mjhjmtu

I noted apt output is different when executed alone in Bash shell and when output is redirected to files.

For example:

$ apt install ./*.deb --simulate 1>111.txt 2>222.txt

adding redirection results in text WARNING: apt does not have a stable CLI interface. Use with caution in scripts. in file for stream 2, whereas running apt w/out redirection does not display this text in shell window.

Why output depends on redirection? Maybe apt sees redirection as its` parameter? How can I write redirection to be unnoticed to other part of shell command line?

P.S. I saw it in Ubuntu 18 and apt is specific app, but maybe the issue is general to Unix, so I put only shell in tag.

apt has been designed historically with interactive command-line use in mind, hence the coded warning, which has been done on purpose in/by apt and not by the shell.

However, apt is not necessarily parsing the command line; the shell has already done that job by the time it calls apt. What apt is doing is detecting that the stdout stream has been changed/redirected.

As for using apt in scripts, you have got the similar older command apt-get which does does more or less the same thing, and does not give that warning when stdout is redirected.

TLDR Is not the shell writing the error message in stderr, it is the apt command.

The fact that you are able to capture the apt stderr output in the shell is an artifact of Unix being a multitasking OS, and does not mean it is the shell producing that output.

PS. Doing an strace at the command, it can be seen apt writing that message:

strace apt get install bash > a
....
write(2, "n", 1
) = 1
write(2, "WARNING: ", 9WARNING: ) = 9
write(2, "apt", 3apt) = 3
write(2, " ", 1 ) = 1
write(2, "does not have a stable CLI inter"..., 38does not have a stable CLI interface. ) = 38
write(2, "Use with caution in scripts.", 28Use with caution in scripts.) = 28
write(2, "n", 1
) = 1
write(2, "n", 1
) = 1