What do these lines of Unix/Linux do?

Question

I am a Unix/Linux shell script newbie and I have been asked to look at a script which contains the lines below. The following details in this question are vague but the person who wrote this code left no documentation and has since demised. Can anyone advise what they actually do?

There are two specific pieces of code. The first is simply line source polys.sh where polys.sh is a text file with contents:

failure="020o 040a"
success="002[a-d] 003[a-r] 004[a-s] 005[a-u]

Representing various parameters, I think, to do with the calculations the shell script performs. The nature of the calculations is, I am told, not important because the aim is to just get the script running.

The second piece of code is below and the relevant lines are delimited by Start and Stop comments. What I can tell you is that: $arg1 is blank, $opt1 is also blank, $poly is the path and name of a text file and ./search I believe to be a folder.

if [ $search == "yes" ]
    then
      # Search stage for squares containing zeros
      #

      # Start.

      output="$outputs/search/"`basename $poly` 
      ./search $opt1 $arg1 < $poly 2>&1 | tee $output
      if tail -n1 $output | grep -v "success"

      # End.

      then
        echo "SEARCH FAILURE" >> $output
        continue
      fi
      # Save approximations
      #
      echo -n "SEARCH SUCCESS " >> $output
      cat /tmp/iters >> $output
      cp /tmp/zeros $inputs/search/`basename $poly`
    else
      echo "No search"
    fi

Answer 1

EDIT Initial disclaimer as advised by Mr. Charles Duffy:

The below explanations assume you won't hit expansion-related bugs; please correct your code as advised by shellcheck.net to be assured that these explanations are correct

• source polys.sh includes the code from the script polys.sh , which is a file in the same folder as the file sourcing it (hence just the filename, without its path).

• Within that file:

   failure="020o 040a" success="002[ad] 003[ar] 004[as] 005[au]" 

  are two variable declarations; the variable $failure is set to "020o 040a" and $success to "002[ad] 003[ar] 004[as] 005[au]" . As the file was source d, these two variables are available in your script (do echo "$failure" and echo "$success" to see for yourself).

• output="$outputs/search/`basename $poly`" has two parts to explain:

   "$outputs/search/" 

  sets the variable $output to "$outputs/search/" , ie, to the value of the variable $outputs , appended by the string "/search/" .,

   `basename $poly` 

  anything in backticks is a command substitution, which interprets and runs the command returning its output, and the command basename $poly gets the base file or folder name from $poly , if it is a file path (eg, basename $poly for poly="/dev/file.txt" yields file.txt ); the output is appended as a string. to "$outputs/search/" .

• ./search $opt1 $arg1 < $poly 2>&1 | tee $output ./search $opt1 $arg1 < $poly 2>&1 | tee $output is two commands, separated by a pipe | :

   ./search $opt1 $arg1 < $poly 2>&1 

  runs the executable file ./search ( ./ is shorthand for the current script's directory) with two arguments, $opt1 and $opt2 variables. $poly is the variable name which should represent a file path, of which the file path has its content redirected to the command (using < ). The output of all errors ( stderr , as 2 ) is redirected ( > ) to the standard output ( stdout , or &2 , the ampersand represents this is a file descriptor, not a file path, otherwise it would redirect output to a file named 2 ).

   tee $output 

tee pipes outputs stdin to stdout and to arguments as file paths. So tee "/home/nick/output" would save the stdin to a file at "/home/nick/output" , as well as the stdout .

• if tail -n1 $output | grep -v "success"

   tail -n1 $output 

  gets the last line of the file at the "$output" variable's value.

   grep -v "success" 

  searches for any non-match ( -v inverts the match) in the last line from tail -n1 of "success" in a line (eg, if the last line is "fail" , it would pass the if statement as it does not contain "success" )