Run commands from a text file through a bash script

Question

I am attempting to write a script that will read through a text file, and then execute every line that begins with the word "run" or "chk" as a command. This is what I have thus far:

#!/bin/bash
counter=1
for i in $@
do
    while read -r line 
    do
    if [[ ${line:0:4} == "run " ]]
    then
        echo "Now running line $counter" 
        ${line:4:${#line}}
    elif [[ ${line:0:4} == "chk " ]]
    then
        echo "Now checking line $counter" 
        ${line:4:${#line}}
    elif [[ ${line:0:2} == "# " ]]
    then
        echo "Line $counter is a comment"
    else
        echo "Line $counter: '$line' is an invalid line"
    fi
    counter=$((counter+1))
    done<$i
done

However, when I feed it a text file with, for example the commands

run echo > temp.txt

It does not actually create a file called temp.txt, it just echoes "> temp.txt" back to the stdout. It also does a similar thing when I attempt to do something like

run program arguments > filename.txt

It does not put the output of the program in a file as I want, but it rather tries to treat the '>' as a file name.

I know this is a super specific and probably obvious thing, but I am very new to bash and all shell scripting.

Thanks

Most special characters are not processed when expanding variables. You need to use eval to do all the normal shell parsing. — Barmar
– Barmar, Commented Feb 4, 2017 at 18:39
eval executes a string just as if it were a normal shell command. — Barmar
– Barmar, Commented Feb 4, 2017 at 18:43
$@ needs to be enclosed in double quotes as "$@" to prevent word splitting and globbing. — codeforester
– codeforester, Commented Feb 4, 2017 at 18:53
Say one of the commands is an if statement that spans multiple lines. Is there a way to read it as one command instead of the while loop reading each line as a separate command? — Sean Morgan
– Sean Morgan, Commented Feb 4, 2017 at 19:42

Barmar · Accepted Answer · 2017-02-04 18:40:35Z

2

You need to use eval to do all the normal shell parsing of the variable:

eval "${line:4}"

You also don't need :${#line}. If you leave out the length, it defaults to the rest of the string.

answered Feb 4, 2017 at 18:40

Barmar

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4 Comments

Fred Over a year ago

It might be useful commenting on what eval will do with the quoting and potential expansions on its arguments.

Barmar Over a year ago

@Fred It just treats them exactly the same as if they were in the script itself.

Sean Morgan Over a year ago

I see, that makes sense. Thank you

Fred Over a year ago

@Barmar I was not really clear with my comment. The potential difficulty is when one (for any reason) tries to make up that string as part of a shell statement (i.e. a variable assignment). It can be a bit frustrating when you are not knowledgeable enough. It is not directly related to the question though.

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