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Reserve Words

Here is the truth about Bash reserved words: they are not commands sitting somewhere in /bin – they are part of the language, recognised before anything is looked up. You cannot name a function if, and type if will tell you why.

What it is, in plain words

Reserved words (keywords) are tokens Bash treats specially when they appear where a command name would. The POSIX set plus Bash extensions includes:

!    case  coproc  do    done
elif else  esac    fi    for
function   if      in    select
then until while   {     }
time [[    ]]

They form the control-flow and compound-command syntax: if … then … fi, for … do … done, case … esac, { …;}, [[ … ]].

Why it matters

Seeing if as syntax, not a program, explains a whole class of errors:

• If (capital I) is not if – the shell looks for a command named If.
• alias if=… does not do what you want.
• Putting if in a variable and expanding it does not recreate an if statement.

It also tells you what not to name your scripts and functions.

An everyday analogy: words that hold the sentence together

In English, “the”, “if”, “and” aren’t names of things – they structure the sentence. You can’t nickname a friend “If” and expect grammar to ignore it. Reserved words are the grammar of Bash.

A hands-on example

type if for [[ time
# if is a shell keyword
# for is a shell keyword
# [[ is a shell keyword
# time is a shell keyword

# These are *not* reserved – they are builtins or commands:
type cd echo test [
# cd is a shell builtin
# test is a shell builtin
# [ is a shell builtin

Keywords vs builtins vs externals:

# Compound commands – notice the keywords
if [[ -f /etc/os-release ]]; then
  while read -r line; do
    case "$line" in
      NAME=*) echo "${line#NAME=}" ;;
    esac
  done < /etc/os-release
fi

# { } groups commands in the current shell; ( ) in a subshell
{ echo a; echo b; } > /tmp/out
( cd /tmp && pwd )   # parent directory unchanged

function is optional Bash sugar; name() { …; } is portable. Prefer the portable form.

! as a reserved word negates a pipeline’s exit status: if ! grep -q foo file; then …. (History expansion also uses !, which is a different mechanism – disable with set +H in scripts.)

Word boundaries and when a keyword is a keyword

Reserved words are only special in command position. After an = or as an argument they are ordinary text:

echo if for while          # prints: if for while
alias_name=if              # fine – not command position
# if=foo                   # syntax error: 'if' is reserved here

This is why type and careful quoting matter when generating scripts, and why syntax highlighters colour these words differently – they really are a different kind of token.

Common misconceptions

• "[ and [[ are the same." [ is a builtin (alias of test); [[ is a keyword with richer parsing and no word-splitting surprises.
• "time is /usr/bin/time." The keyword time times pipelines; the external has different flags. Use command time or \time to force the external.
• “You can export a keyword.” No. Keywords aren’t variables.
• "{} needs no spaces." { echo hi; } needs a space after { and a semicolon (or newline) before }.
• “Reserved words are Bash-only.” Most listed above are POSIX; [[, function, coproc, select are Bash/ksh extensions.

Connections

• BASH, Bash if, Bash case, Bash Variables.
• Bash Cookbook.

if	then	elif	else	fi	time

for	in	until	while	do	done

case	esac	coproc	select	function

{	}	[[	]]	!