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Laurence MorganAbout 2 min


Conditional statement to execute different blocks of code depending on the result of the condition


Conditional control flow

if can be utilized both as a method as well as a standalone function. As a method, the conditional state is derived from the calling function (eg if the previous function succeeds then the condition is true).


Function if:

if { code-block } then {
    # true
} else {
    # false

Method if:

command -> if {
    # true
} else {
    # false

Negative Function if:

!if { code-block } then {
    # false

Negative Method if:

command -> !if {
    # false

Please Note:

the then and else statements are optional. So the first usage could also be written as:

if { code-block } {
    # true
} {
    # false

However the practice of omitting those statements isn't recommended beyond writing short one liners in the interactive command prompt.


Check if a file exists:

if { g somefile.txt } then {
    out "File exists"

...or does not exist (both ways are valid):

!if { g somefile.txt } then {
    out "File does not exist"
if { g somefile.txt } else {
    out "File does not exist"


Pipelines and Output

The conditional block can contain entire pipelines - even multiple lines of code let alone a single pipeline - as well as solitary commands as demonstrated in the examples above. However the conditional block does not output STDOUT nor STDERR to the rest of the pipeline so you don't have to worry about redirecting the output streams to null.

If you require output from the conditional blocks STDOUT then you will need to use either a Murex named pipe to redirect the output, or test or debug flags (depending on your use case) if you only need to occasionally inspect the conditionals output.

Exit Numbers

When evaluating a command or code block, if will treat an exit number less than 0 as true, and one greater than 0 as false. When the exit number is 0, if will examine the stdout of the command or code block. If there is no output, or the output is one of the following strings, if will evaluate the command or code block as false. Otherwise, it will be considered true.

  • 0
  • disabled
  • fail
  • failed
  • false
  • no
  • null
  • off


  • if
  • !if

See Also

  • ! (not): Reads the STDIN and exit number from previous process and not's it's condition
  • and: Returns true or false depending on whether multiple conditions are met
  • catch: Handles the exception code raised by try or trypipe
  • debug: Debugging information
  • false: Returns a false value
  • or: Returns true or false depending on whether one code-block out of multiple ones supplied is successful or unsuccessful.
  • switch: Blocks of cascading conditionals
  • test: Murex's test framework - define tests, run tests and debug shell scripts
  • true: Returns a true value
  • try: Handles non-zero exits inside a block of code
  • trypipe: Checks for non-zero exits of each function in a pipeline

This document was generated from builtins/core/structs/if_doc.yamlopen in new window.

Last update:
Contributors: Laurence Morgan,Laurence Morgan,Laurence,Olivier Refalo,tiymat