In Bash scripts, I frequently find this pattern useful, where I first print the command I'm about to execute, then I execute the command:

echo 'Running this cmd: ls -1 "$HOME/temp/some folder with spaces'
ls -1 "$HOME/temp/some folder with spaces"

echo 'Running this cmd: df -h'
df -h

# etc.

Notice the single quotes in the echo command to prevent variable expansion there! The idea is that I want to print the cmd I'm running, exactly as I will type and run the command, then run it!

How do I wrap this up into a function?

Wrapping the command up into a standard bash array, and then printing and calling it, like this, sort-of works:

# Print and run the passed-in command
# USAGE:
#       cmd_array=(ls -a -l -F /)
#       print_and_run_cmd cmd_array
# See:
# 1. My answer on how to pass regular "indexed" and associative arrays by reference:
#    https://stackoverflow.com/a/71060036/4561887 and
# 1. My answer on how to pass associative arrays: https://stackoverflow.com/a/71060913/4561887
print_and_run_cmd() {
    local -n array_reference="$1"
    echo "Running cmd:  ${cmd_array[@]}"
    # run the command by calling all elements of the command array at once
    ${cmd_array[@]}
}

For simple commands like this it works fine:

Usage:

cmd_array=(ls -a -l -F /)
print_and_run_cmd cmd_array

Output:

Running cmd:  ls -a -l -F /
(all output of that cmd is here)

But for more-complicated commands it is broken!:

Usage:

cmd_array=(ls -1 "$HOME/temp/some folder with spaces")
print_and_run_cmd cmd_array

Desired output:

Running cmd: ls -1 "$HOME/temp/some folder with spaces"
(all output of that command should be here)

Actual Output:

Running cmd:  ls -1 /home/gabriel/temp/some folder with spaces
ls: cannot access '/home/gabriel/temp/some': No such file or directory
ls: cannot access 'folder': No such file or directory
ls: cannot access 'with': No such file or directory
ls: cannot access 'spaces': No such file or directory

The first problem, as you can see, is that $HOME got expanded in the Running cmd: line, when it shouldn't have, and the double quotes around that path argument were removed, and the 2nd problem is that the command doesn't actually run.

How do I fix these 2 problems?

References:

1. my bash demo program where I have this print_and_run_cmd function: https://github.com/ElectricRCAircraftGuy/eRCaGuy_hello_world/blob/master/bash/argument_parsing__3_advanced__gen_prog_template.sh
2. where I first documented how to pass bash arrays by reference, as I do in that function:
   1. Passing arrays as parameters in bash
   2. How to pass an associative array as argument to a function in Bash?

CodePudding user response：

Check your scripts with shellcheck:

Line 2:
    local -n array_reference="$1"
             ^-- SC2034 (warning): array_reference appears unused. Verify use (or export if used externally).
 
Line 3:
    echo "Running cmd:  ${cmd_array[@]}"
                        ^-- SC2145 (error): Argument mixes string and array. Use * or separate argument.
                        ^-- SC2154 (warning): cmd_array is referenced but not assigned.
 
Line 5:
    ${cmd_array[@]}
    ^-- SC2068 (error): Double quote array expansions to avoid re-splitting elements.

You might want to research https://github.com/koalaman/shellcheck/wiki/SC2068 . We fix all errors and we get:

print_and_run_cmd() {
    local -n array_reference="$1"
    echo "Running cmd:  ${array_reference[*]}"
    # run the command by calling all elements of the command array at once
    "${array_reference[@]}"
}

For me it's odd to pass an array by reference in this case. I would pass the actual values. I often do:

prun() {
   # in the style of set -x
   # print to stderr, so output can be captured
   echo "  $*" >&2
   # or echo "  ${*@Q}" >&2
   # or echo " $(printf " %q" "$@")" >&2
   # or echo " $(/bin/printf " %q" "$@")" >&2
   "$@"
}
prun "${cmd_array[@]}"

How do I fix these 2 problems?

Incorporate into your workflow linters, formatters and static analysis tools, like shellcheck, and check the problems they point out.

And quote variable expansion. It's "${array[@]}".

CodePudding user response：

You can achieve what you want with DEBUG trap :

#!/bin/bash
 
trap 'trap_saved_command="${BASH_COMMAND#* }"' DEBUG
print_and_run_cmd(){
    echo "Running this cmd: $trap_saved_command"
    "$@"
}
print_and_run_cmd ls -1 "$HOME/temp/some folder with spaces"
# output ->
# Running this cmd: ls -1 "$HOME/temp/some folder with spaces"
# ...