Why does bash prohibit a shell function named `df`?

Question

I am using GNU bash, version 4.4.12(1)-release (x86_64-pc-linux-gnu).

I have the following lines in one of my startup files:

df() { printf "Hello, world!\n" }

When source that file, I get this error:

-bash: sh/interactive.sh: line 109: syntax error near unexpected token `(' -bash: sh/interactive.sh: line 109: `df() {'

However, if I change the function name from df to dir or ef or anything_else I don't get the error.

I'm assuming that df is somehow a reserved word, but when I checked this list of reserved words in bash I couldn't find it. (And I don't think it deserves to be one, anyway!)

So, can anyone shed some light on this? Why does bash prohibit me from defining a shell function named df?

@JoshLee: That's an issue, but it looks like something else is going wrong before parsing reaches that point. — user2357112
– user2357112, Commented Sep 27, 2019 at 23:38
That said, I can't reproduce the reported problem. The ! causes problems, but naming a function df seems to work fine. — user2357112
– user2357112, Commented Sep 27, 2019 at 23:41

that other guy · Accepted Answer · 2019-09-27 23:48:47Z

This happens because you've previously defined an alias for this name. Aliases are simple string prefix substitutions, and therefore interfere with function definitions:

$ alias foo='foo --bar' $ foo() { echo "Hello"; } bash: syntax error near unexpected token `('

This is equivalent to (and fails with the same error as)

$ foo --bar() { echo "Hello"; } bash: syntax error near unexpected token `('

To declare a function with a name that's been overridden with an alias, you can use the function keyword:

$ alias foo='foo --bar' $ function foo() { echo "Hello, $1"; } $ foo Hello, --bar

I tried to verify this, in a different window, and realized you're entirely correct! I had been playing with df-as-alias before moving to df-as-function, and sure enough there was a legacy alias in the shell window where I was having the problem. Great catch, thanks!

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