What does ${x} mean?

Shell Programming and Scripting
1

In POSIX shell, what is the difference between $x and ${x} ?

Example:

#!/bin/sh

x=1

echo $x
echo ${x}

exit 0

They both print "1".

2

man sh :

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3

The { and } are very useful for clarity. It is also very useful for these cases too:-

  • You want to refer to command line parameter ten or over. Some OSes would interpret $10 as actually the first parameter suffixed with a zero. You would need to refer to ${10} instead.
  • You might want to refer to a variable and put in a suffix of your own, then $var_suffix will not work, but ${var}_suffix will. The _suffix will be treated as though it is part of the variable name with the first format, but is just literal text appended after the value of the variable in the latter.

The second case might be a bit obscure, so perhaps a demonstration on the command line is better:-

$ a=Hello
$ a_b=world
$ echo $a
Hello
$ echo $a_b
world
$ echo ${a}_b
Hello_b

I hope that this helps and that I haven't confused things.
Robin

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