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
• 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
• then – Then clause
• until – Until loop
• while – While loop

Currently, [[ and function are only recognized as reserved words; their functionality is not yet implemented.

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

• ]] – End of a double bracket command
• namespace – Namespace declaration
• select – Select command
• time – Time command

These four words are not reserved in yash-rs now, but may be in the future.

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

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 not found
  |

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

$ { echo Hello; }
Hello