.TH = 1
.CT 1 shell
.SH NAME
=, ==, =p, ==p \- redo previous shell command
.SH SYNOPSIS
.B =
[
.I pattern
]
[
.I substitution ...
]
.br
.B ==
[
.I pattern
]
[
.I substitution ...
]
.br
.B =p
[
.I pattern
]
[
.I substitution ...
]
.br
.B ==p
[
.I pattern
]
[
.I substitution ...
]
.SH DESCRIPTION
The 
.I =
command provides a simple history mechanism for the shell,
.IR sh (1).
The environment variable
.BR HISTORY ,
if set,
names a file to which
the shell appends the text of each command before execution.
.I =
searches the history file for the most recent
command that matches the
.I pattern,
performs the
.IR substitutions,
and executes it.
The
.I pattern
must agree with an initial substring of the original
command except for variations in spacing.
If no
.I pattern
is specified, the most recent command is selected.
If no
substitution
is specified, the command is executed without modification.
.PP
.PP
Substitutions have the form
.IP
.IB old = new
.LP
specifying that the string
.I old
in the command is to be replaced by
.I new.
Substitutions are made in order
and operate on the first match.
.PP
The
.I ==
command is identical to
.IR = ,
but allows the substituted command to be edited before running.
The command is printed,
and a modification request is read from the terminal.
Generally each character in the request specifies how to
modify the character immediately above it:
.TP
.B #
Delete the character.
.PD 0
.TP
.B %
Replace the character with a space.
.TP
.B ^
Insert the rest of the request line before the character.
.TP
.B $
Replace the characters in the command from this position on
with the rest of the request line.
.IP "space or tab"
Leave the character(s) unchanged.
.TP
.B =
Must be the first and only edit character.
Back up to the next most recent
match in the history file
and try again.
.IP "any other"
This character replaces the one above it.
.PD
.PP
If the request line is longer than the command,
the overhang is appended to the command.
.PP
.I =p
and
.I ==p
behave like
.I =
and
.IR == ,
except that they print the command on
their standard output instead of executing it.