Shell - Write Variable Contents to a File

Shell - Write variable contents to a file

Use the echo command:

var="text to append";
destdir=/some/directory/path/filename

if [ -f "$destdir" ]
then 
    echo "$var" > "$destdir"
fi

The if tests that $destdir represents a file.

The > appends the text after truncating the file. If you only want to append the text in $var to the file existing contents, then use >> instead:

echo "$var" >> "$destdir"

The cp command is used for copying files (to files), not for writing text to a file.

Writing variables to file with bash

Enclose the strings you want to write within single quotes to avoid variable replacement.

> FOO=bar
> echo "$FOO"
bar
> echo '$FOO'
$FOO
> 

How to read a file into a variable in shell?

In cross-platform, lowest-common-denominator sh you use:

#!/bin/sh
value=`cat config.txt`
echo "$value"

In bash or zsh, to read a whole file into a variable without invoking cat:

#!/bin/bash
value=$(<config.txt)
echo "$value"

Invoking cat in bash or zsh to slurp a file would be considered a Useless Use of Cat.

Note that it is not necessary to quote the command substitution to preserve newlines.

See: Bash Hacker's Wiki - Command substitution - Specialities.

Write variables to a file which contains the variabe name and value

You can use sed to do the job.

!/bin/bash

echo "Read"
var1=$(grep -Po "(?<=^var1=).*" data.txt)

echo "Do Something"
var2=${var1}

echo "Write"
sed -i "/var2/ s/.*/var2=${var2}/" data.txt # replace the line contains 'var2' with var2=2

I assume you use GNU sed, for BSD sed, you need to feed an extra suffix to -i.

Open and write data to text file using Bash?

The short answer:

echo "some data for the file" >> fileName

However, echo doesn't deal with end of line characters (EOFs) in an ideal way. So, if you're gonna append more than one line, do it with printf:

printf "some data for the file\nAnd a new line" >> fileName

The >> and > operators are very useful for redirecting output of commands, they work with multiple other bash commands.

Need to assign the contents of a text file to a variable in a bash script

In bash, $ (< answer.txt) is equivalent to $ (cat answer.txt), but built in and thus faster and safer. See the bash manual.

I suspect you're running this print:

NAME  
    run-mailcap, see, edit, compose, print − execute programs via entries in the mailcap file

File content into unix variable with newlines

The assignment does not remove the newline characters, it's actually the echo doing this. You need simply put quotes around the string to maintain those newlines:

echo "$testvar"

This will give the result you want. See the following transcript for a demo:

pax> cat num1.txt ; x=$(cat num1.txt)
line 1
line 2

pax> echo $x ; echo '===' ; echo "$x"
line 1 line 2
===
line 1
line 2

---

The reason why newlines are replaced with spaces is not entirely to do with the echo command, rather it's a combination of things.

When given a command line, bash splits it into words according to the documentation for the IFS variable:

IFS: The Internal Field Separator that is used for word splitting after expansion ... the default value is <space><tab><newline>.

That specifies that, by default, any of those three characters can be used to split your command into individual words. After that, the word separators are gone, all you have left is a list of words.

Combine that with the echo documentation (a bash internal command), and you'll see why the spaces are output:

echo [-neE] [arg ...]: Output the args, separated by spaces, followed by a newline.

When you use echo "$x", it forces the entire x variable to be a single word according to bash, hence it's not split. You can see that with:

pax> function count {
...>    echo $#
...> }
pax> count 1 2 3
3
pax> count a b c d
4
pax> count $x
4
pax> count "$x"
1

Here, the count function simply prints out the number of arguments given. The 1 2 3 and a b c d variants show it in action.

Then we try it with the two variations on the x variable. The one without quotes shows that there are four words, "test", "1", "test" and "2". Adding the quotes makes it one single word "test 1\ntest 2".

How to echo $variable into text file within a loop

If you use

echo " $i" >> fail

It will append it to the file.

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