Is it possible to simplify this BASH eval expression? - bash

Given the following Bash shell script excerpt:
# The intent is to take the PATH env variable, break it up into its parts, making them
# appear to be command line args (i.e., `$1`, `$2`, ...), and then for this example, just
# echo the parts in space delimited form, but we can imagine that we may want to do other
# things with them - this is just sample usage
# Important Requirement/Constraint
# ================================
# Please do not alter the "PATH to $1, $2, $3, ..." portion of the answer or replace the
# Bash ".." range construct with the output of the "seq" command exec'd in a subshell.
# Preferably, the answer should simply consist of the simplification of the last line of
# code - the "eval eval ..." . Also, please don't simplify by collapsing the whole thing
# to just echo "$#" since we may want to work with only some of the parts, and not
# necessarily the first parts, of the path. That is to say that the 1 and $# in the
# {1..$#} range could be replaced with other shell variables or expr., potentially
# Test case
PATH=/usr/local/bin:/usr/bin:/bin
# The code being examined follows
# Set ':' as the input field separator of the path
IFS=: # Or, more appropriately if in a function: local IFS=:
# Parse the PATH environment variable and break it up into its components
set $PATH
# This is the line we want to simplify, if possible, without losing functionality of
# course (see the comment that follows for details)
eval eval echo '\'$(eval 'echo "\${1..$#}"')
# Some notes and explanations regarding the functionality and underlying intent of the
# preceding line:
# - We start by dynamically creating the following construct: ${1..3}
# since $# is 3 for our example
# - Use Bash to expand that construct to: $1 $2 $3
# these vars contain the parsed parts of the PATH
# - Finally, display the three parts of the PATH using echo: echo $1 $2 $3
# - This causes the following text to be sent to STDOUT:
# /usr/local/bin /usr/bin /bin
So, can the eval eval... line in the preceding code be simplified, but still produce the desired output, which for the above example is:
/usr/local/bin /usr/bin /bin
I am thinking along the lines of a solution that would replace some of the echo commands with input/output redirection (perhaps) or maybe a reordering/collapsing of sorts that would lead to the need for fewer eval commands than are used in the example.

but still produce the desired output,
/usr/local/bin /usr/bin /bin
Just:
echo "${PATH//:/ }"
The intent is to take the PATH env variable, break it up into its parts, making them
appear to be command line args (i.e., $1, $2, ...), and then for this example, just
echo the parts in space delimited form, but we can imagine that we may want to do other
things with them - this is just sample usage
I do not trust unquoted shell expansions.
IFS=':' read -ra patharr <<<"$PATH"
set -- "${patharr[#]}"
IFS=' '; printf "%s\n" "${patharr[*]}"

echo "${PATH}" | tr ':' '\n' > stack
count=1
echo "#/bin/sh-" | tr '-' '\n' >> stack2
while read line
do
echo "path${count}=${line}" >> stack2
count=$(($count+1))
done < stack
source stack2
Now you've got every section of the path, in its' own named variable.

Sticking close to the original, you can do
IFS=:
set $PATH
echo "$#"
If you don't want to change IFS and PATH, you can do
set $(sed 's/[^=]*=//;s/:/ /g' <<< ${PATH})
echo "$#"

