Build List Iterators With the Linux "foreach" Command

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When programming, the foreach command implements a loop where the loop variables assume the values from one or more lists. In the simplest case there is one loop variable, varname, and one list, list, that is a list of values to assign to varname. The body argument is a Tcl script. For each element of list (in order from first to last), foreach assigns the contents of the element to varname as if the lindex command had been used to extract the element, then calls the Tcl interpreter to execute body.


foreach varname list body 
foreach varlist1 list1 ?varlist2 list2 ...body


In a general case, there can be more than one value list (e.g., list1 and list2), and each value list can be associated with a list of loop variables (e.g., varlist1 and varlist2). During each iteration of the loop, the variables of each varlist are assigned consecutive values from the corresponding list. Values in each list are used in order from first to last, and each value is used exactly once. The total number of loop iterations is large enough to use up all the values from all the value lists. If a value list does not contain enough elements for each of its loop variables in each iteration, empty values are used for the missing elements.

The break and continue statements may be invoked inside body, with the same effect as in the for command. The foreach returns an empty string.  


The following loop uses i and j as loop variables to iterate over pairs of elements of a single list.

set x {}
foreach {i j} {a b c d e f} {
 lappend x $j $i
# The value of x is "b a d c f e"
# There are 3 iterations of the loop.

The next loop uses i and j to iterate over two lists in parallel.

set x {}
foreach i {a b c} j {d e f g} {
 lappend x $i $j
# The value of x is "a d b e c f {} g"
# There are 4 iterations of the loop.

The two forms are combined in the following example.

set x {}
foreach i {a b c} {j k} {d e f g} {
 lappend x $i $j $k
# The value of x is "a d e b f g c {} {}"
# There are 3 iterations of the loop.


Several related commands work with foreach including:

  • for(n)
  • while(n)
  • break(n)
  • continue(n)

Because Linux distribution and kernel-release levels vary, use the man command (% man) to see how a command is specifically used on your particular computer.

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