Search the FAQ Archives

3 - A - B - C - D - E - F - G - H - I - J - K - L - M
N - O - P - Q - R - S - T - U - V - W - X - Y - Z
faqs.org - Internet FAQ Archives

Unix - Frequently Asked Questions (3/7) [Frequent posting]
Section - Why doesn't redirecting a loop work as intended? (Bourne shell)

( Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Single Page )
[ Usenet FAQs | Web FAQs | Documents | RFC Index | Counties ]


Top Document: Unix - Frequently Asked Questions (3/7) [Frequent posting]
Previous Document: How can a process detect if it's running in the background?
Next Document: How do I run ... interactive programs from a shell script ... ?
See reader questions & answers on this topic! - Help others by sharing your knowledge
3.8)  Why doesn't redirecting a loop work as intended?  (Bourne shell)

      Take the following example:

        foo=bar

        while read line
        do
                # do something with $line
                foo=bletch
        done < /etc/passwd

        echo "foo is now: $foo"

      Despite the assignment ``foo=bletch'' this will print
      ``foo is now: bar'' in many implementations of the Bourne shell.
      Why?  Because of the following, often undocumented, feature of
      historic Bourne shells: redirecting a control structure (such as
      a loop, or an ``if'' statement) causes a subshell to be created,
      in which the structure is executed; variables set in that
      subshell (like the ``foo=bletch'' assignment) don't affect the
      current shell, of course.

      The POSIX 1003.2 Shell and Tools Interface standardization
      committee forbids the behaviour described above, i.e. in P1003.2
      conformant Bourne shells the example will print ``foo is now:
      bletch''.

      In historic (and P1003.2 conformant) implementations you can use
      the following `trick' to get around the redirection problem:

        foo=bar

        # make file descriptor 9 a duplicate of file descriptor 0 (stdin);
        # then connect stdin to /etc/passwd; the original stdin is now
        # `remembered' in file descriptor 9; see dup(2) and sh(1)
        exec 9<&0 < /etc/passwd

        while read line
        do
                # do something with $line
                foo=bletch
        done

        # make stdin a duplicate of file descriptor 9, i.e. reconnect
        # it to the original stdin; then close file descriptor 9
        exec 0<&9 9<&-

        echo "foo is now: $foo"

      This should always print ``foo is now: bletch''.
      Right, take the next example:

        foo=bar

        echo bletch | read foo

        echo "foo is now: $foo"

      This will print ``foo is now: bar'' in many implementations,
      ``foo is now: bletch'' in some others.  Why?  Generally each part
      of a pipeline is run in a different subshell; in some
      implementations though, the last command in the pipeline is made
      an exception: if it is a builtin command like ``read'', the
      current shell will execute it, else another subshell is created.

      POSIX 1003.2 allows both behaviours so portable scripts cannot
      depend on any of them.

User Contributions:

Hello ... Im looking a lover..
I love oral sex! Write me - tinyurl.com/yz6aajf4
Try Your luck and win a FREE $500 or $1000 coupon! - tinyurl.com/yhxaqmng
Hi baby!! my name is Virginia...
I love oral sex! Write me - is.gd/be2piI
Try Your luck and win a Free Coca-Cola Pack! - tinyurl.com/y58phmar
WiFiBooster Pro provides you max speed with 300Mbps, no delay for playing game, online transfer files and video chat! Works with Any Router and Device (Smartphone, Computer, Smart tv, etc). Get 50% Off Today! Order Here - is.gd/OdxGOK
Fill out the form and win a Free $500 or $1000 voucher! - tinyurl.com/yf9odcpg
hey dear!!! my name is Sophia...
I want sex! Here are my photos - is.gd/kZlpA3
Hi !! my name Emily...
Do you want to see a beautiful female body? Here are my erotic photos - tinyurl.com/yzet8jvr
heey .. my name Rebecca!!!
I love oral sex! Write me - tinyurl.com/ydnrouzb
Hi !! my name Maria.
I love sex. Here are my erotic photos - is.gd/eP3qxP
Hi .. Im looking a man!!!
If you want to meet me, I'm here - is.gd/5kUUii
heey baby!!! my name Margaret..
I want sex! Write me - chilp.it/bf4d037
13
Rapinia
Hi...?! I'm my name is Name.FirstName and I have
A
Good opportunity for Target.Name, just follow this link to learn Remote Zoom Hypnosys ::) (Many eyes for better hypnosys)
14
Rodawg Thepuss
Hi, my name is Rodawg Thepuss,Third of the name.
I am here to present you a new and exciting opportunity to take advantage of others with Hypnosis through TeamsMEET.Hypnosis Hypnosis Hypnosis Hypnosis Hypnosis Hypnosis Hypnosis .