Related

String expansion - escaped quoted variable to value

To get started, here's the script I'm running to get the offending string:
# sed finds all sourced file paths from inputted file.
#
# while reads each match output from sed to $SOURCEFILE variable.
# Each should be a file path, or a variable that represents a file path.
# Any variables found should be expanded to the full path.
#
# echo and calls are used for demonstractive purposes only
# I intend to do something else with the path once it's expanded.
PATH_SOME_SCRIPT="/path/to/bash/script"
while read -r SOURCEFILE; do
echo "$SOURCEFILE"
"$SOURCEFILE"
$SOURCEFILE
done < <(cat $PATH_SOME_SCRIPT | sed -n -e "s/^\(source\|\.\|\$include\) //p")
You may also wish to use the following to test this out as mock data:
[ /path/to/bash/script ]
#!/bin/bash
source "$HOME/bash_file"
source "$GLOBAL_VAR_SCRIPT_PATH"
echo "No cow powers here"
For the tl;dr crew, basically the while loop spits out the following on the mock data:
"$HOME/bash_file"
bash: "$HOME/bash_file": no such file or directory
bash: "$HOME/bash_file": no such file or directory
"$GLOBAL_VAR_SCRIPT_PATH"
"$GLOBAL_VAR_SCRIPT_PATH": command not found
"$GLOBAL_VAR_SCRIPT_PATH": command not found
My question is, can you get the variable to expand correctly, e.g., print "/home//bash_file" and "/expanded/variable/path"? I should also state that although eval works I do not intend to use it because of its potential insecurities.
Protip that any variable value used in cat | sed would be available globally, including to the calling script, so it's not because the script cannot call the variable value.
FIRST SOLUTION ATTEMPT
Using anubhava's envsubst solution:
SOMEVARIABLE="/home/nick/.some_path"
while read -r SOURCEFILE; do
echo "$SOURCEFILE"
envsubst <<< "$SOURCEFILE";
done < <(echo -e "\"\$SOMEVARIABLE\"\n\"$HOME/.another_file\"")
This outputs the following:
"$SOMEVARIABLE"
""
"/home/nick/.another_file"
"/home/nick/.another_file"
Unfortunately, it does not expand the variable! Oh dear :(
SECOND SOLUTION ATTEMPT
Based upon the first attempt:
export SOMEVARIABLE="/home/nick/.some_path"
while read -r SOURCEFILE; do
echo "$SOURCEFILE"
envsubst <<< "$SOURCEFILE";
done < <(echo -e "\"\$SOMEVARIABLE\"\n\"$HOME/.another_file\"")
unset SOMEVARIABLE
which produces the results we wanted without eval and without messing with global variables (for too long anyway), hoorah!
Good runner-ups were further suggested using eval (although potentially unsafe) which can be found in this answer and here (link courtesy of anubhava's extended comments).
My question is, can you get the variable to expand correctly, e.g., print "/home//bash_file" and "/expanded/variable/path"?
Yes you can use envsubst program, that substitutes the values of environment variables:
while read -r sourceFile; do
envsubst <<< "$sourceFile"
done < <(sed -n "s/^\(source\|\.\|\$include\) //p" "$PATH_SOME_SCRIPT")
I think you are asking how to recursively expand variables in bash. Try
expanded=$(eval echo $SOURCEFILE)
inside your loop. eval runs the expanded command you give it. Since $SOURCEFILE isn't in quotes, it will be expanded to, e.g., $HOME/whatever. Then the eval will expand the $HOME before passing it to echo. echo will print the result, and expanded=$(...) will put the printed result in $expanded.

How to read stdin when no arguments are passed?

