Shell Programming and Scripting

@Aia:

Revised AHA!

When in doubt, single quote it out* --

*unless the shell needs to see the variable to substitute. In those cases, the game of double quote, scape, multiple single quotes would be on.

(Teamwork )

I almost always single quote sed commands because it makes it obvious to me when I'm reading it that the shell won't do anything with it, but to effectively write shell scripts, you must learn how single quotes, double quotes, and backslash escapes are processed and know when they are required.

Note that the command:

Code:

sed -i 's/\${HOUSING}/casing/g' ./some.file

(which seems to do what you want), and the command Aia suggested:

Code:

sed -i s\/\$\{HOUSING\}\/casing\/g ./some.file

are not equivalent. Aia's suggestion has several unneeded escapes and is missing some that are crucial.

For this substitute command, the only characters that are special to the shell in this context (so they need to be quoted or escaped) are the backslash and the dollar sign. So, the following three commands are all equivalent to the command you want with minimal single quotes, minimal double quotes, and minimal backslash escapes:

Code:

sed -i s/'\$'{HOUSING}/casing/g ./some.file
sed -i s/"\\$"{HOUSING}/casing/g ./some.file
sed -i s/\\\${HOUSING}/casing/g ./some.file

Some versions of sed do what you want with or without the backslash in the 1st two commands above and with only one backslash in the 3rd command (including the sed on MAC OS X), but any sed that does what you want without those backslashes does not conform to the standards.

@Don,

I am afraid you read too much on my example. It was only intended as a visual representation that quotes were not really necessary. Completely, unrelated.

Now, it is good if you felt you could use it to further show differences.

Seeing the difference is easy. Compare the 1st three lines of output (showing the operands that I said would produce equivalent sed behavior) with the last line (showing the operand that you suggested) from the command:

Code:

printf '%s\n' s/'\$'{HOUSING}/casing/g s/"\\$"{HOUSING}/casing/g s/\\\${HOUSING}/casing/g s\/\$\{HOUSING\}\/casing\/g

which produces the output:

Code:

s/\${HOUSING}/casing/g
s/\${HOUSING}/casing/g
s/\${HOUSING}/casing/g
s/${HOUSING}/casing/g

Do you see the missing backslash?

You don't need to escape the slashes (/) or the braces ({ and }) because slash doesn't have any special meaning to the shell and the braces are keywords (not tokens) in the shell. If the braces had been surrounded by whitespace characters, they would have needed to be escaped or quoted as well.