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/ dev / stdin with shelter

I need a Bash script that can accept input from a file or stdin, e.g. grepfor example

$ cat hw.txt
Hello world

$ grep wor hw.txt
Hello world

$ echo 'Hello world' | grep wor
Hello world

$ grep wor <<< 'Hello world'
Hello world

everything works beautifully. However with the following script

read b < "${1-/dev/stdin}"
echo $b

It fails if you use herestring

$ hw.sh hw.txt
Hello world

$ echo 'Hello world' | hw.sh
Hello world

$ hw.sh <<< 'Hello world'
/opt/a/hw.sh: line 1: /dev/stdin: No such file or directory
+5
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4 answers

Using /dev/stdinthis way can be problematic because you are trying to get the stdin descriptor using the name in the file system ( /dev/stdin), and not using the file descriptor that bash already passed to you as stdin (file descriptor 0).

Here is a small script to test:

#!/bin/bash

echo "INFO: Listing of /dev"
ls -al /dev/stdin

echo "INFO: Listing of /proc/self/fd"
ls -al /proc/self/fd

echo "INFO: Contents of /tmp/sh-thd*"
cat /tmp/sh-thd*

read b < "${1-/dev/stdin}"
echo "b: $b"

In my cygwin installation, this leads to the following:

./s <<< 'Hello world'


$ ./s <<< 'Hello world'
INFO: Listing of /dev
lrwxrwxrwx 1 austin None 15 Jan 23  2012 /dev/stdin -> /proc/self/fd/0
INFO: Listing of /proc/self/fd
total 0
dr-xr-xr-x 2 austin None 0 Mar 11 14:27 .
dr-xr-xr-x 3 austin None 0 Mar 11 14:27 ..
lrwxrwxrwx 1 austin None 0 Mar 11 14:27 0 -> /tmp/sh-thd-1362969584
lrwxrwxrwx 1 austin None 0 Mar 11 14:27 1 -> /dev/tty0
lrwxrwxrwx 1 austin None 0 Mar 11 14:27 2 -> /dev/tty0
lrwxrwxrwx 1 austin None 0 Mar 11 14:27 3 -> /proc/5736/fd
INFO: Contents of /tmp/sh-thd*
cat: /tmp/sh-thd*: No such file or directory
./s: line 12: /dev/stdin: No such file or directory
b:

, bash HERE (/tmp/sh-thd-1362969584) 0, stdin. , /dev/stdin. , 0, /dev/stdin.

Linux ./s script , , :

INFO: Listing of /dev
lrwxrwxrwx 1 root root 15 Mar 11 09:26 /dev/stdin -> /proc/self/fd/0
INFO: Listing of /proc/self/fd
total 0
dr-x------ 2 austin austin  0 Mar 11 14:30 .
dr-xr-xr-x 7 austin austin  0 Mar 11 14:30 ..
lr-x------ 1 austin austin 64 Mar 11 14:30 0 -> /tmp/sh-thd-1362965400 (deleted) <---- /dev/stdin not found
lrwx------ 1 austin austin 64 Mar 11 14:30 1 -> /dev/pts/12
lrwx------ 1 austin austin 64 Mar 11 14:30 2 -> /dev/pts/12
lr-x------ 1 austin austin 64 Mar 11 14:30 3 -> /proc/10659/fd
INFO: Contents of /tmp/sh-thd*
cat: /tmp/sh-thd*: No such file or directory
b: Hello world

script, stdin, , /dev/stdin.

if [ -n "$1" ]; then
    read b < "$1"
else
    read b
fi
+7

bash (, /dev/stdin), , . script #!/bin/bash , /dev/stdin , script /bin/sh, , , /dev/stdin .

( , , , Austin answer.)

+1
$ cat ts.sh 
read b < "${1-/dev/stdin}"
echo $b

$ ./ts.sh <<< 'hello world'
hello world

No problem for me. I am using bash 4.2.42 on Mac OS X.

0
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You have a typo here

read b < "${1-/dev/stdin}"

Try

read b < "${1:-/dev/stdin}"
-1
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