LinuxQuestions.org - [SOLVED] Shell script, what does this line do: "cat -- << EOF >${CreateScript

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Shell script, what does this line do: "cat -- << EOF >${CreateScript_DeployDirs}"

Hello,

I'm analyzing a script from someone else and I'm asking what does this line do?

In short I have made a test script, trying to understand how it works..., but it is not clear to me:

Code:

set -euo pipefail

LocalPath="/home" 

TempPath="nkamp/$(date +%Y-%m-%d)"



function DeployReleaseKit() {



        local CreateScript_DeployDirs=${LocalPath}/${TempPath}/linux/deploy

        local BackupScript_Previous="BackupScript_dir"



        #Deploy the releasekit

        echo "1. Start function start deploy Releasekit"



        cat -- << EOF >${CreateScript_DeployDirs}

#!/bin/bash        

echo -n

echo "2. indien getoond is de local variabel ${CreateScript_DeployDirs} een waarde"

exit 0

EOF



        cat -- << EOF >${BackupScript_Previous}



echo -n

echo " 3. indien dit getoond wordt heeft de variabele ${BackupScript_Previous} een waarde!" 

EOF



        echo "Dit is het eind van de functie - DeployReleaseKit -"



}



echo "Roep de functie DeployReleaseKit aan" #  var: CreateScript_DeployDirs: kan niet = local - BackupScript_Previous: ${BackupScript_Previous}" 

DeployReleaseKit

echo "De functie deployReleaseKit is aangeroepen" #  var: CreateScript_DeployDirs: kan niet = local - BackupScript_Previous: ${BackupScript_Previous} " 



exit 0

~

My expectations is that, if the variable {CreateScript_DeployDirs} has a value than the part till the first EOF is not executed. If it does not have a value than the part is till EOF is executed. Or just the other way around.

But this is the result:

Quote:

[nkamp@192 ~]$ ./test.sh
Roep de functie DeployReleaseKit aan
1. Start function start deploy Releasekit
./test.sh: line 14: /home/nkamp/2020-09-02/linux/deploy: Is a directory

[nkamp@192 ~]$ ./test.sh
Roep de functie DeployReleaseKit aan
1. Start function start deploy Releasekit
./test.sh: line 14: //linux/deploy: No such file or directory

This was not what I expected. (first result: the vars. ${LocalPath} and ${TempPath} does have a right value. Second result not)

I hope that someone can explain how this work and why and/or when such a construction should be used.

Nico.

cat will write the text between EOFs into the named file. If that was possible (=writing that file).

This does not do anything, there is nothing to cat.

Code:

cat -- << EOF > ${CreateScript_DeployDirs}

>

Couple of Examples:

Code:

a=1

cat << EOF

$a

$(($a + 1))

$(($a * 5))

EOF

1

2

5





b=2

cat << EOF > test.txt

c=$(($b * 3))

d=$(($c / 2))

EOF



cat test.txt

c=6

d=3

That's a little bit of a mess. Is this what you are trying to do?

Code:

cat -- << EOF > test.txt      

echo -n

echo "2. indien getoond is de local variabel ${CreateScript_DeployDirs} een waarde"

exit 0

EOF



cat test.txt

echo -n

echo "2. indien getoond is de local variabel /home/nkamp/2020-09-02/linux/deploy een waarde"

exit 0

Edit:

Code:

LocalPath="/home" 

TempPath="nkamp/$(date +%Y-%m-%d)"



echo "$LocalPath"

/home



echo "$TempPath"

nkamp/2020-09-02



CreateScript_DeployDirs=${LocalPath}/${TempPath}/linux/deploy

echo "$CreateScript_DeployDirs"

/home/nkamp/2020-09-02/linux/deploy

Ok, where did you make that directory path so that you can write to it? So far all that you have is a variable.

Hello,

Now I see it what this part is doing:

Code:

cat -- << EOF >${CreateScript_DeployDirs}

#!/bin/bash        

echo -n

echo "2. indien getoond is de local variabel ${CreateScript_DeployDirs} een waarde"

exit 0

EOF

This is my 'simple' example try to understand what this is doing. But I see now in the 'real' scripts which I'm analyzing do have in other variables the script (*.sh) names.

But I was not aware what that this was possible to write in a file with cat -- << EOF >${variable}...EOF. Very powerfull from my point of view.

Thanks.

Nico