[SOLVED] bash redirection & other

doru · 01-12-2018, 08:01 AM

My GNU bash, version 4.2.46(2)-release (x86_64-redhat-linux-gnu) manual and the bash itself puzzle me on a number of issues.

REDIRECTION

Code:

$ : 7<test.txt # does nothing, it should have created /dev/fd/7 
$ ll /dev/fd/
total 0
lrwx------. 1 user user 64 Jan 12 15:16 0 -> /dev/pts/2
lrwx------. 1 user user 64 Jan 12 15:16 1 -> /dev/pts/2
lrwx------. 1 user user 64 Jan 12 15:16 2 -> /dev/pts/2
lr-x------. 1 user user 64 Jan 12 15:16 3 -> /proc/3401/fd
$ : {my}<test.txt # works, this was supposed to be a special case of the above 
$ ll /dev/fd/
total 0
lrwx------. 1 user user 64 Jan 12 15:16 0 -> /dev/pts/2
lrwx------. 1 user user 64 Jan 12 15:16 1 -> /dev/pts/2
lr-x------. 1 user user 64 Jan 12 15:16 10 -> /home/user/tmp/test.txt
lrwx------. 1 user user 64 Jan 12 15:16 2 -> /dev/pts/2
lr-x------. 1 user user 64 Jan 12 15:16 3 -> /proc/3402/fd
$ : 10<&- # does nothing, it was supposed to delete /dev/fd/10
$ ll /dev/fd/
total 0
lrwx------. 1 user user 64 Jan 12 15:17 0 -> /dev/pts/2
lrwx------. 1 user user 64 Jan 12 15:17 1 -> /dev/pts/2
lr-x------. 1 user user 64 Jan 12 15:17 10 -> /home/user/tmp/test.txt
lrwx------. 1 user user 64 Jan 12 15:17 2 -> /dev/pts/2
lr-x------. 1 user user 64 Jan 12 15:17 3 -> /proc/3407/fd
$ : 11<&10- # deletes 10 as long as 11 does not exist, it was supposed to move 10 in 11 
$ ll /dev/fd/
total 0
lrwx------. 1 user user 64 Jan 12 15:17 0 -> /dev/pts/2
lrwx------. 1 user user 64 Jan 12 15:17 1 -> /dev/pts/2
lrwx------. 1 user user 64 Jan 12 15:17 2 -> /dev/pts/2
lr-x------. 1 user user 64 Jan 12 15:17 3 -> /proc/3408/fd

Simple Commands

Code:

$ a=1 echo $a # returns an empty line instead of '1'

Arrays

"Any element of an array may be referenced using ${name[subscript]}. The braces are required to avoid conflicts
with pathname expansion. "
Braces are required to include [subscript] in name. Conflicts with pathname expansion do not appear:

Code:

$ echo $my[1]
[1]
$ touch my1
$ echo $my[1]
[1]

Shell Variables

How can I verify the content of IFS? I'd like to see it.

pan64 · 01-12-2018, 08:17 AM

what do you really want now? are these statements, or issues, do you want to ask something or ???
see http://www.tldp.org/LDP/abs/html/arrays.html about arrays (do not forget about { } )!
about redirection:
http://www.tldp.org/LDP/abs/html/io-redirection.html
http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO-3.html
about IFS probably:
echo "${IFS}" | od -xc

doru · 01-12-2018, 10:30 AM

Why does it not work as it was supposed to? Do I misread the manual? Is the manual inconsistent with the actual program?
How do I perform those tasks? How do I create a redirect to a file descriptor I choose, not to a file descriptor chosen by bash, bigger or equal to 10?
How do I close a file descriptor without using some number of a file descriptor which does not exist?
How do I duplicate a file descriptor in this version of bash?

I understand that bash can not be modified for backward compatibility reasons, but the manual certainly can be modified.

pan64 · 01-12-2018, 10:36 AM

which documentation/manual do you use?

doru · 01-12-2018, 10:37 AM

Your links lead to much clearer explanations than my bash manual, but still they do not clarify my questions. Maybe the features I can not find have just been removed from my bash?! That would be very bad for portability. And n<&- still does not work, as I show above. Does it work on your system? Is my example correct? I mean, was it supposed to work as expected? Maybe the file descriptor is closed but I still see it? No, it still works.

doru · 01-12-2018, 10:39 AM

Bash is Copyright (C) 1989-2011 by the Free Software Foundation, Inc.
GNU Bash-4.2 2010 December 28

It came with bash on Centos 7.

Thank you for your help.

Please try those commands on your version.

MadeInGermany · 01-12-2018, 11:16 AM

Regarding the /dev/fd/, this is highly dependend of the kernel version what happens there.
What matters is that

Code:

: 7<test.txt

has really temporary associated a descriptor 7 with the file test.txt.
That this is implemented by /dev/fd/7 is an assumption (mostly true), and that /dev/fd/7 remains is another assumption.