Script doesn't work when I want to use standard input when there are no arguments (files) passed. Is there any way how to use stdin instead of a file in this code?
I tried this:
if [ ! -n $1 ] # check if argument exists
then
$1=$(</dev/stdin) # if not use stdin as an argument
fi
var="$1"
while read line
do
... # find the longest line
done <"$var"
For a general case of wanting to read a value from stdin when a parameter is missing, this will work.
$ echo param | script.sh
$ script.sh param
script.sh
#!/bin/bash
set -- "${1:-$(</dev/stdin)}" "${#:2}"
echo $1
Just substitute bash's specially interpreted /dev/stdin as the filename:
VAR=$1
while read blah; do
...
done < "${VAR:-/dev/stdin}"
(Note that bash will actually use that special file /dev/stdin if built for an OS that offers it, but since bash 2.04 will work around that file's absence on systems that do not support it.)
pilcrow's answer provides an elegant solution; this is an explanation of why the OP's approach didn't work.
The main problem with the OP's approach was the attempt to assign to positional parameter $1 with $1=..., which won't work.
The LHS is expanded by the shell to the value of $1, and the result is interpreted as the name of the variable to assign to - clearly, not the intent.
The only way to assign to $1 in bash is via the set builtin.
The caveat is that set invariably sets all positional parameters, so you have to include the other ones as well, if any.
set -- "${1:-/dev/stdin}" "${#:2}" # "${#:2}" expands to all remaining parameters
(If you expect only at most 1 argument, set -- "${1:-/dev/stdin}" will do.)
The above also corrects a secondary problem with the OP's approach: the attempt to store the contents rather than the filename of stdin in $1, since < is used.
${1:-/dev/stdin} is an application of bash parameter expansion that says: return the value of $1, unless $1 is undefined (no argument was passed) or its value is the empty string (""or '' was passed). The variation ${1-/dev/stdin} (no :) would only return /dev/stdin if $1 is undefined (if it contains any value, even the empty string, it would be returned).
If we put it all together:
# Default to filename '/dev/stdin' (stdin), if none was specified.
set -- "${1:-/dev/stdin}" "${#:2}"
while read -r line; do
... # find the longest line
done < "$1"
But, of course, the much simpler approach would be to use ${1:-/dev/stdin} as the filename directly:
while read -r line; do
... # find the longest line
done < "${1:-/dev/stdin}"
or, via an intermediate variable:
filename=${1:-/dev/stdin}
while read -r line; do
... # find the longest line
done < "$filename"
Variables are assigned a value by Var=Value and that variable is used by e.g. echo $Var. In your case, that would amount to
1=$(</dev/stdin)
when assigning the standard input. However, I do not think that variable names are allowed to start with a digit character. See the question bash read from file or stdin for ways to solve this.
Here is my version of script:
#!/bin/bash
file=${1--} # POSIX-compliant; ${1:--} can be used either.
while IFS= read -r line; do
printf '%s\n' "$line"
done < <(cat -- "$file")
If file is not present in the argument, read the from standard input.
See more examples: How to read from file or stdin in bash? at stackoverflow SE

bash printf: ignore parameters or access by index?

I have a bash function that fetches values (using curl and cut) and creates a file name from them. Now I want to support a second naming scheme that needs a different set of parameters.
Example:
#!/bin/bash
TEMPLATE="%02i. %s.txt"
foo() {
a="Imagine these to"
b="be set dynamically"
c="42"
filename="$(printf "$TEMPLATE" "$c" "$a")"
# second: filename="$a - $b.txt"
# or: filename="$(printf "%s - %s.txt" "$a" "$b")"
echo "$filename"
# generate file
}
# actual script loops over:
foo
One of the values is a number that should be padded with leading zeros if required, thus printf in the current implementation.
Is there a way to implement this with just setting a different template globally? This would require that the template can access parameters by index or at least skip some of them.
If not, what are my alternatives? The template is to be chosen by command line parameter and does not change after initialization.
What does not work:
bash man page suggests that zero length output is not possible (to skip values)
C's printf man page mentions a "%m$" construct, which apparently is not supported in bash
the function itself generates the values, so it cannot receive the full filename as parameter
If you need to skip an argument, you can use %.s. Examples:
$ printf "%.s%s\n" "Bonjour" "Hello"
Hello
$ printf "%s%.s\n" "Bonjour" "Hello"
Bonjour
AFAIK, you can't access arguments by index.
If you need to store the formated string in a variable, please don't use a subshell as in:
$ variable=$(printf "%.s%s" "Bonjour" "Hello")
$ echo "$variable"
Hello
Instead, use the -v option to printf (type help printf to have the details) as in:
$ printf -v variable "%.s%s" "Bonjour" "Hello"
$ echo "$variable"
Hello
Also, if your template can come from user input, I would advise you to add -- just before it (this is to end the options of the command, just in case a user wants to start a template with a dash). Hence I would replace you line
filename="$(printf "$TEMPLATE" "$c" "$a")"
with
printf -v filename -- "$TEMPLATE" "$c" "$a"
Finally, having upper case variable names is considered bad bash practice.

Bash Templating: How to build configuration files from templates with Bash?

