Concatenate with parameter in command substitution

Question

If I want to set a value like:

key="some"
value=$(cat ${filename} | grep ${key}"string" | cut -d 'string' -f2)

What do I do to get the ${key}"string" to work? It doesn't handle the ${key} variable to concatenate it with string. I want to do:

cat $filename | grep "somestring" | cut...

`key=some; echo 'somestring' | grep -o "${key}s"` outputs `somes` — Ruslan Osmanov, Oct 29 '16 at 12:34
Use one call to `awk`: `awk -v k="$key" '$0 ~ k"string" {print $2}' "$filename"` — chepner, Oct 29 '16 at 12:59
@chepner: It looks like the desired logic is: output what comes _after_ the match, so you can't use the default `FS`; `awk -F "${key}string" 'NF >= 2 { print $2 }'` may work (assuming just 1 match per line and escaping of regex metacharacters in the field-separator string, if needed). — mklement0, Oct 29 '16 at 13:27
Note that you should avoid the [Useless Use of `cat` — UUoC](https://stackoverflow.com/questions/11710552/useless-use-of-cat/11710888#11710888). — Jonathan Leffler, Oct 30 '16 at 03:13

score 3 · Answer 1 · edited May 23 '17 at 12:33

To properly concatenate variable reference ${key} with literal "string", embed the variable reference inside the double-quoted string:

key="some"
... | grep "${key}string" | ...

^{Note how enclosing the variable name in {...} is actually required now, to tell the shell where the variable name ends. chepner points out in a comment elsewhere:

"${key} is the canonical way to perform parameter expansion. The braces can optionally be dropped when they aren't necessary to enclose an expansion operator or to disambiguate the parameter name. "}

With the specific sample value "some", your concatenation (${key}"string") would have worked the same, but not generally, because using ${key} unquoted makes it subject to shell expansions, notably word splitting, which will break the grep command if the value of variable ${key} happens to contain whitespace.

The main problem with your code, however, is that you're passing a multi-character string to cut -d in order to specify a delimiter, whereas only a single character is supported.

If you have GNU grep, you can try the following:

key="some"
value=$(grep -Po "${key}string"'\K.*$' "${filename}")

This will return everything after "${key}string" from matching lines, which is what I presume your intent was.
The assumption is that "${key}string" matches at most once per line.

The small caveat is that you now may have to escape regular-expression metacharacters such as . in the value of ${key} and also the literal part ("string") to make it work as a regular expression.

With the same assumption and caveat, you can try this POSIX-compliant awk solution:

key="some"
value=$(awk -F "${key}string" 'NF >= 2 { print $2 }')

That said, if there could be multiple matches, and you needed the strings between the matches on each line, it is possible to adapt this solution.

Concatenate with parameter in command substitution

1 Answers1