shell - Subshell created using (...) behaves differently from bash -c '...' -


i use set -e inside (...) on command line doesn't exit subshell when false command executed.

code 1:

( set -e; echo "$bashpid: start"; false; echo "foobar"; date; ) &&    echo "$bashpid: ok" || echo "$bashpid: nope" 9136: start foobar wed, apr 29, 2015  7:14:24 pm 7292: ok

however if use bash -c subshell creation behaves expect.

code 2:

bash -c 'set -e; echo "$bashpid: start"; false; echo "foobar"; date;' &&   echo "$bashpid: ok" || echo "$bashpid: nope" 7880: start 7292: nope

interestingly if remove && , || part after subshell (...) behaves fine.

code 3:

( set -e; echo "$bashpid: start"; false; echo "foobar"; date; ) 5940: start

so conclusions getting are:

  1. (...) behaves differently bash -c
  2. (...) && false behaves differently (...) , changes behavior of subshell well.

am making obvious mistake in interpreting strange (at least me) behavior?

the behavior of set -e && intentional.

it explicitly doesn't trigger when command part of && sequence.

from posix spec:

-e

when option on, when command fails (for of reasons listed in consequences of shell errors or returning exit status greater zero), shell shall exit following exceptions: failure of individual command in multi-command pipeline shall not cause shell exit. failure of pipeline shall considered.

the -e setting shall ignored when executing compound list following while, until, if, or elif reserved word, pipeline beginning ! reserved word, or command of and-or list other last.

if exit status of compound command other subshell command result of failure while -e being ignored, -e shall not apply command.

this requirement applies shell environment , each subshell environment separately. example, in:

set -e; (false; echo one) | cat; echo 2

the false command causes subshell exit without executing echo one; however, echo 2 executed because exit status of pipeline (false; echo one) | cat zero.

i assume difference here whether shell knows fact.

(...) presumably because current shell executes , bash -c presumably doesn't because that's external process (or something).

that last bit speculation , mind sub-shell behavior here entirely unhelpful , makes set -e unreliable (which why many people suggest avoiding it).

though seems might provide sort of "escape hatch" reliability problem if explicitly run shell doesn't have issue composition of scripts.


Comments

Post a Comment

Popular posts from this blog

php - failed to open stream: HTTP request failed! HTTP/1.0 400 Bad Request -

this code i'm using find latitude , longitude of place: function coords($address){ echo $url="http://maps.googleapis.com/maps/api/geocode/json?address=".$address; $json = file_get_contents(urlencode($url)); $data = json_decode($json, true); print_r($data['results'][0]['geometry']['location']['lat']); print_r($data['results'][0]['geometry']['location']['lng']); } however returns warning: failed open stream: http request failed! http/1.0 400 bad request if use above code in simple procedure works fine not in function. note: have tried curl_init()... method produced same result? instead of encoding whole url encode address part. following code work fine $add='jamshoro phase 2'; $url="http://maps.googleapis.com/maps/api/geocode/json?address=".urlencode($add); $json = file_get_contents($url); $data = json_decode($json, true); print_r($data['r...
Read more

command line - Use qwinsta in PowerShell ISE -

i trying use qwinsta , rwinsta powershell code. when try run error "the term 'qwinsta' not recognized name of cmdlet, function, script file, or operable program." need able remotely on several machines without having add .dll's or modules other machines. tried terminal services powershell module, need able put on remote machines , not able that. within powershell ise how run qwinsta? not looking parse information gather it. following code have tried (which had found on site also): function get-tssessions { param( $computername = "localhost" ) qwinsta /server:$computername | #parse output foreach-object { $_.trim() -replace "\s+","," } | #convert objects convertfrom-csv } get-tssessions -computername "localhost" | ft -autosize best have running powershell ise (x86). when run command qwinsta there same error you. of course assuming have 64-bit os. th...
Read more

java - Incorrect order of records in M-M relationship in hibernate -

Image
i have 4 records in bridge table in mysql database. using @manytomany in hibernate, accurate order of employees incorrect incorrect of projects. following required material figure out doing mistake. this source. @entity @table(name="project") public class projectbean { @id @generatedvalue @column(name="project_id") private int projectid; @column(name="title") private string projecttitle; @manytomany(mappedby="projects") private collection<employeebean> employees; public projectbean() { employees = new arraylist<employeebean>(); } //getters & setters } @entity @table(name="employee") public class employeebean { @id @generatedvalue @column(name="employee_id") private int employee_id; @column(name="first_name") private string ...
Read more