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While loop stops reading after the first line in Bash
(5 answers)
Closed 2 years ago.
I made a bash script for my personal usage which sets up selenium webdriver with appropriate options . Here is its raw link - https://del.dog/raw/edivamubos
If i execute this script using curl after writing it to a file first like..
curl https://del.dog/raw/edivamubos -o test.sh && \
chmod u+x test.sh && \
bash test.sh
The script works perfectly as its intended to work
But usually i like to execute scripts directly using curl , so when i do..
curl https://del.dog/raw/edivamubos | bash
The script works very weirdly , it keeps repeating line 22,23 and 29 infinitely on loop. i couldnt beleive it as first so i tested this 3,4 times and can confirm it.
Now
what is the reason for same script acting differently in both cases ?
How do i fix it ( ie make it work correctly even after executing it directly without writing to a file )
Edit -
If someone want they can quickly test this in google colab ( in case someone intending to test but don't want to install any packages locally ) . I am mentioning this thing because you won't be able to reproduce this properly in any bash IDE.
When you pipe the script to bash, this command (line 24):
read -p "Enter your input : " input
reads the next line (i.e. line 25, case $input in) because bash's stdin is connected to curl's stdout, and read reads from the same descriptor as bash.
To avoid that, the developer can change the script so that all input is read from /dev/tty (i.e. the controlling terminal). E.g.:
read -p 'prompt' input </dev/tty
Or the user can use one of the below, so that read reads from the terminal, not the descriptor it was read from.
bash -c "$(curl link)"
bash <(curl link)
Related
I am logging into a remote server using SSH client. I have written a script that will execute two commands on the server.But, as the first command executes a bash script that calls "bash" command at the end. This results in execution of only one command not the other.
I cannot edit the first script to comment or remove the bash call.
i have written following script:
abc.sh
#!/bin/bash
command1="sudo -u user_abc -H /abc/xyz/start_shell.sh"
command2=".try1.sh"
$command1 && $command2
Only command 1 is getting executed not the second as the "bash" call is creating a new process the second command is not executing.
Solution 1
Since you can execute start_shell.sh you must have read permissions. Therefore, you could copy the script, modify it such that it doesn't call bash anymore, and execute the modified version.
I think this would be the best solution. If you really really really have to use start_shell.sh as is, then you could try one of the following solutions.
Solution 2
Try closing stdin using <&-. An interactive bash session will exit immediately if there is no stdin.
sudo -u user_abc -H /abc/xyz/start_shell.sh <&-; ./try1.sh
Solution 3
Change the order if both commands are independent.
./try1.sh; sudo -u user_abc -H /abc/xyz/start_shell.sh
Specifics:
I'm trying to build a bash script which needs to do a couple of things.
Firstly, it needs to run a third party script that I cannot manipulate. This script will build a project and then start a node server which outputs data to the terminal continually. This process needs to continue indefinitely so I can't have any exit codes.
Secondly, I need to wait for a specific line of output from the first script, namely 'Started your app.'.
Once that line has been output to the terminal, I need to launch a separate set of commands, either from another subscript or from an if or while block, which will change a few lines of code in the project that was built by the first script to resolve some dependencies for a later step.
So, how can I capture the output of the first subscript and use that to run another set of commands when a particular line is output to the terminal, all while allowing the first script to run in the terminal, and without using timers and without creating a huge file from the output of subscript1 as it will run indefinitely?
Pseudo-code:
#!/usr/bin/env bash
# This script needs to stay running & will output to the terminal (at some point)
# a string that we need to wait/watch for to launch subscript2
sh subscript1
# This can't run until subscript1 has output a particular string to the terminal
# This could be another script, or an if or while block
sh subscript2
I have been beating my head against my desk for hours trying to get this to work. Any help would be appreciated!
I think this is a bad idea — much better to have subscript1 changed to be automation-friendly — but in theory you can write:
sh subscript1 \
| {
while IFS= read -r line ; do
printf '%s\n' "$line"
if [[ "$line" = 'Started your app.' ]] ; then
sh subscript2 &
break
fi
done
cat
}
I'm coding a Bash script to automate tasks across multiple servers.
I am logging to a Centos 7 machine over SSH to run some editor (nano, vi, ...)
ssh -tt centos#... '/bb/Conf edit'
The /bb/Conf edit is basically just vi /bb/conf.yaml.
When I run the SSH command from my shell, it works fine. However, when the same SSH command is ran from a Bash script inside a while read ...; do loop, the editor has wrong size (80x40 I guess) and seems to ignore the keys I press - i.e. in nano, Ctrl+x doesn't do anything. The only key that works is Ctrl+c which closes the connection.
I thought this is something related to the TERM variable, as per this, so I tried to add export TERM=xterm or TERM=rxvt to /bb/Conf or the place calling the SSH. The variable is in fact set in the target environment (I've tried echo $TERM right before vi). But the terminal still misbehaved.
Then I have tried to put just that single command ssh ... to a new script. When running that, the editor worked fine.
After a while I found out that it works outside a while read loop, but not inside. I assume that the editors do some stdin/stdout magic and then read somehow breaks that.
Is there a way to run an editor like vi or nano from within a loop?
(The purpose in my case is to allow the users to edit files on multiple servers.)
That's because both read and ssh are reading from the same input stream. The solution is to use a different file descriptor for the while read loop:
while IFS= read -r -u3 line; do
ssh ...
done 3< file
Here, we're using file descriptor 3 instead of stdin.
Lengthy pipelines can be hard to read and maintain, but you can use whitespace constructively: newlines are allowed following | and && and ||. Also, parentheses introduce a subshell which contains an arbitrary script, so indentation helps.
while read -u3 line; do
: do stuff here that needs to read from stdin
done 3< <(
command 1 of the pipeline |
command 2 |
command 3
)
That's clean and readable. The downside is that it puts the last part of the pipeline (the while loop) first, so the code kind of flows backwards.
Content of my remote file:
#!/bin/sh
read foo && echo "$foo"
What I’m doing locally in my terminal:
$ curl -fLSs http://example.com/my-remote-file.sh | sh
What I’m expecting:
The downloaded script should prompt the user to enter something and wait for the users input.
What actually happens:
The downloaded script skips the read command and continues executing.
Question:
How can I prompt the user to input something from a script that is downloaded via curl? I know that sh <(curl -fLSs http://example.com/my-remote-file.sh) is working, but this is not POSIX compliant and I’d like to achieve that.
The problem is that stdin is reading from curl, rather than the keyboard. You need to change your script to instruct it to read specifically from the terminal, like this:
read input < /dev/tty
echo $input
I'm creating a script that I want people to run with
curl -sSL install.domain.com | bash
As RVM, oh-my-zsh and many others does,
However, I'm having issues because my script is interactive (it uses read and select, and the user is not being prompted, the script just skip those steps as is being executed with a |.
I've tried adding {} to my whole code.
I was thinking in ask the script to download himself again and put in tmp folder, and execute from there, not sure if that will work.
You can explicitly tell read to read from the terminal using
read var < /dev/tty
The solution i found is ask user to run it like:
bash <( curl -sSL install.domain.com )
This way script is passed as an argument and standard input remains untouched.
This is a non-problem, as users should not be executing code directly from the stream. The user should be downloading the code to a file first, then verifying that the script they receive has, for example, an MD5 hash that matches the hash you provide on your website. Only after confirming that the script they receive is the script you sent should they execute it.
$ curl -sSL install.domain.com > installer.sh
$ md5sum installer.bash # Does the output match the hash posted on install.domain.com?
$ bash installer.bash