Reserved words

Some words have special meaning in shell syntax. These reserved words must be quoted to use them literally. The reserved words are:

• ! – Negation
• { – Start of a grouping
• } – End of a grouping
• [[ – Start of a double bracket command
• ]] – End of a double bracket command (since 3.3.0)
• case – Case command
• do – Start of a loop or conditional block
• done – End of a loop or conditional block
• elif – Else if clause
• else – Else clause
• esac – End of a case command
• fi – End of an if command
• for – For loop
• function – Function definition
• if – If command
• in – Delimiter for a for loop and case command
• namespace – Namespace declaration (since 3.3.0)
• select – Select command (since 3.3.0)
• then – Then clause
• until – Until loop
• while – While loop

The reserved words [[, ]], function, namespace, and select are recognized but not yet implemented. Using these reserved words will result in a syntax error.

Additionally, the POSIX standard allows for the following optional reserved words:

• time – Time command
• Any words that end with a colon (:)

These words are not reserved in yash-rs now, but may be in the future. (Since 3.3.0) The portable option rejects a command name ending with a : (the lone : colon built-in is exempt).

Where are reserved words recognized?

Reserved words are recognized in these contexts:

• As the first word of a command
• As a word following any reserved word other than case, for, or in
• in as the third word in a for loop or case command
• do as the third word in a for loop

As an extension, yash-rs additionally recognizes a reserved word immediately after a subshell or a redirection, which POSIX does not, so such scripts are not portable. (Since 3.3.0) The portable option rejects a reserved word in this position; insert ; or a newline before it.

Examples

This example uses the reserved words for, in, do, and done in a for loop:

$ for i in 1 2 3; do echo $i; done
1
2
3

In the following example, {, do, and } are not reserved words because they are not the first word of the command:

$ echo { do re mi }
{ do re mi }

Reserved words are recognized only when they appear as a whole word. In this example, { and } are not reserved words because they are part of {echo and Hello}:

$ {echo Hello}
error: cannot execute external utility "{echo"
 --> <stdin>:1:1
  |
1 | {echo Hello}
  | ^^^^^ utility "{echo" not found

To use { and } as reserved words, write them as separate words:

$ { echo Hello; }
Hello

Per the extension described above, a reserved word is also recognized right after a subshell or a redirection. Here, the } closes the grouping even though it immediately follows the subshell ( … ):

$ { ( echo Hello ) }
Hello

This is not portable; inserting a separator, as in { ( echo Hello ); }, makes it portable.