Reusing output from last command in Bash

Question

Is the output of a Bash command stored in any register? E.g. something similar to $? capturing the output instead of the exit status.

I could assign the

Answer 1

Yeah, why type extra lines each time; agreed. You can redirect the returned from a command to input by pipeline, but redirecting printed output to input (1>&0) is nope, at least not for multiple line outputs. Also you won't want to write a function again and again in each file for the same. So let's try something else.

A general solution for running the desired command only once and getting multi-line printed output of the command in an array variable line-by-line.

If you are not exporting the files anywhere and intend to use it locally only, you can have Terminal set-up the function declaration. You have to add the function in ~/.bashrc file or in ~/.profile file. In second case, you need to enable Run command as login shell from Edit>Preferences>yourProfile>Command.

Make a simple function, say:

get_prev()
{
    # option 1: create an executable with the command(s) and run it
    echo $* > /tmp/exe
    bash /tmp/exe > /tmp/out
    
    # option 2: if your command is single command (no-pipe, no semi-colons), still it may not run correct in some exceptions
    #echo `$*` > /tmp/out
    
    # return the command(s) outputs line by line
    IFS=$(echo -en "\n\b")
    arr=()
    exec 3

So what we did was print the whole command to a temporary file /tmp/exe and run it and save the output to another file /tmp/out and then read the contents of the /tmp/out file line-by-line to an array.

In main script:

#run your command:
cmd="echo hey ya; echo hey hi; printf `expr 10 + 10`'\n' ; printf $((10 + 20))'\n'"
get_prev $cmd
#or simply
get_prev "echo hey ya; echo hey hi; printf `expr 10 + 10`'\n' ; printf $((10 + 20))'\n'"

Now, bash saves the variable even outside previous scope, so the arr variable created in get_prev function is accessible even outside the function in the main script:

#get previous command outputs in arr
for((i=0; i<${#arr[@]}; i++)); do echo ${arr[i]}; done

Edit:
I'm using the following code in my implementation:

get_prev()
{
    usage()
    {
        echo "Usage: alphabet [ -h | --help ]
        [ -s | --sep SEP ]
        [ -v | --var VAR ] \"command\""
    }
    
    ARGS=$(getopt -a -n alphabet -o hs:v: --long help,sep:,var: -- "$@")
    if [ $? -ne 0 ]; then usage; return 2; fi
    eval set -- $ARGS
    
    local var="arr"
    IFS=$(echo -en '\n\b')
    for arg in $*
    do
        case $arg in
            -h|--help)
                usage
                echo " -h, --help : opens this help"
                echo " -s, --sep  : specify the separator, newline by default"
                echo " -v, --var  : variable name to put result into, arr by default"
                echo "  command   : command to execute. Enclose in quotes if multiple lines or pipelines are used."
                shift
                return 0
                ;;
            -s|--sep)
                shift
                IFS=$(echo -en $1)
                shift
                ;;
            -v|--var)
                shift
                var=$1
                shift
                ;;
            -|--)
                shift
                ;;
            *)
                cmd=$option
                ;;
        esac
    done
    if [ ${#} -eq 0 ]; then usage; return 1; fi
    
    echo $* > /tmp/exe
    ERROR=$( { bash /tmp/exe > /tmp/out; } 2>&1 )
    if [ $ERROR ]; then echo $ERROR; return 1; fi
    
    local a=()
    exec 3

Ive been using this to print space-separated outputs of multiple/pipelined/both commands as line separated:

get_prev -s " " -v myarr "cmd1 | cmd2; cmd3 | cmd4"

For example:

get_prev -s ' ' -v myarr whereis python # or "whereis python"
# can also be achieved (in this case) by
whereis python | tr ' ' '\n'

Now tr command is useful at other places as well, such as

echo $PATH | tr ':' '\n'

But for multiple/piped commands... you know now. :)

-Himanshu