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How to run read after a pipe in Bash?
In this case, read is running within our primary shell, and our output-producing command runs in the subshell. The < (…) syntax creates a subshell and connects its output to a pipe, which we redirect into the input of read with the ordinary < operation. Because read ran in our main shell the variables are set correctly.
Why does read die at the end of a shell in Bash?
bash runs the right-hand side of a pipeline in a subshell context, so changes to variables (which is what read does) are not preserved — they die when the subshell does, at the end of the command. Instead, you can use process substitution: In this case, read is running within our primary shell, and our output-producing command runs in the subshell.
Is there a bug with Bash and ksh?
This is not a bash bug as POSIX allows both bash and ksh behaviors, leading to the unfortunate discrepancy you are observing. Additionally, each command of a multi-command pipeline is in a subshell environment; as an extension, however, any or all commands in a pipeline may be executed in the current environment.
Where did the term ” pipes ” come from?
According to Dennis Ritche’s article Prophetic Petroglyphs, pipes originated from a 1964 internal memo by Malcolm Douglas McIlroy, at the time when they were working on Multics operating system. Quote:
How does the read builtin in Bash work?
The read builtin reads one line of data (text, user input, …) from standard input or a supplied filedescriptor number into one or more variables named by . Since Bash 4.3-alpha, read skips any NUL ( ASCII code 0) characters in input.
Why is the last line missing in my bash script?
We had a bug where, when run via the automated process that sequences different scripts together, the last line wasn’t being seen. The code used to iterate over the lines in the file (name stored in DATAFILE was We could run the script from the command line and it would see every line in the file, including the last one, just fine.
How to read command line arguments from a pipe?
A smart script that can both read data from PIPE and command line arguments: Explanation: When a script receives any data via pipe, then the /dev/stdin (or /proc/self/fd/0) will be a symlink to a pipe. The bash [ [ -p option can check it it is a pipe or not.
What happens when you pipe data into a shell script?
In your case, your script provides its standard input for each command that it runs. A simple example script: Piping data into your shell script causes cat to read that data, since cat inherits its standard input from your script.
Where does a variable occur in a pipe in Bash?
By default, and on Bash v4.1- invariably, any variable creations / modifications in a (multi-segment) pipeline happen in a subshell, so that the result will not be visible to the calling shell.
Why do I put the command read into a subshell while using pipeline?
However, now comes the subshell issue: commands in a pipeline are run in a subshell, so setting a variable in them doesn’t affect the parent shell. So the echo command has to be in the same subshell as the read s. So: df . | { read a; read a b; echo “$a”; }.