Questions tagged [subshell]
The subshell tag has no usage guidance.
199
questions
10
votes
1
answer
582
views
Different behaviour of subshells and && in bash vs zsh
My understanding of running commands in subshells is that the current shell is forked, and the subshell then does a further fork and exec on the desired commands.
I see strange behaviour when running ...
- 101
1
vote
0
answers
5
views
Queue interactive commands and attend to then in sequence
I want to have a script that adds commands to some kind of queue. I then want to run those commands interactively one at a time. More commands should be able to be added to the queue while I'm ...
- 11
0
votes
1
answer
34
views
How to pass more than one argument from `find -exec` to a `bash -c` sub-shell?
Is there a way to pass more than one argument from find -exec to a bash -c sub-shell?
Given this working invocation which searches all videos matching a glob pattern (eg "*dogs.mp4) and then ...
- 1,425
7
votes
1
answer
478
views
Why wait in this script is not executed after all subshells?
In this script, that pulls all git repositories:
#!/bin/bash
find / -type d -name .git 2>/dev/null |
while read gitFolder; do
if [[ $gitFolder == *"/Temp/"* ]]; then
...
- 199
0
votes
0
answers
29
views
How to know all subshells are executed and finished? [duplicate]
I have this script to pull all of the git repositories on my machine:
#!/bin/bash
find / -type d -name .git 2>&- |
while read gitFolder; do
if [[ $gitFolder == *"/Temp/"* ]]; ...
- 199
0
votes
0
answers
31
views
How to clear the terminal in script while restoring entered commands after script is finished?
I'm currently writing a CLI file manager written in POSIX Shell.
I want a cleared screen while running the script to not distract the user while browsing through the files.
After the user quits the ...
- 168
1
vote
1
answer
43
views
Subshell (subprocess/child process) with its own $BASHPID not listed by `ps`
I don't understand the following:
mkfifo p;
$ (>p ps -f | >>p echo $BASHPID) &
[1] 983527
$ cat p
983529
PID PPID C STIME TTY TIME CMD
981815 165343 0 19:57 pts/27 00:...
- 234
0
votes
2
answers
145
views
Example of subshell without child-shell process
I just recently learned that "subshell" is not the same as "child shell process" (see for example What is the exact difference between a "subshell" and a "child ...
- 234
1
vote
1
answer
111
views
Setting a trap for INT doesn't work in a subshell
$ bash -c "trap \"echo INT\" INT; sleep 3" & pid=$!; sleep 1; kill -INT $pid; wait
[1] 27811
INT
[1]+ Done bash -c "trap \"echo INT\" INT; ...
- 2,893
0
votes
2
answers
141
views
bash: assign to variable using a subshell, but exit main script if assignment fails
I'd like to have a bash script that has allows me to call a function to see if some data is contained in a file, and if it isn't fail the main script, kind of like the following, which is simplified ...
- 1,327
1
vote
2
answers
149
views
Capture stdout of a crashing program and shell's "segfault error" message
I have a compiled program main.bin that is supposed to print:
hello world
another line
This program generates a segmentation fault right after printing the first line.
If I run it in the shell (zsh), ...
- 303
0
votes
0
answers
20
views
Is there any difference between bash -c "command" vs. (command)? [duplicate]
bash -c runs a command in a subshell:
bash -c "cd /tmp && ls"
So does enclosing the command in parentheses:
(cd /tmp && ls)
Are these two techniques identical? If not, how ...
- 484
1
vote
1
answer
65
views
Killing `sleep` in a subshell
In bash, I'm observing that calling kill on a sleeping process won't kill it if I start out with a subshell. Can you help me understand why?
function foo () {
# Start a loop with `sleep` in the ...
- 347
0
votes
1
answer
271
views
Parenthesis and curly brackets not working as expected in cron
I have set the following crontab for the root user:
root@curie:/usr/home/username # crontab -l
# /etc/crontab - root's crontab for FreeBSD
#
# $FreeBSD: release/10.0.0/en_US.ISO8859-1/books/handbook/...
- 11
0
votes
2
answers
120
views
Difference between $0 and $SHELL?
I am currently playing CTF in OverTheWire website . I was stuck on 32nd Level [ https://overthewire.org/wargames/bandit/bandit33.html ] to get password for next Level . Command to access Level 32 - ...
- 9
1
vote
1
answer
89
views
zsh: Halt when command not found
I want to stop the zsh script running when it encounters a command not found error.
I know of command_not_found_handler, but this function only allows me to print a message, not halt the parent shell.
...