I'm writing a script to automate creating configuration files for Apache and PHP for my own webserver. I don't want to use any GUIs like CPanel or ISPConfig.
I have some templates of Apache and PHP configuration files. Bash script needs to read templates, make variable substitution and output parsed templates into some folder. What is the best way to do that? I can think of several ways. Which one is the best or may be there are some better ways to do that? I want to do that in pure Bash (it's easy in PHP for example)
How to replace ${} placeholders in a text file?
template.txt:
The number is ${i}
The word is ${word}
script.sh:
#!/bin/sh
#set variables
i=1
word="dog"
#read in template one line at the time, and replace variables
#(more natural (and efficient) way, thanks to Jonathan Leffler)
while read line
do
eval echo "$line"
done < "./template.txt"
BTW, how do I redirect output to external file here? Do I need to escape something if variables contain, say, quotes?
Using cat & sed for replacing each variable with its value:
Given template.txt (see above)
Command:
cat template.txt | sed -e "s/\${i}/1/" | sed -e "s/\${word}/dog/"
Seems bad to me because of the need to escape many different symbols and with many variables the line will be tooooo long.
Can you think of some other elegant and safe solution?
Try envsubst
$ cat envsubst-template.txt
Variable FOO is (${FOO}).
Variable BAR is (${BAR}).
$ FOO=myfoo
$ BAR=mybar
$ export FOO BAR
$ cat envsubst-template.txt | envsubst
Variable FOO is (myfoo).
Variable BAR is (mybar).
A heredoc is a builtin way to template a conf file.
STATUS_URI="/hows-it-goin"; MONITOR_IP="10.10.2.15";
cat >/etc/apache2/conf.d/mod_status.conf <<EOF
<Location ${STATUS_URI}>
SetHandler server-status
Order deny,allow
Deny from all
Allow from ${MONITOR_IP}
</Location>
EOF
Regarding yottsa's answer: envsubst was new to me. Fantastic.
You can use this:
perl -p -i -e 's/\$\{([^}]+)\}/defined $ENV{$1} ? $ENV{$1} : $&/eg' < template.txt
to replace all ${...} strings with corresponding enviroment variables (do not forget to export them before running this script).
For pure bash this should work (assuming that variables do not contain ${...} strings):
#!/bin/bash
while read -r line ; do
while [[ "$line" =~ (\$\{[a-zA-Z_][a-zA-Z_0-9]*\}) ]] ; do
LHS=${BASH_REMATCH[1]}
RHS="$(eval echo "\"$LHS\"")"
line=${line//$LHS/$RHS}
done
echo "$line"
done
. Solution that does not hang if RHS references some variable that references itself:
#!/bin/bash
line="$(cat; echo -n a)"
end_offset=${#line}
while [[ "${line:0:$end_offset}" =~ (.*)(\$\{([a-zA-Z_][a-zA-Z_0-9]*)\})(.*) ]] ; do
PRE="${BASH_REMATCH[1]}"
POST="${BASH_REMATCH[4]}${line:$end_offset:${#line}}"
VARNAME="${BASH_REMATCH[3]}"
eval 'VARVAL="$'$VARNAME'"'
line="$PRE$VARVAL$POST"
end_offset=${#PRE}
done
echo -n "${line:0:-1}"
WARNING: I do not know a way to correctly handle input with NULs in bash or preserve the amount of trailing newlines. Last variant is presented as it is because shells “love” binary input:
read will interpret backslashes.
read -r will not interpret backslashes, but still will drop the last line if it does not end with a newline.
"$(…)" will strip as many trailing newlines as there are present, so I end … with ; echo -n a and use echo -n "${line:0:-1}": this drops the last character (which is a) and preserves as many trailing newlines as there was in the input (including no).
I agree with using sed: it is the best tool for search/replace. Here is my approach:
$ cat template.txt
the number is ${i}
the dog's name is ${name}
$ cat replace.sed
s/${i}/5/
s/${name}/Fido/
$ sed -f replace.sed template.txt > out.txt
$ cat out.txt
the number is 5
the dog's name is Fido
I have a bash solution like mogsie but with heredoc instead of herestring to allow you to avoid escaping double quotes
eval "cat <<EOF
$(<template.txt)
EOF
" 2> /dev/null
Try eval
