What is EOF in a shell script

Ever wondered what is <<EOF in Linux shell scripts? Basically <<EOF tells the shell that you are going to enter a multiline string until the "tag" EOF. You can name this tag as you want, it's often EOF or STOP.

EOF Parameter Example
Save EOF Output
EOF Examples

EOF Parameter Example

Let's take below shell script example

sqlplus / as sysdba << EOF
select name, open_mode from v$database;
archive log list;
EOF

The sqlplus command will start sqlplus but now we need to pass sql queries that are not understood by Linux interpreter. The <<EOF parameter will pass sql queries as user inputs directly to sql prompt.

This is done until same EOF parameters is encountered. Some rules to follow:

The tag can be any string, uppercase or lowercase, though most people use uppercase by convention
The tag will not be considered as a Here tag if there are other words in that line. In this case, it will merely be considered part of the string. The tag should be by itself on a separate line, to be considered a tag
The tag should have no leading or trailing spaces in that line to be considered a tag. Otherwise it will be considered as part of the string

Save EOF Output

You can save the EOF output to another file

sqlplus / as sysdba << EOF > output.log
select name, open_mode from v$database;
archive log list;
EOF

This will create a new file output.log in current location which will contain output sql queries.

EOF Examples

Print variable value with EOF

a=10
cat << EOF
$a
EOF

Print multi-line output to a file

cat << EOF > print.sh
#!/bin/bash
echo \$PWD
echo $PWD
EOF

Run rman backup via shell script

rman target / << EOF > backup.log
run
{
crosscheck backup;
backup database plus archivelog;
}
EOF

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