12.3.6         Command substitution

You can use the result of one command as an argument to another command. This is called command substitution:

For Local variables (i.e. variable used inside scripts) examples are:

set a 2

set b [expr $a*16]

set deltat [expr $time2 - $time1]

In the above a and b are not a tags.  They are local variables used inside a script.

For Tags (i.e. IO Tags, Bock:parameters, constant Tags, Calculation Tags, Accumulation Tags, System Tags, local tags and %DAQ tags) examples are:

SETVAL tag1=@ tag2

SETVAL tag1={GETVAL tag2}

SETVAL tag1=[GETVAL tag2]

SETVAL anatag1=[expr [GETVAL anatag2] * 10]

SETVAL tag2=[expr [GETVAL tag1]+[GETVAL tag2] ]

set deltat [expr [GETVAL tag1]+[GETVAL tag2] ]


In the above example, Tag1, tag2, anatag1, anatag2 are all Tags. deltat is a local variable.

When a square bracket [ appears in a command, Tcl treats everything between it and the matching end bracket ] as a single command. Tcl evaluates the nested command and substitutes its result into the enclosing command in place of the bracketed text.

Quotes " " also force command substitution.

See Braces or Quotes or Brackets

and 12.10 Example Scripts