --quiet
• --V