rman: can't open target

If you have Oracle database on Linux, when you run rman utility to connect to database, you can get an error "rman: can't open target":
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[oracle@db1 /]$ rman target /
rman: can't open target
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In this case check, which rman is used, it can be from another direcroty:
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[oracle@db1 /]$ which rman
/usr/X11R6/bin/rman
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If it is, more likely, that in you PATH environment variable $ORACLE_HOME is not on the first place:
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[oracle@db1 /]$ echo $PATH
/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/oracle/OraHome/bin
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All, that you need now, is to change PATH variable, set is to be $ORACLE_HOME at the first place. To do it for you login profile, in case of bash shell, modify ~/.bash_profile file. For example:
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...
export ORACLE_HOME=/oracle/product/10.2/DB1/bin
export PATH=$ORACLE_HOME:$PATH
...
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Good luck!