I think eval works really well. It handles templates with linebreaks, whitespace, and all sorts of bash stuff. If you have full control over the templates themselves of course:
$ cat template.txt
variable1 = ${variable1}
variable2 = $variable2
my-ip = \"$(curl -s ifconfig.me)\"
$ echo $variable1
AAA
$ echo $variable2
BBB
$ eval "echo \"$(<template.txt)\"" 2> /dev/null
variable1 = AAA
variable2 = BBB
my-ip = "11.22.33.44"
This method should be used with care, of course, since eval can execute arbitrary code. Running this as root is pretty much out of the question. Quotes in the template need to be escaped, otherwise they will be eaten by eval.
You can also use here documents if you prefer cat to echo
$ eval "cat <<< \"$(<template.txt)\"" 2> /dev/null
#plockc provoded a solution that avoids the bash quote escaping issue:
$ eval "cat <<EOF
$(<template.txt)
EOF
" 2> /dev/null
Edit: Removed part about running this as root using sudo...
Edit: Added comment about how quotes need to be escaped, added plockc's solution to the mix!
Edit Jan 6, 2017
I needed to keep double quotes in my configuration file so double escaping double quotes with sed helps:
render_template() {
eval "echo \"$(sed 's/\"/\\\\"/g' $1)\""
}
I can't think of keeping trailing new lines, but empty lines in between are kept.
Although it is an old topic, IMO I found out more elegant solution here: http://pempek.net/articles/2013/07/08/bash-sh-as-template-engine/
#!/bin/sh
# render a template configuration file
# expand variables + preserve formatting
render_template() {
eval "echo \"$(cat $1)\""
}
user="Gregory"
render_template /path/to/template.txt > path/to/configuration_file
All credits to Grégory Pakosz.
Instead of reinventing the wheel go with envsubst
Can be used in almost any scenario, for instance building configuration files from environment variables in docker containers.
If on mac make sure you have homebrew then link it from gettext:
brew install gettext
brew link --force gettext
./template.cfg
# We put env variables into placeholders here
this_variable_1 = ${SOME_VARIABLE_1}
this_variable_2 = ${SOME_VARIABLE_2}
./.env:
SOME_VARIABLE_1=value_1
SOME_VARIABLE_2=value_2
./configure.sh
#!/bin/bash
cat template.cfg | envsubst > whatever.cfg
Now just use it:
# make script executable
chmod +x ./configure.sh
# source your variables
. .env
# export your variables
# In practice you may not have to manually export variables
# if your solution depends on tools that utilise .env file
# automatically like pipenv etc.
export SOME_VARIABLE_1 SOME_VARIABLE_2
# Create your config file
./configure.sh
I'd have done it this way, probably less efficient, but easier to read/maintain.
TEMPLATE='/path/to/template.file'
OUTPUT='/path/to/output.file'
while read LINE; do
echo $LINE |
sed 's/VARONE/NEWVALA/g' |
sed 's/VARTWO/NEWVALB/g' |
sed 's/VARTHR/NEWVALC/g' >> $OUTPUT
done < $TEMPLATE
If you want to use Jinja2 templates, see this project: j2cli.
It supports:
Templates from JSON, INI, YAML files and input streams
Templating from environment variables
A longer but more robust version of the accepted answer:
perl -pe 's;(\\*)(\$([a-zA-Z_][a-zA-Z_0-9]*)|\$\{([a-zA-Z_][a-zA-Z_0-9]*)\})?;substr($1,0,int(length($1)/2)).($2&&length($1)%2?$2:$ENV{$3||$4});eg' template.txt
This expands all instances of $VAR or ${VAR} to their environment values (or, if they're undefined, the empty string).
It properly escapes backslashes, and accepts a backslash-escaped $ to inhibit substitution (unlike envsubst, which, it turns out, doesn't do this).
So, if your environment is:
FOO=bar
BAZ=kenny
TARGET=backslashes
NOPE=engi
and your template is:
Two ${TARGET} walk into a \\$FOO. \\\\
\\\$FOO says, "Delete C:\\Windows\\System32, it's a virus."
$BAZ replies, "\${NOPE}s."
the result would be:
Two backslashes walk into a \bar. \\
\$FOO says, "Delete C:\Windows\System32, it's a virus."
kenny replies, "${NOPE}s."
