Difference between and in Bash

Question

I have two questions and could use some help understanding them    What is the difference between     and      I understand that    means running command in separate shell and placing   means passing the value to variable  Can someone help me in understanding this  Please correct me if I am wrong  If we can use for   i 0 i lt 10 i      do echo  i  done and it works fine then why can t I use it as while   i 0 i lt 10 i      do echo  i  done  What is the difference in execution cycle for both

User · Answer

your understanding is right  For detailed info on    see bash ref - parameter expansion  for  and  while  have different syntax and offer different styles of programmer control for an iteration  Most non-asm languages offer a similar syntax     With while  you would probably write i 0  while    i -lt 10    do echo  i  i     i   1     done in essence manage everything about the iteration yourself

User · Answer

means   first evaluate this  and then evaluate the rest of the line    Ex    echo   pwd  myFile txt   will be interpreted as  echo  my path myFile txt   On the other hand     expands a variable   Ex   MY VAR toto echo   MY VAR  myFile txt   will be interpreted as  echo toto myFile txt     Why can t I use it as bash  while   i 0 i lt 10 i      do echo  i  done   I m afraid the answer is just that the bash syntax for while just isn t the same as the syntax for for

User · Answer

The syntax is token-level  so the meaning of the dollar sign depends on the token it s in  The expression   command  is a modern synonym for  command  which stands for command substitution  it means run command and put its output here  So  echo  Today is   date   A fine day     will run the date command and include its output in the argument to echo  The parentheses are unrelated to the syntax for running a command in a subshell  although they have something in common  the command substitution also runs in a separate subshell    By contrast    variable  is just a disambiguation mechanism  so you can say   var text when you mean the contents of the variable var  followed by text  as opposed to  vartext which means the contents of the variable vartext    The while loop expects a single argument which should evaluate to true or false  or actually multiple  where the last one s truth value is examined -- thanks Jonathan Leffler for pointing this out   when it s false  the loop is no longer executed  The for loop iterates over a list of items and binds each to a loop variable in turn  the syntax you refer to is one  rather generalized  way to express a loop over a range of arithmetic values   A for loop like that can be rephrased as a while loop  The expression  for   init  check  step    do     body done   is equivalent to  init while check  do     body     step done   It makes sense to keep all the loop control in one place for legibility  but as you can see when it s expressed like this  the for loop does quite a bit more than the while loop   Of course  this syntax is Bash-specific  classic Bourne shell only has  for variable in token1 token2      do      Somewhat more elegantly  you could avoid the echo in the first example as long as you are sure that your argument string doesn t contain any   format codes   date   Today is  c  A fine day     Avoiding a process where you can is an important consideration  even though it doesn t make a lot of difference in this isolated example

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