4

Here is my script:

echo "Defining aliases"
echo $$
echo "
s=screen
v=vlock -a
j=jobs
" | sed '/^$/d' | while read alias
do
  echo $$
  alias "$alias"
  alias | wc -l
done

And here is its output:

parthian@computer$ . aliases 
Defining aliases
3744
3744
4
3744
5
3744
6
parthian@computer$ ps -p $$
PID TTY          TIME CMD
3744 pts/1    00:00:03 bash
parthian@computer$ alias | wc -l
3

I'm flummoxed. This is some of the most straightforward code I've ever written, and it's not doing what it's supposed to do.

EDIT: When I change the echo lines from echo $$ to echo $$ $BASHPID, the second PID ($BASHPID) is different inside the loop, but $$ remains the same throughout. Could this be the issue?

Improve this question
2
  • And it can't be that I'm not making enough blood sacrifices to Stallman's beard, because I sacrificed my last oxen just yesterday. Commented Jun 24, 2015 at 23:33
  • 1
    By putting the alias definitions on the RHS of a pipe (... | while read ...), you are implicitly confining their scope to a subshell, I think: see mywiki.wooledge.org/BashFAQ/024 Commented Jun 24, 2015 at 23:44

1 Answer 1

Reset to default
4

Don’t believe everything a computer tells you.

$ ps -p $$
PID TTY          TIME CMD
3744 pts/1    00:00:03 bash
$ ps -t pts/1
PID TTY          TIME CMD
3744 pts/1    00:00:03 bash
4285 pts/1    00:00:00 ps
$ echo foo | while read x
do
        ps -t pts/1
done
PID TTY          TIME CMD
3744 pts/1    00:00:03 bash
4287 pts/1    00:00:00 bash
4288 pts/1    00:00:00 ps

$ echo "$$, $BASHPID"
3744, 3744
$ echo foo | while read x
do
        echo "$$, $BASHPID"
        ps -t pts/1
done
3744, 4292
PID TTY          TIME CMD
3744 pts/1    00:00:03 bash
4292 pts/1    00:00:00 bash
4293 pts/1    00:00:00 ps

When you do some_command | while …, the shell puts the while loop into a subshell, but $$ continues to report the PID of the main (parent) process.  In bash, $BASHPID reveals the truth.

Consider doing

while read alias
do
  alias "$alias"
done << EOF
s=screen
v=vlock -a
j=jobs
EOF
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3
  • Arrghhh.  I didn't see your edit until I had finished typing my answer.  Hope it helps anyway. Commented Jun 25, 2015 at 0:02
  • It does. Seems silly, but it hadn't occurred to me to use a heredoc- thanks! Commented Jun 25, 2015 at 0:16
  • 1
    It is also mentioned in bash man page: "[$]$ Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the current shell, not the subshell." May be it also applies to subshells created for pipes Commented Jun 25, 2015 at 1:12

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