ANSI-C Quoting¶
You've probably seen that weird syntax $'...' somewhere in a Bash script before.
That syntax is called ANSI-C quoting. This page explains what that is.
What is ANSI-C quoting?¶
ANSI-C quoting is a bash feature with special syntax that allows you to use backslash escape sequences that you'd use in C and represent non-printable characters cleanly. It also allows for writing characters using octal/hex notation.
Note: ANSI-C quoting is not POSIX-compliant. If you care.
Syntax¶
The syntax for ANSI-C quoting is:
$'...'
This acts like a single-quoted string, but escape sequences are interpreted like in the C language.
Why use them?¶
These types of strings allow you to use certain characters/escapes in variables that would otherwise be interpreted incorrectly.
For instance, if you were trying to use a newline in a variable:
newline="\n"
printf "Test%s" "$newline"
# Output:
# Test\n
However, if you ANSI-C quote the string:
newline=$'\n'
printf "Test%s" "$newline"
# Output:
# Test
This goes for any other escape sequence you're trying to use in a variable.
Common Escape Sequences¶
Some of the most common escape sequences that would go in an ANSI-C quoted string:
\n: newline\t: Tab\a: Alert (bell)- '\': A literal backslash
\': A literal single quote
You can also use hexadecimal/octal values here.
Examples¶
You can use ANSI-C quoting to embed byte values directly.
For example, using hexadecimal and octal values:
printf $'\x48\x65\x6c\x6c\x6f\n' # Hexadecimal
# Output:
# Hello
printf $'\101\102\103\n' # Octal
# Output:
# ABC
ANSI-C quoting can also be used to strip characters (i.e., newlines) from strings.
output=$(printf "Hello\n\n")
clean="${output//$'\n'/}"
Note: Using ANSI-C quoted strings inside parameter expansions requires the
extquoteshell option to be enabled (this is enabled by default though).
If it's not enabled, you can enable it withshopt -s extquote