- 1,287
0
votes
0
answers
19
views
Only execute subshell script if column 2 in Input.csv has value > 0
I have a long linux shell script (.sh) that has some R subshells. I only want to execute a subshell WEurope.sh if column 2 value in Input.csv has a value >0.
Here is an example of Input.csv ...
- 1
0
votes
2
answers
77
views
How to get caller function call method in bash
Based on the following code below, is it possible to obtain the value of $caller_method from the pseudocode function below whether a function's caller has made a function call normally eg: mytest 1 or ...
- 335
0
votes
1
answer
148
views
How can a subshell return data in a variable (not file) and output distinct logs at the same time?
Say I've got the following javascript:
const something = doSomething();
function doSomething() {
console.log("About to do something");
const a = ...
0
votes
2
answers
1k
views
continue: only meaningful in a `for', `while', or `until' loop
I have a loop that checks for certain criteria for whether or not to skip to the next iteration (A). I realized that if I invoke a function (skip) that calls continue, it is as if it is called in a ...
- 479
1
vote
0
answers
44
views
How to return a value from a job?
I want to spin up some jobs within a script to do some work in parallel. I will know when the work is done by counting the number of active jobs. The issue I'm having is that I want to also return a ...
- 2,145
0
votes
0
answers
89
views
How to display banner text inside the subshell along with the results of whatever is being run in subshell
I'm trying to figure out how to display text inside a subshell along with the results of whatever is being run in the subshell, i.e: test=$(sudo echo foo)
I've already set up a lecture_file so that a ...
1
vote
2
answers
50
views
Check if the script was already run in the current shell session
I have a script that's interactive if started without arguments, and I'd like it to print an ASCII-art logo the first time it's run in the current shell session. Is this possible to detect somehow?
I ...
- 233
0
votes
0
answers
335
views
Capturing output of sudo commands run over ssh (without password prompt)?
If I run this command, the output is what I expect, listing the directory /root on my server.
~ bash -c "ssh -t myserver 'sudo ls /root'"
Enter passphrase for key '/home/james/.ssh/ed25519':...
- 629
0
votes
1
answer
246
views
How are sudo permissions granted across parent/children processes?
I was having a look at how sudo grants work across parent/children processes, however, I'm confused.
If I open a terminal (emulator, in a graphic environment), and execute:
$ sudo bash -c "sudo -...
- 778
0
votes
0
answers
101
views
Does ( ) really spawns a subshell? [duplicate]
I saw this video which explains that when running a command in parentheses it actually runs the command in a subshell, which is a child process of the original shell. Running the following experiment:
...
- 264
1
vote
0
answers
39
views
How to capture the output of a Python program running in a subshell? [duplicate]
I'm trying to run a python program at startup by putting the following lines into etc/rc.local:
(cd /home/pi/CODE/weather-project/weather-station &&
python main.py) &>> log.txt &
...
- 111
2
votes
0
answers
76
views
Is the log-in shell the parent of shells in gnome terminal?
I have somehow thought that all shells that are invoked by a terminal emulator or any other way while a user is logged in are subshells of the log-in shell. Is this correct?
If it matters, I am ...
- 153
0
votes
1
answer
553
views
Launching a program via `xdg-open` from a subshell without blocking
I've noticed that calling xdg-open from a subshell will reliably block until the launched process is closed. I suspect there may be a reason for this, but I'm not sure as to why. For example, ...
- 186
4
votes
3
answers
632
views
Why must I put the command read into a subshell while using pipeline [duplicate]
The command df . can show us which device we are on. For example,
me@ubuntu1804:~$ df .
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sdb1 61664044 8510340 49991644 15% /home
...
- 2,809
1
vote
1
answer
844
views
How to continue running the script when a script called in the first script exited with an error code?
I am trying to write a bash script that will run a second script repeatedly until it fails and capture the stdout and stderr of that second script. I have managed to deal with most problems ...
- 13
0
votes
0
answers
108
views
Why source same script twice is exiting the shell?
I have written a bash script to create a git worktree- cr_wt
#!/bin/bash
unset CDET_ID
unset WT_DIR
source ~/.bashrc
usage()
{
echo "Usage: $0 [ -c CDET_ID ] [ -d Directory Name ]"
...
- 205
1
vote
0
answers
266
views
In bash, can I create an anonymous read-write pipe to a subshell?
I'm trying to pass a "resolution service" to a bash script as read-write file descriptor attached to another script.
So given a "service" like capitalize:
#!/usr/bin/env bash
while ...
- 233
1
vote
2
answers
216
views
Isn't a subshell created when I run the `bash` command? [duplicate]
Isn't a subshell created when I run the bash command? For example, after executing bash, I cannot access the value of a non-exported variable. In this case, is the environment I switch to with the ...
