Using (, ) or {, } braces in Bash behaviour

[Sangal-Arun]

Hi,

Read the following first.

command group.
(a=hello; echo $a)

A listing of commands within parentheses starts a subshell.
Variables inside parentheses, within the subshell, are not visible to the rest of the script. The parent process, the script, cannot read variables created in the child process, the subshell.
a=123
( a=321; )

echo "a = $a" # a = 123
# "a" within parentheses acts like a local variable.


{}
Block of code [curly brackets]. Also referred to as an inline group, this construct, in effect, creates an anonymous function (a function without a name). However, unlike in a "standard" function, the variables inside a code block remain visible to the remainder of the script.
bash$ { local a;
a=123; }
bash: local: can only be used in a
function

a=123
{ a=321; }
echo "a = $a" # a = 321 (value inside code block)

# Thanks, S.C.

Now, if you know the difference between (, ) (creates child/sub shell)
and {, } (don't creates a child/sub shell and execute shell command statements in the same parent shell from where it's started) then why "c" variable in the following script is not getting c=AA.BB in the final echo statement.

am i missing something?

Script and its output:
===========
[/efare1/Users/qabuild/aksutil/1] $ vim giga.aks.sh

[/efare1/Users/qabuild/aksutil/1] $ chmod 700 giga.aks.sh

[/efare1/Users/qabuild/aksutil/1] $ cat giga.aks.sh|grep -v ^$

#!/bin/bash
a=A
b=B
c=$a.$b
echo $c
echo
{
 echo giga1
 a=AA
 echo giga2
 b=BB
 echo giga3
 c=$a.$b
 echo $c
 echo
} | tee -a giga.aks.out
echo =======
echo $c

[/efare1/Users/qabuild/aksutil/1] $ ./giga.aks.sh
A.B

giga1
giga2
giga3
AA.BB

=======
A.B

[/efare1/Users/qabuild/aksutil/1] $ ll -tr|tail -2
-rwx------ 1 qabuild denccefs 168 Sep 22 18:15 giga.aks.sh*
-rw-rw-r-- 1 qabuild denccefs 25 Sep 22 18:15 giga.aks.out

[/efare1/Users/qabuild/aksutil/1] $ cat giga.aks.out
giga1
giga2
giga3
AA.BB


[/efare1/Users/qabuild/aksutil/1] $

===========

[tetsujin]

I think that pipelining the output of a command has a similar effect to using parens around it - it creates a subshell in which variable assignments become local to that temporary environment.

It doesn't strike me as particularly desirable behavior from a perspective of language design - but in terms of how the shell works (creating pipelines and forking processes to handle each end of the pipeline) it kind of makes sense.

[Sangal-Arun]

Or it's the SCOPE of { , } brackets. When i took the opening curly brace after #!/bin/bash line and the closing curly brace at the end of the line, i got the following.. which seems both tee and variable behaviour will work.

[/efare1/Users/qabuild/aksutil/1] $ ./giga.aks.sh
A.B

giga1
giga2
giga3
AA.BB

=======
AA.BB

[/efare1/Users/qabuild/aksutil/1] $ cat ./giga.aks.out
A.B

giga1
giga2
giga3
AA.BB

=======
AA.BB

[tetsujin]

If you take out the curly braces from the script you wrote, it should fail because of a line starting with a pipe character... So I don't understand what kind of change you made when you say you removed the curly braces.

There's something wrong with the output you provide for "cat./giga.aks.out" - it should never include "A.B" or "====="...

Also note that since you're using the -a argument to "tee", previous runs' data will still be in the output file, so even if a particular run didn't add anything to the log file, there will still be something there...