${stuff} in shell

jhwilliams · 06-28-2007, 02:57 PM

What does putting ${ } around statements in shell do? Sometimes I see it done with legitimate variables, and other times with expressions. What's the deal? Thanks

acid_kewpie · 06-28-2007, 03:05 PM

it makes quoting variables easier, for example you can't do "echo thisisthenumber$itest" where $i is a variable, but "echo thisisstillthe${i}test" will substitute just fine. it's also used when you're doing regular expression substitutions directly in bash, i'd give you an example but i can never find them online, as mos people just use grep or awk instead. so basially evaluates the expression in the curly brackets in terms of bash variables and other bash functions.

aikempshall · 06-28-2007, 03:13 PM

It's used when embedded amongst other text like -

%echo ${TERM}like
vt100like

%echo $TERM
vt100

%echo ${TERM}
vt100

I find it good practice to always enclose variables with braces

try

%echo $TERMlike

and see what you get.

Alex

PTrenholme · 06-28-2007, 03:29 PM

From info bash:

Code:

       (list) list is executed in a subshell environment  (see  COMMAND  EXECUTION  ENVIRONMENT
              below).   Variable assignments and builtin commands that affect the shell’s envi-
              ronment do not remain in effect after the command completes.  The  return  status
              is the exit status of list.

       { list; }
              list  is  simply  executed in the current shell environment.  list must be termi-
              nated with a newline or semicolon.  This is known as a group command.  The return
              status  is the exit status of list.  Note that unlike the metacharacters ( and ),
              { and } are reserved words and must occur where a reserved word is  permitted  to
              be recognized.  Since they do not cause a word break, they must be separated from
              list by whitespace.

druuna · 06-28-2007, 03:34 PM

Hi,

Here are some links to bash's parameter expansion, which makes use of the ${...} construct (I believe this is what acid_kewpie was referring to):

Bash Reference manual: 3.5.3 Shell Parameter Expansion
The Deep, Dark Secrets of Bash
bash String Manipulations

Using bash internals instead of external programs (sed,awk, grep etc), is faster and resource friendlier.

Hope this helps.

pixellany · 06-28-2007, 03:50 PM

Also in regular expression, { } are used to repeat a previous expression. e.g.:

cat somefile|egrep '[0-9]{1,3}\.'

Looks for 1 to 3 occurences of a numeral, followed by a "." (part of the expression used to find IP addresses)