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bashstdoutstderrio-redirection

can't get specific line from bash output

I'm trying simple bash script:

/usr/libexec/java_home -V

output is:

Matching Java Virtual Machines (3):
11.0.1, x86_64: "Java SE 11.0.1"    /Library/Java/JavaVirtualMachines/jdk-11.0.1.jdk/Contents/Home
10.0.1, x86_64: "Java SE 10.0.1"    /Library/Java/JavaVirtualMachines/jdk-10.0.1.jdk/Contents/Home
1.8.0_192, x86_64:  "Java SE 8" /Library/Java/JavaVirtualMachines/jdk1.8.0_192.jdk/Contents/Home

Seems to be 4 lines, but if I try to get second line:

/usr/libexec/java_home -V | sed -n 2p

The output is the same. No additional line. If I try the first one - I got the second:

/usr/libexec/java_home -V | sed -n 1p

Output:

Matching Java Virtual Machines (3):
    11.0.1, x86_64: "Java SE 11.0.1"    /Library/Java/JavaVirtualMachines/jdk-11.0.1.jdk/Contents/Home
    10.0.1, x86_64: "Java SE 10.0.1"    /Library/Java/JavaVirtualMachines/jdk-10.0.1.jdk/Contents/Home
    1.8.0_192, x86_64:  "Java SE 8" /Library/Java/JavaVirtualMachines/jdk1.8.0_192.jdk/Contents/Home

/Library/Java/JavaVirtualMachines/jdk-11.0.1.jdk/Contents/Home

If I assign output to array and count it's size:

array=( $(/usr/libexec/java_home -V) )
array_size=${#array[@]}
echo $array_size

it shows size = 1:

Matching Java Virtual Machines (3):
    11.0.1, x86_64: "Java SE 11.0.1"    /Library/Java/JavaVirtualMachines/jdk-11.0.1.jdk/Contents/Home
    10.0.1, x86_64: "Java SE 10.0.1"    /Library/Java/JavaVirtualMachines/jdk-10.0.1.jdk/Contents/Home
    1.8.0_192, x86_64:  "Java SE 8" /Library/Java/JavaVirtualMachines/jdk1.8.0_192.jdk/Contents/Home

1

What am I doing wrong?

Solution

  • What appears to be happening is that your script, /usr/libexec/java_home -V is outputting some, or all, of its output to stderr instead of writing to stdout. Since the shell pipe ('|') connects the stdout of the previous command to stdin of the command that follows, any output to stderr by /usr/libexec/java_home -V just gets written to your screen and is never processed by sed making it look like sed wasn't working.

    To test and correct the problem is that is the case, just redirect stderr from your script to stdout and then pipe the result to sed, making sure sed receives all of the output from your script, e.g.

    $ /usr/libexec/java_home -V 2>&1 | sed -n 1p

    Glad it helped.