重击：使用反引号在引号中使用空格执行命令

Question

I've written the following bash function. It is supposed to print a command, run it and print its output, and print a short description.

#!/bin/bash

print_and_run_command() {
    COMMAND="$1"
    DESCRIPTION="$2"
    OUTPUT="`$COMMAND`"
    printf "%-30s %-30s %s\n" "$COMMAND" "$OUTPUT" "$DESCRIPTION"
}

print_and_run_command "date +%S" "Second"
print_and_run_command" date +%H" "Hour"
print_and_run_command "date +%Y-%m-%dT%H:%M:%S%z" "T-separated rfc 3339 / ISO 8601"

Each of the date commands works well when executed from a shell. However, when running the script, a strange error happens:

date +%S                       34                             Second
./blah.sh: line 11: print_and_run_command date +%H: command not found
date +%Y-%m-%dT%H:%M:%S%z      2013-07-30T14:20:34-0700       T-separated rfc 3339 / ISO 8601

Running date +%H fails, even though date +%S and the complicated date +%Y-%m-%dT%H:%M:%S%z work just fine.

Any ideas what is failing the print_and_run_command" date +%H" "Hour" ?

Answer 1

Well, let set -x help us to see what is actually going on.

+ print_and_run_command 'date +%S' Second
+ COMMAND='date +%S'
+ DESCRIPTION=Second
++ date +%S
+ OUTPUT=13
+ printf '%-30s %-30s %s\n' 'date +%S' 13 Second
date +%S                       13                             Second
+ 'print_and_run_command date +%H' Hour
./oh.sh: line 13: print_and_run_command date +%H: command not found
+ print_and_run_command 'date +%Y-%m-%dT%H:%M:%S%z' 'T-separated rfc 3339 / ISO 8601'
+ COMMAND='date +%Y-%m-%dT%H:%M:%S%z'
+ DESCRIPTION='T-separated rfc 3339 / ISO 8601'
++ date +%Y-%m-%dT%H:%M:%S%z
+ OUTPUT=2013-07-31T01:24:13+0400
+ printf '%-30s %-30s %s\n' 'date +%Y-%m-%dT%H:%M:%S%z' 2013-07-31T01:24:13+0400 'T-separated rfc 3339 / ISO 8601'
date +%Y-%m-%dT%H:%M:%S%z      2013-07-31T01:24:13+0400       T-separated rfc 3339 / ISO 8601
+ set +x

So here is your error:

print_and_run_command" date +%H" "Hour"

It should be

print_and_run_command "date +%H" "Hour"

print_and_run_command" date +%H" "Hour" fails because of print_and_run_command" date +%H" expanded to print_and_run_command date +%H which is an unknown command for shell.