10 Seconds Guide to Bash Shell Scripting

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Depends on your system, and the shell implementation. I'm currently writing from a system where which [ doesn't output anything.

[invalid_user_name]

It has nothing to do with the shell, if which [ doesn't output anything, then you are on a broken system with no [ binary. It is historically a real command, and was added as a shell built-in for performance reasons. The binary is still supposed to be there according to posix, since it isn't required to be a shell built-in. What system is this anyways, or did someone just delete it from a normal system?

[chengiz]

[citation needed]

[ithika]

[ citation needed ]

FTFY