How to compose external command in julia using concatenation of different variables?

Question

i'd like to do something like :

arg= " -l "
cmd = "ls $arg "
run(cmd)

but i can find no simple solution to do that :

$ julia
               _
   _       _ _(_)_     |  A fresh approach to technical computing
  (_)     | (_) (_)    |  Documentation: https://docs.julialang.org
   _ _   _| |_  __ _   |  Type "?help" for help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 0.6.0 (2017-06-19 13:05 UTC)
 _/ |\__'_|_|_|\__'_|  |  Official http://julialang.org/ release
|__/                   |  x86_64-pc-linux-gnu

julia> arg=" -l "
" -l "

julia> cmd=`ls $arg`
`ls ' -l '`

julia> run(cmd)
ls: cannot access  -l : No such file or directory
ERROR: failed process: Process(`ls ' -l '`, ProcessExited(2)) [2]
Stacktrace:
 [1] pipeline_error(::Base.Process) at ./process.jl:682
 [2] run(::Cmd) at ./process.jl:651

julia> cmd="ls $arg"
"ls  -l "

julia> run(`$cmd`)
ERROR: could not spawn `'ls  -l '`: no such file or directory (ENOENT)
Stacktrace:
 [1] _jl_spawn(::String, ::Array{String,1}, ::Ptr{Void}, ::Base.Process, ::RawFD, ::RawFD, ::RawFD) at ./process.jl:360
 [2] #373 at ./process.jl:512 [inlined]
 [3] setup_stdio(::Base.##373#374{Cmd}, ::Tuple{RawFD,RawFD,RawFD}) at ./process.jl:499
 [4] #spawn#372(::Nullable{Base.ProcessChain}, ::Function, ::Cmd, ::Tuple{RawFD,RawFD,RawFD}) at ./process.jl:511
 [5] run(::Cmd) at ./process.jl:650

should i split the resulting string to separate each part (cause there is no shell doing the job ?)

by the way, how to get the exit status of the command ?

thanks a lot

Try arg = "-l" instead of arg = " -l ". The spaces confused ls. Also, look at Running External Programs from the manual. — Dan Getz
– Dan Getz, Commented Aug 2, 2017 at 10:49
i think it's the same problem. you mean like that : julia> arg="-l" "-l" julia> cmd="ls $arg" "ls -l" julia> run($cmd) ERROR: could not spawn 'ls -l': no such file or directory (ENOENT), — Laurent Facq
– Laurent Facq, Commented Aug 2, 2017 at 12:18
no better... (backticks are remove in my comment :( ) julia> arg=" -l " " -l " julia> cmd=ls $arg ls ' -l ' julia> run(cmd) ls: impossible d'accéder à -l : Aucun fichier ou dossier de ce type ERROR: failed process: Process(ls ' -l ', ProcessExited(2)) [2] Stacktrace: [1] pipeline_error(::Base.Process) at ./process.jl:682 [2] run(::Cmd) at ./process.jl:651 — Laurent Facq
– Laurent Facq, Commented Aug 2, 2017 at 12:21
Just cut and paste the following: arg = "-l" ; cmd = `ls $arg` ; run(cmd) — Dan Getz
– Dan Getz, Commented Aug 2, 2017 at 12:24

Dan Getz · Accepted Answer · 2017-08-02 12:28:44Z

1

Try:

arg = "-l"
cmd = `ls $arg`
run(cmd)

And read running external programs to understand more.

answered Aug 2, 2017 at 12:28

Dan Getz

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5 Comments

Laurent Facq Over a year ago

ok ! sorry for my mistakes... and it works with more variables too :) arga = "-l" ; argb = "-a" ; cmd = `ls $arga $argb` ; run(cmd)

Laurent Facq Over a year ago

i searched the Julia doc about running external programs but did not found about the exit status... any idea ?

Dan Getz Over a year ago

In the doc linked, there is a paragraph with The run method itself returns nothing, and throws an ErrorException if the external command fails to run successfully. Should apply (though I haven't tried it now)

Laurent Facq Over a year ago

ok. so it's still a bit hard to use for real cases. it's confusing too not to be able to use something like : arg = "-l -a"

Dan Getz Over a year ago

Perhaps it is only confusing at the beginning. You might want to try: args = ["-l","-a"] ; cmd = `ls $args` ; run(cmd). This also enables you to have spaces in arguments and filenames, something which a normal shell command would make difficult. Another solution is: cmd = "ls -l -r" ; run(`bash -c $cmd`) which uses the shell to run command. But the last solution depends on the specific shell and OS.

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