Why does bash "echo [t]" result in "t" not "[t]"

Question

This happens for the character t, and the value root. Quite perplexing

$ echo [s] [s] $ echo [t] t $ echo [ t ] [ t ] $ echo [root] t 

Answer 1

[] denotes a character class, and you have a file named t in your current directory.

The following should explain it further:

$ ls $ echo [t] [t] $ touch t $ echo [t] t $ echo [root] t $ touch r $ echo [root] r t 

---

If you want to echo something within [], escape the [:

echo \[$var] 

Observe the difference now:

$ echo \[root] [root] 

or, as Glenn Jackman points out, quote it:

$ echo '[root]' [root] $ echo "[root]" [root] 

---

Shell Command Language tells that the following characters are special to the shell depending upon the context:

*   ?   [   #   ~   =   % 

Moreover, following characters must be quoted if they are to represent themselves:

|  &  ;  <  >  (  )  $  `  \  "  '  <space>  <tab>  <newline> 

---

You could also use printf to determine which characters in a given input need to be escaped if you are not quoting the arguments. For your example, i.e. [s]:

$ printf "%q" "[s]" \[s\] 

Another example:

$ printf "%q" "[0-9]|[a-z]|.*?$|1<2>3|(foo)" \[0-9\]\|\[a-z\]\|.\*\?\$\|1\<2\>3\|\(foo\)