Shell – “Touching” standard out

Programs typically buffer their output for efficiency. That is, they accumulate output in a memory area (called a buffer), and they actually get the output out only when the buffer is full or at certain key points in the program. When the program ends normally, it flushes the output buffer (i.e. prints out any data that's left in it). When it segfaults, the content of the buffer is lost.

You don't observe this effect when running the program directly in a terminal because the behavior is different when the program's output is connected to a terminal (as opposed to a regular file or a pipe). In a terminal, the default behavior is to flush the buffer at the end of each line. Therefore you'll see every complete line that's produced up to the point when the program segfaults.

You can force the program to run in a terminal and collect its output. The simplest way is to run script. There are a number of annoyances that you'll need to work around:

• script adds a header line to the transcript file, which you'll need to remove afterwards.
• script doesn't return the status code of the command, so you'll need to save it somewhere if you want to know about the segfault or any other error.
• script will cause normal output and error out; you'd better save the error output to a separate file.

export FONT="foo"
script -q -c '
    ttf2afm "$FONT.ttf" 2>"$FONT.ttf2afm-err";
    echo $? >"$FONT.ttf2afm-status"
' "$FONT.ttf2afm-typescript"
tail -n +2 <"$FONT.ttf2afm-typescript" >"foo.afm"
rm "$FONT.ttf2afm-typescript"
if [ "$(cat "$FONT.ttf2afm-status")" -ne 0 ]; then
  echo 1>&2 "Warning: ttf2afm failed"
  cat "$FONT.ttf2afm-err"
fi

Try a script like this:

#!/bin/bash
echo "to fd1" >&1
echo "to fd2" >&2
echo "to screen" >$(tty)

When you call it, it looks like this:

user@host:~# ./script
to fd1
to fd2
to screen
user@host:~# ./script 1>/dev/null
to fd2
to screen
user@host:~# ./script 2>/dev/null
to fd1
to screen
user@host:~# ./script > /dev/null 2>&1
to screen