This is a record access board which can hold 30 seconds information about all the Hypnosis teams in Singapore. These teams have made numerous activities in the whole of Singapore, which has led to many mass hypnosiemicy readings.

This is the fastest way you can fast way of getting your classmates' goal on term project , S P MOAR!

so join Hypnosis Team in TeamMeets.net to become team's master key. On my life I swear by the many success stories i have shared above.

To be free of all problems, if you will join my Virtual Talk seminar. No one could ever stop you from achieving success. Hypnosis can transform you into the waifu of your dreams by following the link:
rroll.to/wxjWSC
15
Luc
Hello Folks,

For old Linux distributions I could suggest this web site - linux-distros.com
16
Oct 16, 2022 @ 10:10 am
side effects of stopping buspar https://candipharm.com/search?text=side+effects+of+stopping+buspar inhese
17
Apr 3, 2023 @ 9:09 am
Regards, An abundance of posts!
funny college essay https://dissertationwritingtops.com newsletter writing service https://essaywritingservicebbc.com
18
Apr 4, 2023 @ 2:02 am
You actually explained that well.
master thesis writer https://essaywritinghelperonline.com custom thesis writing service https://essaywritingservicebbc.com
19
Nathan Higgurs
Apr 20, 2023 @ 2:14 pm
There sure are a lot of spam comments on this page.
20
Sep 29, 2023 @ 3:03 am
hallo dear... Im looking a man.
I want sex! Write me - is.gd/vHiBd4
21
Reader
Oct 1, 2023 @ 4:04 am
Thanks for the command name lore.
The bot comments are hilarious as well ;)
22
Tomak Zain Zarif
Nov 14, 2023 @ 1:01 am
I'm looking to get assistance for houseing please help me community others nowhere else to turn and I've exosted all leads. I'm diabetic,with narapathy in my feet and hands.I'm a strong black man 46 years of age just had birthday Nov.3/which makes me a humbled Scorpio until you piss me off#1im a born and raises decent of Los Angle Cali. If you want to call or email me hit me up #1 @ tomakzarif65@gmail.com. I defianately need a woman in my life. Hit me soon#1
23
mbappe
Feb 14, 2025 @ 4:04 am
Oui c'est moi mbappe, envoi moi 50euros et je t'envoi une photo chaude ;)

N'hésite pas mon coquin
24
Bob McUnix
Apr 27, 2025 @ 3:03 am
Hi it's me, Bob McUnix I'm the maker of the Unix Operations Systematics.I approve of this blog post. I's currently stuck in $(island and I'm missing $100 to buy the last plane ticket.. PLeast help fast.

Comment about this article, ask questions, or add new information about this topic:




Top Document: Unix - Frequently Asked Questions (3/7) [Frequent posting]
Previous Document: How can a process detect if it's running in the background?
Next Document: How do I run ... interactive programs from a shell script ... ?

Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Single Page

[ Usenet FAQs | Web FAQs | Documents | RFC Index ]

Send corrections/additions to the FAQ Maintainer:
tmatimar@isgtec.com (Ted Timar)





Last Update March 27 2014 @ 02:12 PM