- 1,072
1
vote
2
answers
132
views
Ordering output when two subshells write to stdout
I have a command of the below form
input | tee >(subshell) | mainshell
Both subshell and mainshell writes to stdout. So their output is not synchronised. E.g.
echo "Hello\nWorld" | tee &...
- 4,039
1
vote
1
answer
138
views
PID of a nested command in a subshell
Okay, I've got a program I'm trying to fix, for lengthy reasons a rewrite isn't the greatest option... So, I've been up for 8 hours trying to solve this and I'm completely stumped.
I'm trying to get ...
- 11
1
vote
0
answers
66
views
`set -e` doesn't abort the script when a sub-shell fails as part of `export` [duplicate]
When running the following script:
#!/bin/bash
set -e
export X=$(false)
echo That did not abort
X=$(false)
echo That did abort
It outputs:
That did not abort
So export X= doesn't abort the execution,...
- 111
0
votes
2
answers
58
views
Another called script not going into background even given &
I have 2 scripts.
I'm calling my script2 as sh scriptpath/script2.sh & inside script1
which compresses files using a combination of find, xargs and gzip commands on 16 files at a time. It is ...
- 5
1
vote
1
answer
201
views
What is the difference between executing in subshells vs using & vs coproc command?
I was recently reading about subshells in linux. An example used was sleep 10.
(sleep 10) - subshell
sleep 10& - background process
coproc sleep 10
It was written that the first command ...
- 13
4
votes
1
answer
65
views
Can I determine if the current function has been invoked via command substitution?
foo() {
if [[ something ]]; then
echo "The foo function was called directly."
else
echo "The foo function was invoked via command substitution."
fi
}
Call foo ...
- 143
0
votes
2
answers
462
views
$! is empty when running command in bash with -c or in subshell
I am trying to get the pid of the previous command with $!, but that only works in the 'main' shell:
$ echo "test" | echo $!
4436
$
If I start a subshell, then it's empty:
$ bash
$ echo &...
1
vote
2
answers
236
views
less stops my script; why is that and how to avoid?
I have this Bash script named as s in current directory:
#!/bin/bash
pipe_test() {
( set -m; (
$1
); set +m ) |
(
$2
)
}
pipe_test "$1" "$2"
If I call e.g.
./s yes ...
- 528
1
vote
1
answer
140
views
Differences between (command) and command in bash
While investigating the problem of redirecting the stderr of the bash keyword time and the standard solution to the problem, I came across another question.
The description man bash gives of the (list)...
- 89
0
votes
1
answer
365
views
Why does Bash run scripts in subshells?
Why does the Bash shell run scripts in subshells? What is the advantage of doing so?
- 1,431
0
votes
0
answers
46
views
How a command which is meant to output indefinitely, when piped to another command that waits for a certain amount of input, knows when to terminate?
I am baffled by how piping in bash works.
Take a random string generator, for example:
tr -dc 'A-Z0-9' </dev/urandom
spawned tr process that reads from /dev/urandom indefinitely, as far as /dev/...
- 1
1
vote
1
answer
248
views
Different behavior of progress bars: local vs. remote ssh bash script
I am having issues to get minimal progress bars working in local scripts which are executed remotely via ssh. Depending on the command I get other undesired progress indicators (lines of dots with ...
- 13
0
votes
1
answer
357
views
How to pass a subprocess to a command with bash
I'm trying to pass multiple outputs to a command using sub-processes.
This is (a shortened version of) my command:
cat "$1"
one=cat "$1"
The output of command <(echo "foo") is
foo
/dev/fd/63: ...
- 3
0
votes
1
answer
437
views
How to exit the parent script as failed when the child script fails
I have a script which calls another script.
Now even if the second script fails it's still succeeding original script.
Below is the first part of the script:
#OBJ_TYP=`echo $Job_detail_corr | awk -...
- 21
4
votes
1
answer
385
views
Why is $! returning the wrong process ID for a subshell running in background?
$(sleep 5) &
echo $!
sleep 1
echo "done"
The above outputs:
$ ./test.sh
7483
done
However if I search for sleep in ps aux I see:
ps aux | rg sleep
chris 7484 0.0 0.0 132788 1432 pts/6 ...
- 4,025
0
votes
2
answers
333
views
Why grep --include with curly braces fail in subshell? [duplicate]
This works in shell:
grep -R --exclude-dir=storage --exclude-dir=node-modules --include=*.{scss,css} loader.gif
But this fails:
sh -c "grep -R --exclude-dir=storage --exclude-dir=node-modules --...
- 1