@devnull's helpful answer explains why your code didn't work as expected: command substitution always returns a single string (possibly composed of multiple lines).
However, simply putting (...) around a command substitution to create an array of lines will only work as expected if the lines output by the command do not have embedded spaces - otherwise, each individual (whitespace-separated) word will become its own array element.
Capturing command output lines at once, in an array:
To capture the lines output by an arbitrary command in an array, use the following:
- bash < 4 (e.g., on OSX as of OS X 10.9.2): use
read -a
IFS=$'\n' read -rd '' -a linesArray <<<"$(grep "[a-z]" failedfiles.txt)"
- bash >= 4: use
readarray:
readarray -t linesArray <<<"$(grep "[a-z]" failedfiles.txt)"
Note:
<<< initiates a so-called here-string, which pipes the string to its right (which happens to be the result of a command substitution here) into the command on the left via stdin.
- While
command <<< string is functionally equivalent to echo string | command in principle, the crucial difference is that the latter creates subshells, which make variable assignments in command pointless - they are localized to each subshell.
- An alternative to combining here-strings with command substitution is [input] process substitution -
<(...) - which, simply put, allows using a command's output as if it were an input file; the equivalent of <<<"$(command)" is < <(command).
read: -a reads into an array, and IFS=$'\n' ensures that every line is considered a separate field and thus read into its own array element; -d '' ensures that ALL lines are read at once (before breaking them into fields); -r turns interpretation of escape sequence in the input off.readarray (also callable as mapfile) directly breaks input lines into an array of lines; -t ensures that the terminating \n is NOT included in the array elements.
Looping over command output lines:
If there is no need to capture all lines in an array at once and looping over a command's output line by line is sufficient, use the following:
while IFS= read -r line; do
# ...
done < <(grep "[a-z]" failedfiles.txt)
IFS= ensures that each line is read unmodified in terms of whitespace; remove it to have leading and trailing whitespace trimmed.-r ensures that the lines are read 'raw' in that substrings in the input that look like escape sequences - e.g., \t - are NOT interpreted as such.- Note the use of [input] process substitution (explained above) to provide the command output as input to the
read loop.