If you only want to escape backslashes before $ (you could write "C:\Windows\System32" in a template unchanged), use this slightly-modified version:
perl -pe 's;(\\*)(\$([a-zA-Z_][a-zA-Z_0-9]*)|\$\{([a-zA-Z_][a-zA-Z_0-9]*)\});substr($1,0,int(length($1)/2)).(length($1)%2?$2:$ENV{$3||$4});eg' template.txt
Here's another pure bash solution:
it's using heredoc, so:
complexity doesn't increase because of additionaly required syntax
template can include bash code
that also allows you to indent stuff properly. See below.
it doesn't use eval, so:
no problems with the rendering of trailing empty lines
no problems with quotes in the template
$ cat code
#!/bin/bash
LISTING=$( ls )
cat_template() {
echo "cat << EOT"
cat "$1"
echo EOT
}
cat_template template | LISTING="$LISTING" bash
Input:
$ cat template (with trailing newlines and double quotes)
<html>
<head>
</head>
<body>
<p>"directory listing"
<pre>
$( echo "$LISTING" | sed 's/^/ /' )
<pre>
</p>
</body>
</html>
Output:
<html>
<head>
</head>
<body>
<p>"directory listing"
<pre>
code
template
<pre>
</p>
</body>
</html>
Here is another solution: generate a bash script with all the variables and the contents of the template file, that script would look like this:
word=dog
i=1
cat << EOF
the number is ${i}
the word is ${word}
EOF
If we feed this script into bash it would produce the desired output:
the number is 1
the word is dog
Here is how to generate that script and feed that script into bash:
(
# Variables
echo word=dog
echo i=1
# add the template
echo "cat << EOF"
cat template.txt
echo EOF
) | bash
Discussion
The parentheses opens a sub shell, its purpose is to group together all the output generated
Within the sub shell, we generate all the variable declarations
Also in the sub shell, we generate the cat command with HEREDOC
Finally, we feed the sub shell output to bash and produce the desired output
If you want to redirect this output into a file, replace the last line with:
) | bash > output.txt
Taking the answer from ZyX using pure bash but with new style regex matching and indirect parameter substitution it becomes:
#!/bin/bash
regex='\$\{([a-zA-Z_][a-zA-Z_0-9]*)\}'
while read line; do
while [[ "$line" =~ $regex ]]; do
param="${BASH_REMATCH[1]}"
line=${line//${BASH_REMATCH[0]}/${!param}}
done
echo $line
done
If using Perl is an option and you're content with basing expansions on environment variables only (as opposed to all shell variables), consider Stuart P. Bentley's robust answer.
This answer aims to provide a bash-only solution that - despite use of eval - should be safe to use.
The goals are:
Support expansion of both ${name} and $name variable references.
Prevent all other expansions:
command substitutions ($(...) and legacy syntax `...`)
arithmetic substitutions ($((...)) and legacy syntax $[...]).
Allow selective suppression of variable expansion by prefixing with \ (\${name}).
Preserve special chars. in the input, notably " and \ instances.
Allow input either via arguments or via stdin.
Function expandVars():
expandVars() {
local txtToEval=$* txtToEvalEscaped
# If no arguments were passed, process stdin input.
(( $# == 0 )) && IFS= read -r -d '' txtToEval
# Disable command substitutions and arithmetic expansions to prevent execution
# of arbitrary commands.
# Note that selectively allowing $((...)) or $[...] to enable arithmetic
# expressions is NOT safe, because command substitutions could be embedded in them.
# If you fully trust or control the input, you can remove the `tr` calls below
IFS= read -r -d '' txtToEvalEscaped < <(printf %s "$txtToEval" | tr '`([' '\1\2\3')
# Pass the string to `eval`, escaping embedded double quotes first.
# `printf %s` ensures that the string is printed without interpretation
# (after processing by by bash).
# The `tr` command reconverts the previously escaped chars. back to their
# literal original.
eval printf %s "\"${txtToEvalEscaped//\"/\\\"}\"" | tr '\1\2\3' '`(['
