Why doesn't if [ echo $foo | grep -q bar ] work? - bash

Question

I'm trying to read user input and compare it against a stored value as follows:
read confirm
if [ echo $confirm | grep -q y ]; then
...
fi
However, this results in a pair of errors:
[: missing `]'
grep: ]: No such file or directory
Why does this happen, and what's the appropriate alternative?

Answer 1

Short Answer
For your immediate use case, you simply want:
if echo "$confirm" | grep -q y; then
...or its much more efficient equivalent (if your shell is bash):
if [[ $confirm = *y* ]]; then
...or its much more efficient equivalent (for any POSIX shell):
case $confirm in *y*) echo "Put your code for the yes branch here" ;; esac
Why was the original wrong?
[ is not part of if syntax: if simply takes a (potentially compound) command as its argument before the then. [ is different name for the test command, which runs checks on its arguments; however, if what you want to test is the exit status of grep -q, then the test command doesn't need to be invoked for this purpose at all.
If you put a | inside a [ command, that makes your compound command a pipeline, and starts a new simple command. Arguments after the | are thus no longer passed to [.
With your original code:
if [ echo $confirm | grep -q y ]; then
...this was running two commands, with a pipeline between them:
[ echo $confirm # first command
grep -q y ] # second command
Since [ requires that its last argument be ], it reported that that mandatory argument was missing; and since grep treats extra arguments as filenames to read, it complained that no file named ] could be found.
Also, [ "$foo" ] checks whether the contents of foo is nonempty. Since the output of grep -q is always empty, [ "$(echo "$confirm" | grep -q y)" ], while syntactically correct, would always evaluate to false, even while exit status of grep -q changes to indicate whether a match was found. ([ "$(echo "$confirm" | grep y)" ], by contrast, is an alternative that emits a correct result - using [ ] to test whether the output from grep is or is not empty -- but is much less efficient than the best-practice approaches).
Formal if syntax
From help if:
if: if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [ else COMMANDS; ] fi
The if COMMANDS list is executed. If its exit status is zero, then the
then COMMANDS list is executed. Otherwise, each elif COMMANDS list is
executed in turn, and if its exit status is zero, the corresponding
then COMMANDS list is executed and the if command completes. Otherwise,
the else COMMANDS list is executed, if present. The exit status of the
entire construct is the exit status of the last command executed, or zero
if no condition tested true.
Notably, if takes a list of COMMANDS, and no [ is included in the syntax specification.