Learning scripting -- functions capturing output

Shell Programming and Scripting
1

Hi, everyone! First post here. I just assembled my first Linux computer and I'm trying to teach myself shell scripting. (Xubuntu and bash, respectively, but I'm not tied to either.)

I'm writing a program to deal with different kinds of files (to be executed from a text editor -- LaTeX'ing a .tex, compiling a .c or .cpp, etc.). This is partially because I want to have those functions, but also as a learning exercise.

To avoid redundancy I'd like to collect the handling for each type of file together, regardless of how I determined its type (by extension, header, or some other method). In a Windows batch file I'd use GOTO:

IF "%1"=="help" GOTO Help
. . .
:Help
. . .
GOTO end
. . .
:end

but it looks like there's no goto in bash.* At first it seemed simple: shell scripts support functions, so just make a function for each:

function Help {
. . .
}
. . .
if [ "$1" = "help" ]; then
  Help
. . .

but functions capture whatever I echo.

  1. If I wanted to implement this with functions, how can I get around the capturing? I'd guess that one of the redirection commands would work -- but I've had no luck so far. (It's only my first day, be gentle with me here!)
  2. Subjectively, what's the 'right' or 'best' way to do what I want? Functions seem somewhat heavyweight when in many cases I'll just want to do a single line.
  • Yes, I'm familiar with Dijkstra's diatribe.
2

A function that includes echoes should still echo to the terminal.
Yes for one liners functions are no normally used, for your example a case statement might be more elegant, e.g.:

case "$1" in
  "help")
     echo "This is how you use this script, valid options are help and doit."
  ;;
  "doit")
      echo "Do your stuff here"
  ;;
  *)
      echo "You did not supply a valid option, try: $0 help."
      exit 99
  ;;
esac

If you wanted to display the help info from "help" and * then that would benefit from a function.

HTH

3

I experimented around a bit. They do echo to stdout if they don't have an exit value:

function foo {
    echo -n hi 

    # Uncomment this line to change the output from "hi again hi " to "again ".
    #exit 0
}
foo
echo again 
foo

but then I can't use them to pass along success/failure information. Right now the relevant part of the script would be something like this (short version, in function form):

recognized=1;

exec_c () {
  gcc -O3 "$file" -o"$tfile"
}
exec_png () {
  gpicview "$file"
}
cleanup () {
  echo All done!
}

case "$full_ext" in
  "gp.c")
    gp2c-run "$file"
  ;; *)
    recognized=0
esac
exit_status=$?
if [ recognized -eq 1 ]; then
  cleanup
  exit $exit_status
fi

recognized=1
case "$ext" in
  "c" | "cpp")
    exec_c
  ;; "png")
    exec_png
  ;; *)
    recognized=0
  ;;
esac
 exit_status=$?

if [ recognized -eq 1 ]; then
  cleanup
  exit $exit_status
else
     head4=`head -c4 "$file"`
  png=`echo -e '\0211PNG'`

  if [ "$head4" = "$png" ]; then
    exec_png
  fi
fi
4

Functions will echo as long as there is some command inside them which writes to stdout. If you want to suppress that, you can redirect output to /dev/null. It has nothing to do with exit code.

You can use "return <exit_code>" to pass around success/failure information. Using "exit <exit_code>" will quit the script altogether. Once you return, you can check $? to check for success/failure.

5

I don't want to suppress output, I want to show it.

I'll try that, thanks. But as you can see (try my example), exit does not exit the script, just the function. (I'm learning by doing; I never would have guessed that it would work that way.)

6

There are no exit commands in the three functions in your example. You are running exit from main body.

7

First example, second comment. You're referring to my code snippet, I'm talking about my short testcase.

8

Short testcase:

$ cat foo
#!/usr/bin/bash

function foo {
    echo -n hi

    # Uncomment this line to change the output from "hi again hi " to "again ".
    exit 0
}
foo
echo again
foo
$
$ foo
hi$
$

??

9

OK, I have a working version now. It's not very polished but hey -- it's my first script! It took me about three hours, but there was a lot of reading in that time.

10

Hmm, you're right. I had a different version (more complicated) that worked as I described, but I deleted that an hour ago as I found better ways to do things. But I'm not crazy -- I did test whether the script was exiting or not.

I'll chalk that up to 'bizarre things happen some times' or something. :eek: