This question already has answers here:
When to wrap quotes around a shell variable?
(5 answers)
Closed 7 years ago.
I want to create some scripts for filling some templates and inserting them into my project folder. I want to use a shell script for this, and the templates are very small so I want to embed them in the shell script. The problem is that echo seems to ignore the line breaks in my string. Either that, or the string doesn't contain line breaks to begin with. Here is an example:
MY_STRING="
Hello, world! This
Is
A
Multi lined
String."
echo -e $MY_STRING
This outputs:
Hello, world! This Is A Multi lined String.
I'm assuming echo is the culprit here. How can I get it to acknowledge the line breaks?
You need double quotes around the variable interpolation.
echo -e "$MY_STRING"
This is an all-too common error. You should get into the habit of always quoting strings, unless you specifically need to split into whitespace-separated tokens or have wildcards expanded.
So to be explicit, the shell will normalize whitespace when it parses your command line. You can see this if you write a simple C program which prints out its argv array.
argv[0]='Hello,'
argv[1]='world!'
argv[2]='This'
argv[3]='Is'
argv[4]='A'
argv[5]='Multi'
argv[6]='lined'
argv[7]='String.'
By contrast, with quoting, the whole string is in argv[0], newlines and all.
For what it's worth, also consider here documents (with cat, not echo):
cat <<"HERE"
foo
Bar
HERE
You can also interpolate a variable in a here document.
cat <<HERE
$MY_STRING
HERE
... although in this particular case, it's hardly what you want.
echo is so nineties. The new (POSIX) kid on the block is printf.
printf '%s\n' "$MY_STRING"
No -e or SYSV vs BSD echo madness and full control over what gets printed where and how wide, escape sequences like in C. Everybody please start using printf now and never look back.
Try this :
echo "$MY_STRING"
Related
I have recently started studying shell script and I'd like to be able to comment out a set of lines in a shell script. I mean like it is in case of C/Java :
/* comment1
comment2
comment3
*/`
How could I do that?
Use : ' to open and ' to close.
For example:
: '
This is a
very neat comment
in bash
'
Multiline comment in bash
: <<'END_COMMENT'
This is a heredoc (<<) redirected to a NOP command (:).
The single quotes around END_COMMENT are important,
because it disables variable resolving and command resolving
within these lines. Without the single-quotes around END_COMMENT,
the following two $() `` commands would get executed:
$(gibberish command)
`rm -fr mydir`
comment1
comment2
comment3
END_COMMENT
Note: I updated this answer based on comments and other answers, so comments prior to May 22nd 2020 may no longer apply. Also I noticed today that some IDE's like VS Code and PyCharm do not recognize a HEREDOC marker that contains spaces, whereas bash has no problem with it, so I'm updating this answer again.
Bash does not provide a builtin syntax for multi-line comment but there are hacks using existing bash syntax that "happen to work now".
Personally I think the simplest (ie least noisy, least weird, easiest to type, most explicit) is to use a quoted HEREDOC, but make it obvious what you are doing, and use the same HEREDOC marker everywhere:
<<'###BLOCK-COMMENT'
line 1
line 2
line 3
line 4
###BLOCK-COMMENT
Single-quoting the HEREDOC marker avoids some shell parsing side-effects, such as weird subsitutions that would cause crash or output, and even parsing of the marker itself. So the single-quotes give you more freedom on the open-close comment marker.
For example the following uses a triple hash which kind of suggests multi-line comment in bash. This would crash the script if the single quotes were absent. Even if you remove ###, the FOO{} would crash the script (or cause bad substitution to be printed if no set -e) if it weren't for the single quotes:
set -e
<<'###BLOCK-COMMENT'
something something ${FOO{}} something
more comment
###BLOCK-COMMENT
ls
You could of course just use
set -e
<<'###'
something something ${FOO{}} something
more comment
###
ls
but the intent of this is definitely less clear to a reader unfamiliar with this trickery.
Note my original answer used '### BLOCK COMMENT', which is fine if you use vanilla vi/vim but today I noticed that PyCharm and VS Code don't recognize the closing marker if it has spaces.
Nowadays any good editor allows you to press ctrl-/ or similar, to un/comment the selection. Everyone definitely understands this:
# something something ${FOO{}} something
# more comment
# yet another line of comment
although admittedly, this is not nearly as convenient as the block comment above if you want to re-fill your paragraphs.
There are surely other techniques, but there doesn't seem to be a "conventional" way to do it. It would be nice if ###> and ###< could be added to bash to indicate start and end of comment block, seems like it could be pretty straightforward.
After reading the other answers here I came up with the below, which IMHO makes it really clear it's a comment. Especially suitable for in-script usage info:
<< ////
Usage:
This script launches a spaceship to the moon. It's doing so by
leveraging the power of the Fifth Element, AKA Leeloo.
Will only work if you're Bruce Willis or a relative of Milla Jovovich.
////
As a programmer, the sequence of slashes immediately registers in my brain as a comment (even though slashes are normally used for line comments).
Of course, "////" is just a string; the number of slashes in the prefix and the suffix must be equal.
I tried the chosen answer, but found when I ran a shell script having it, the whole thing was getting printed to screen (similar to how jupyter notebooks print out everything in '''xx''' quotes) and there was an error message at end. It wasn't doing anything, but: scary. Then I realised while editing it that single-quotes can span multiple lines. So.. lets just assign the block to a variable.
x='
echo "these lines will all become comments."
echo "just make sure you don_t use single-quotes!"
ls -l
date
'
what's your opinion on this one?
function giveitauniquename()
{
so this is a comment
echo "there's no need to further escape apostrophes/etc if you are commenting your code this way"
the drawback is it will be stored in memory as a function as long as your script runs unless you explicitly unset it
only valid-ish bash allowed inside for instance these would not work without the "pound" signs:
1, for #((
2, this #wouldn't work either
function giveitadifferentuniquename()
{
echo nestable
}
}
Here's how I do multiline comments in bash.
This mechanism has two advantages that I appreciate. One is that comments can be nested. The other is that blocks can be enabled by simply commenting out the initiating line.
#!/bin/bash
# : <<'####.block.A'
echo "foo {" 1>&2
fn data1
echo "foo }" 1>&2
: <<'####.block.B'
fn data2 || exit
exit 1
####.block.B
echo "can't happen" 1>&2
####.block.A
In the example above the "B" block is commented out, but the parts of the "A" block that are not the "B" block are not commented out.
Running that example will produce this output:
foo {
./example: line 5: fn: command not found
foo }
can't happen
Simple solution, not much smart:
Temporarily block a part of a script:
if false; then
while you respect syntax a bit, please
do write here (almost) whatever you want.
but when you are
done # write
fi
A bit sophisticated version:
time_of_debug=false # Let's set this variable at the beginning of a script
if $time_of_debug; then # in a middle of the script
echo I keep this code aside until there is the time of debug!
fi
in plain bash
to comment out
a block of code
i do
:||{
block
of code
}
This question already has answers here:
Why a variable assignment replaces tabs with spaces
(2 answers)
Closed 7 years ago.
I'm having some troubles with the printf function in bash.
I wrote a little script on which I pass a name and two letters (such as "sh", "py", "ht") and it creates a file in the current working directory named "name.extension".
For instance, if I execute seed test py a file named test.py is created in the current working dir with the shebang #!/usr/bin/python3.
So far, so good, nothing fancy: I'm learning shell scripting and I thought this could be a simple exercise to test the knowledge gained so far.
The problem is when I want to create an HTML file. This is the function that I use:
creaHtml(){
head='<!--DOCTYPE html-->\n<html>\n\t<head>\n\t\t<meta charset=\"UTF-8\">\n\t</head>\n\t<body>\n\t</body>\n</html>'
percorso=$CARTELLA_CORRENTE/$NOME_FILE.html
printf $head>>$percorso
chmod 755 $percorso
}
If I run, for instance, seed test ht the correct function (creaHtml) is called, test.html is created but if I try to look into it I only see:
<!--DOCTYPE
And nothing else.
This is the trace for that function:
[sviluppo:~/bin]$ seed test ht
+ creaHtml
+ head='<!--DOCTYPE html-->\n<html>\n\t<head>\n\t\t<meta charset=\"UTF-8\">\n\t</head>\n\t<body>\n\t</body>\n</html>'
+ percorso=/home/sviluppo/bin/test.html
+ printf '<!--DOCTYPE' 'html-->\n<html>\n\t<head>\n\t\t<meta' 'charset=\"UTF-8\">\n\t</head>\n\t<body>\n\t</body>\n</html>'
+ chmod 755 /home/sviluppo/bin/test.html
+ set +x
However, if I try to run printf '<!--DOCTYPE html-->\n<html>\n\t<head>\n\t\t<meta charset=\"UTF-8\">\n\t</head>\n\t<body>\n\t</body>\n</html>' from the terminal, I see the correct output: the "skeleton" of an HTML file neatly displayed with indentation and everything. What am I missing here?
Try echo -e instead of printf. printf is for printing formatted strings. Since you didn't protect $head with quotes, bash splits the string to form the command. The first word (before first white space) forms the format string. The rest are just arguments for things you didn't specify to print.
echo -e "$head" > "$percorso"
The -e evaluates your \n into newlines. I changed your >> to > since it looks like you want this to be the whole file, rather than append to any existing file you might have.
You have to be careful with quotes in bash. One thing can become many things. This actually makes it more powerful, but it can be confusing for people learning. Notice that I also put the file name "$percorso" in double quotes too. This evaluates the variable and makes sure that it ends up as one thing. If you use single quotes, it will be one word, but not evaluated. Unlike Python, there is a big difference between single and double quotes.
If you want to use printf for compatibility as #chepner pointed out, just be sure to quote it:
printf "$head" > "$percorso"
Actually that is much simpler anyway.
This question already has answers here:
I just assigned a variable, but echo $variable shows something else
(7 answers)
Closed 7 years ago.
i want to read whole file in a variable.
for example my file is.
file name : Q.txt
My name is Naeem Rehmat.
I am a student of FAST university.
Now i am in 4th semester.
I am learning bash script.
I have this problem.
code:
text=$(cat Q.txt)
echo $text
out put should be like this:
My name is Naeem Rehmat.
I am a student of FAST university.
Now i am in 4th semester.
I am learning bash script.
I have this problem.
Presumably the problem is that your whitespace is incorrect. Use double quotes:
echo "$text"
When you write echo $text without quotes, bash evaluates the string and performs what is known as "field splitting" or "word splitting" before generating the command. To simplify the case, suppose text is the string foo bar. Bash splits that into two "words" and passes "foo" as the first argument to echo, and "bar" as the second. If you use quotes, bash passes only one argument to echo, and that argument contains multiple spaces which echo will print.
Note that it is probably good style to also use quotes in the assignment of text (ie, text="$(cat Q.txt)"), although it is not necessary since field splitting does not occur in a variable assignment.
I want to echo a text like this:
"I'm going to bed at "$'\cc3'"$var"$'\cc'
Sometimes it happens that the $var variable begins with a number and Bash is simply concatenating it or whatever. How could I escape the $var so it is separated but without a space between them?
The ANSI-C Quoting mechanism in Bash uses \cx to generate Control-X. Your use of $'\cc3' generates a Control-C (aka \003 or \x03) character followed by a digit 3.
Superficially, then, you want:
var=01:15
echo "I'm going to bed at "$'\cc'"$var"$'\cc'
which surrounds the time with Control-C characters (though quite why you want that, I'm not clear). If you're after a Unicode character U+0CC3 (KANNADA VOWEL SIGN VOCALIC R — ೃ — if you've got good Unicode support), then you need Bash 4.x and $'\ucc3'.
If you're after something else, you need to explain what you're trying to echo with the ANSI-C Quoting.
You could try sending the control-c using the \nnn format instead of \c:
echo $'I\'m going to bed at \003'"$var"$'\003'
(I changed the quoting slightly just to reduce the the number of context switches used to build the string.)
Or, save the control-c character in a variable:
cc=$'\cc'
echo "I'm going to bed at $cc$var$cc"
I seem to have some difficulty getting what I want to work. Basically, I have a series of variables that are assigned strings with some quotes and \ characters. I want to remove the quotes to embed them inside a json doc, since json hates quotes using python dump methods.
I figured it would be easy. Just determine how to remove the characters easy and then write a simple for loop for the variable substitution, well it didn't work that way.
Here is what I want to do.
There is a variable called "MESSAGE23", it contains the following "com.centrify.tokend.cac", I want to strip out the quotes, which to me is easy, a simple echo $opt | sed "s/\"//g". When I do this from the command line:
$> MESSAGE23="com."apple".cacng.tokend is present"
$> MESSAGE23=`echo $MESSAGE23 | sed "s/\"//g"`
$> com.apple.cacng.tokend is present
This works. I get the properly formatted string.
When I then try to throw this into a loop, all hell breaks loose.
for i to {1..25}; do
MESSAGE$i=`echo $MESSAGE$i | sed "s/\"//g"`
done
This doesn't work (either it throws a bunch of indexes out or nothing), and I'm pretty sure I just don't know enough about arg or eval or other bash substitution variables.
But basically I want to do this for another set of variables with the same problems, where I strip out the quotes and incidentally the "\" too.
Any help would be greatly appreciated.
You can't do that. You could make it work using eval, but that introduces another level of quoting you have to worry about. Is there some reason you can't use an array?
MESSAGE=("this is MESSAGE[0]" "this is MESSAGE[1]")
MESSAGE[2]="I can add more, too!"
for (( i=0; i<${#MESSAGE[#]}; ++i )); do
echo "${MESSAGE[i]}"
done
Otherwise you need something like this:
eval 'echo "$MESSAGE'"$i"'"'
and it just gets worse from there.
First, a couple of preliminary problems: MESSAGE23="com."apple".cacng.tokend is present" will not embed double-quotes in the variable value, use MESSAGE23="com.\"apple\".cacng.tokend is present" or MESSAGE23='com."apple".cacng.tokend is present' instead. Second, you should almost always put double-quotes around variable expansions (e.g. echo "$MESSAGE23") to prevent parsing oddities.
Now, the real problems: the shell doesn't allow variable substitution on the left side of an assignment (i.e. MESSAGE$i=something won't work). Fortunately, it does allow this in a declare statement, so you can use that instead. Also, when the sees $MESSAGE$i it replaces it will the value of $MESSAGE followed by the value of $i; for this you need to use indirect expansion (`${!metavariable}').
for i in {1..25}; do
varname="MESSAGE$i"
declare $varname="$(echo "${!varname}" | tr -d '"')"
done
(Note that I also used tr instead of sed, but that's just my personal preference.)
(Also, note that #Mark Reed's suggestion of an array is really the better way to do this sort of thing.)