在shell中使用转义字符

Question

While reading about Escape characters in shell I tried an example -

echo "The balance for user $USER is: \\$5:00"

and it gave the output

The balance for user me is: $5.00

But when I tried using the escape character \\ with a tab \\t or a newline \\n , it didn't seem to work. I tried using it in double quotes (escape characters retain their meaning in the double quotes) but it still didn't work.

echo "The balance for user $USER is:\\t\\$5:00"

doesn't give the output

The balance for user me is: $5:00

Instead the output was -

The balance for user vyadav is:\\t$5:00

Answer 1

echo command doesn't expand backslash escapes without -e option.

You can use echo -e (please note that this is a gnu extension and not very portable):

echo -e "The balance for user $USER is:\t\$5:00"
The balance for user anubhava is:   $5:00

As per help echo :

-e  enable interpretation of the following backslash escapes

Or better to use printf :

printf "The balance for user %s is:\t\$%s\n" "$USER" "5:00"

Or

printf 'The balance for user %s is:\t$%s\n' "$USER" "5:00"

The balance for user anubhava is:   $5:00

Answer 2

There is a difference between what \\$ is doing and what \\t is doing.

The shell is expanding variables in double-quoted strings when it sees $ so you need to "escape" the $ from the shell (that's what \\$ is doing). You can also use a single-quoted which doesn't expand variables.

\\t on the other hand is "backslash escape" (to use the term the echo man page uses). There's nothing special about \\t (in a double-quoted string) to the shell so it doesn't touch it. And, by default, echo doesn't process "backslash escapes". You need the -e flag to turn that on.

Many people think that echo with arguments is an abomination and shouldn't exist and using the -e flag is easily avoided by using printf instead (which does handle a number of "backslash escapes" in its format string).

So the printf version of this would be (note the lack of \\$ in the format string because of the single quotes).

printf 'The balance for user %s is:\t$5:00' "$USER"