Bash – Why does Bash history not record this command

bashcommand lineprintf

The command printf "#!/bin/bash\n" > t generates an error(bash: !/bin/bash\n": event not found), but hitting the up key does not recall the command so that I can modify it. Why is that?

(What I was trying for here can be accomplished with printf "%s\n" '#!/bin/bash' >t I am trying to learn Bash and really want to understand why bash history doesn't capture the previous printf command.)

Best Answer

The ! character triggers history expansion, as you discovered. This step occurs before a command is saved to the history, so that it can saved into the history with the expansion already done. When an error occurs in history expansion, bash stops processing the command, and so it never gets saved into the history.

History expansion allows previous commands or parts of previous commands to be substituted into the current command. If this was done before saving the command to the history, it could mean something different each time it is executed, because the previous commands are different in each instance. See http://www.gnu.org/software/bash/manual/bashref.html#History-Interaction

For reference, zsh's behavior is different. It will save the command but with the history expansion missing.

Related Solutions

Why Does the Exclamation Mark `!` Sometimes Upset Bash?

The ! character invokes bash's history substitution (enabled by default in interactive shells). When followed by a string (as in your failing example) it tries to expand to the last history event that began with that string. Just like $var gets expanded to the value of that string, !echo would expand to the last echo command in your history.

Space is a breaking character in such expansions. First note how this would work with variables:

# var="like"
# echo "$var"
like
# echo "$"
$
# echo "Do you $var frogs?"
Do you like frogs?       <- as expected, variable name broken at space
# echo "Do you $varfrogs?"
Do you?                  <- $varfrogs not defined, replaced with blank
# echo "Do you $ var frogs?"
Do you $ var frogs?      <- $ not a valid variable name, ignored

The same thing will happen for history expansion. The bang character (!) starts off a history replacement sequence, but only if followed by a string. Following it with a space make it literal bang instead of part of a replace sequence.

You can avoid this kind of replacement for both variable and history expantion by using single quotes. Your first examples used single quotes and so ran fine. Your last examples are in double quotes and thus bash scanned them for expantion sequences before it did anything else. The only reason the first one didn't trip is that that the space is a break character as shown above.

Bash – Using Dashes in Printf

The -- is used to tell the program that whatever follows should not be interpreted as a command line option to printf.

Thus the printf "--" you tried basically ended up as "printf with no arguments" and therefore failed.

Best Answer

Related Solutions

Why Does the Exclamation Mark `!` Sometimes Upset Bash?

Bash – Using Dashes in Printf

Related Question