Bash – For loops in zsh and bash

bashzsh

I have noticed there are two alternative ways of building loops in zsh:

for x (1 2 3); do echo $x; done
for x in 1 2 3; do echo $x; done

They both print:

1
2
3

My question is, why the two syntaxes? Is $x iterating through a different type of object in each of them?

Does bash make a similar distinction?

Addendum:

Why does the following work?:

#!/bin/zsh
a=1
b=2
c=5    

d=(a b c)    
for x in $d; do print $x;done

but this one doesn't?:

#!/bin/zsh
a=1
b=2
c=5

d=(a b c)    
# It complains with "parse error near `$d'"
for x $d; do print $x;done

Best Answer

Several forms of complex commands such as loops have alternate forms in zsh. These forms are mostly inspired by the C shell, which was fairly common when zsh was young but has now disappeared. These alternate forms act exactly like the normal forms, they're just a different syntax. They're slightly shorter, but less clear.

The standard form for the for command is for x in 1 2 3; do echo $x; done, and the standard form for the while command is while test …; do somecommand; done. Ksh, bash and zsh have an alternate form of for: for ((i = 0; i < 42; i++)); do somecommand; done, which mimics the for loops of languages like Pascal or C, to enumerate integers. Other exotic forms that exist in zsh are specific to zsh (but often inspired by csh).

Related Solutions

Arrays and loops in zsh

Section 15.2.1 “Array Subscripts” says that arrays can be index with [exp] (where exp is a numeric expression) and that elements start at index 1 (0 if KSH_ARRAYS is set).

Section 14.3 “Parameter Expansion” says that the syntax ${#array_name} will expand to the number of elements of an array.

Section 6.3 “Complex Commands” gives the syntax for a numeric for loop (as in C):
for (( initExpr ; testExpr ; stepExpr )) do … done.

Putting them all together:

for (( i = 1; i <= $#LOCAL_PATH; i++ )) do
    ( # subshell to contain the effect of the chdir
        cd $LOCAL_PATH[i]
        hg pull $REMOTE_PATH[i]
    )
done

Or, if you are using KSH_ARRAYS, then this:

for (( i = 0; i < ${#LOCAL_PATH[@]}; i++ )) do
    ( # subshell to contain the effect of the chdir
        cd ${LOCAL_PATH[i]}
        hg pull ${REMOTE_PATH[i]}
    )
done

Using KSH_ARRAYS makes arrays start with index 0, requires the use of braces for array expressions, and interprets $array as $array[0] (requiring a change to the array length expression). The syntax changes required by KSH_ARRAY will also work without KSH_ARRAY, but you still have to adjust the logic for the different index range (1 through N versus 0 through N-1).

Bash and Zsh – Double and Triple Variable Substitution

The \ must be used to prevent the expansion of $$ (current process id). For triple substitution, you need double eval, so also more escapes to avoid the unwanted expansions in each eval:

#! /bin/bash
l0=value
l1=l0
l2=l1
l3=l2
l4=l3

echo $l0
eval echo \$$l1
eval eval echo \\$\$$l2
eval eval eval echo \\\\$\\$\$$l3
eval eval eval eval echo \\\\\\\\$\\\\$\\$\$$l4

Addendum:

Best Answer

Related Solutions

Arrays and loops in zsh

Bash and Zsh – Double and Triple Variable Substitution

Related Question