Code:

a=1 echo $a

does not work because $a is evaluated too early.
Also this is a temporary assignment.
The following works

Code:

a=1 sh
$ echo $a
$ exit

Or

Code:

a=1 sh -c 'echo $a'

Another demonstration

Code:

a=2 # the local shell variable
a=1 sh -c 'echo $a'
1
a=1 sh -c "echo $a"
2

Even within " " the shell substitutes $a with 2, then forks, the forked shell sets a=1 in its environment, then execs to sh -c ... and the sh prints the 2.
But in ' ' the shell does not substitute $a so the sh takes it from its environment.
Array handling denotes braces.

Code:

echo ${my[1]}

is the second index (starts with 0).
A demonstration

Code:

my=( first second )
echo "${my[1]} ${my[0]}"
echo "my has ${#my} elements, that are: ${my[@]}"

pan64 · 01-12-2018, 11:21 AM

if I understand well something like 7<test.txt is not specified at all, no such thing documented. That's why I asked where is it from?

MadeInGermany · 01-12-2018, 11:29 AM

For viewing IFS I would redirect it to a file and view it with vi

Code:

$ echo "$IFS" > IFS
$ vi IFS
:set list

The standard IFS consists of

a space character
a tab character
a newline character

rknichols · 01-12-2018, 12:53 PM

Quote:

Originally Posted by pan64

if I understand well something like 7<test.txt is not specified at all, no such thing documented. That's why I asked where is it from?

It runs a command with file descriptor 7 redirected from the named file. In the case of the ":" (no-op) command, or any other command that is not going to read from FD 7, it is pointless. If you want that redirection to be available for the rest of the current script, you use the "exec" command:

Code:

exec 7<test.txt

Quote from the bash manpage:

"Redirecting Input

Redirection of input causes the file whose name results from the expansion of word to be opened for reading on file descriptor n, or the standard input (file descriptor 0) if n is not specified.

The general format for redirecting input is:

[n]<word
"

pan64 · 01-12-2018, 02:14 PM

so the documentation I read was too old.

doru · 01-13-2018, 09:13 AM

Quote:

Originally Posted by MadeInGermany

Regarding the /dev/fd/, this is highly dependend of the kernel version what happens there.
What matters is that

Code:

: 7<test.txt

has really temporary associated a descriptor 7 with the file test.txt.
That this is implemented by /dev/fd/7 is an assumption (mostly true), and that /dev/fd/7 remains is another assumption.

Thank you very much, this is wonderful:

Code:

$ { ll /dev/fd/; ll /dev/fd/ 7<&-; ll /dev/fd/; } 7<test.txt
total 0
lrwx------. 1 safe safe 64 Jan 13 17:05 0 -> /dev/pts/1
lrwx------. 1 safe safe 64 Jan 13 17:05 1 -> /dev/pts/1
lrwx------. 1 safe safe 64 Jan 13 17:05 2 -> /dev/pts/1
lr-x------. 1 safe safe 64 Jan 13 17:05 3 -> /proc/4000/fd
lr-x------. 1 safe safe 64 Jan 13 17:05 7 -> /home/safe/tmp/test.txt
total 0
lrwx------. 1 safe safe 64 Jan 13 17:05 0 -> /dev/pts/1
lrwx------. 1 safe safe 64 Jan 13 17:05 1 -> /dev/pts/1
lrwx------. 1 safe safe 64 Jan 13 17:05 2 -> /dev/pts/1
lr-x------. 1 safe safe 64 Jan 13 17:05 3 -> /proc/4001/fd
total 0
lrwx------. 1 safe safe 64 Jan 13 17:05 0 -> /dev/pts/1
lrwx------. 1 safe safe 64 Jan 13 17:05 1 -> /dev/pts/1
lrwx------. 1 safe safe 64 Jan 13 17:05 2 -> /dev/pts/1
lr-x------. 1 safe safe 64 Jan 13 17:05 3 -> /proc/4002/fd
lr-x------. 1 safe safe 64 Jan 13 17:05 7 -> /home/safe/tmp/test.txt

All your answers are great!

doru · 01-13-2018, 09:28 AM

Quote:

Originally Posted by MadeInGermany

Code:

echo ${my[1]}

is the second index (starts with 0).
A demonstration

Code:

my=( first second )
echo "${my[1]} ${my[0]}"
echo "my has ${#my} elements, that are: ${my[@]}"

${#my} returns the number of characters in my[0], ${#my[*]} returns the number of elements in my.

MadeInGermany · 01-15-2018, 08:06 AM

Thanks for correcting my mistake.
Indeed the #elements is ${#my[*]} or ${#my[@]}.