If Conditional: if

Laurence MorganAbout 2 minBuiltin CommandsLanguage And Scripting

If Conditional: `if`

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

Description

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).

Usage

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.

Examples

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"
}

Detail

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

Synonyms

if
!if

If Conditional: if

# If Conditional: if

# Description

# Usage

# Function if:

# Method if

# Negative Function if:

# Negative Method if:

# Please Note:

# Examples

# Detail

# Pipelines and Output

# Exit Numbers

# Synonyms

# See Also