}
Examples:
$ expandVars '\$HOME="$HOME"; `date` and $(ls)'
$HOME="/home/jdoe"; `date` and $(ls) # only $HOME was expanded
$ printf '\$SHELL=${SHELL}, but "$(( 1 \ 2 ))" will not expand' | expandVars
$SHELL=/bin/bash, but "$(( 1 \ 2 ))" will not expand # only ${SHELL} was expanded
For performance reasons, the function reads stdin input all at once into memory, but it's easy to adapt the function to a line-by-line approach.
Also supports non-basic variable expansions such as ${HOME:0:10}, as long as they contain no embedded command or arithmetic substitutions, such as ${HOME:0:$(echo 10)}
Such embedded substitutions actually BREAK the function (because all $( and ` instances are blindly escaped).
Similarly, malformed variable references such as ${HOME (missing closing }) BREAK the function.
Due to bash's handling of double-quoted strings, backslashes are handled as follows:
\$name prevents expansion.
A single \ not followed by $ is preserved as is.
If you want to represent multiple adjacent \ instances, you must double them; e.g.:
\\ -> \ - the same as just \
\\\\ -> \\
The input mustn't contain the following (rarely used) characters, which are used for internal purposes: 0x1, 0x2, 0x3.
There's a largely hypothetical concern that if bash should introduce new expansion syntax, this function might not prevent such expansions - see below for a solution that doesn't use eval.
If you're looking for a more restrictive solution that only supports ${name} expansions - i.e., with mandatory curly braces, ignoring $name references - see this answer of mine.
Here is an improved version of the bash-only, eval-free solution from the accepted answer:
The improvements are:
Support for expansion of both ${name} and $name variable references.
Support for \-escaping variable references that shouldn't be expanded.
Unlike the eval-based solution above,
non-basic expansions are ignored
malformed variable references are ignored (they don't break the script)
IFS= read -d '' -r lines # read all input from stdin at once
end_offset=${#lines}
while [[ "${lines:0:end_offset}" =~ (.*)\$(\{([a-zA-Z_][a-zA-Z_0-9]*)\}|([a-zA-Z_][a-zA-Z_0-9]*))(.*) ]] ; do
pre=${BASH_REMATCH[1]} # everything before the var. reference
post=${BASH_REMATCH[5]}${lines:end_offset} # everything after
# extract the var. name; it's in the 3rd capture group, if the name is enclosed in {...}, and the 4th otherwise
[[ -n ${BASH_REMATCH[3]} ]] && varName=${BASH_REMATCH[3]} || varName=${BASH_REMATCH[4]}
# Is the var ref. escaped, i.e., prefixed with an odd number of backslashes?
if [[ $pre =~ \\+$ ]] && (( ${#BASH_REMATCH} % 2 )); then
: # no change to $lines, leave escaped var. ref. untouched
else # replace the variable reference with the variable's value using indirect expansion
lines=${pre}${!varName}${post}
fi
end_offset=${#pre}
done
printf %s "$lines"
To follow up on plockc's answer on this page, here is a dash-suitable version, for those of you looking to avoid bashisms.
eval "cat <<EOF >outputfile
$( cat template.in )
EOF
" 2> /dev/null
Try shtpl
Perfect case for shtpl. (project of mine, so it is not widely in use and lacks in documentation. But here is the solution it offers anyhow. May you want to test it.)
Just execute:
$ i=1 word=dog sh -c "$( shtpl template.txt )"
Result is:
the number is 1
the word is dog
Have fun.
This page describes an answer with awk
awk '{while(match($0,"[$]{[^}]*}")) {var=substr($0,RSTART+2,RLENGTH -3);gsub("[$]{"var"}",ENVIRON[var])}}1' < input.txt > output.txt
# Usage: template your_file.conf.template > your_file.conf
template() {
local IFS line
while IFS=$'\n\r' read -r line ; do
line=${line//\\/\\\\} # escape backslashes
line=${line//\"/\\\"} # escape "
line=${line//\`/\\\`} # escape `
line=${line//\$/\\\$} # escape $
line=${line//\\\${/\${} # de-escape ${ - allows variable substitution: ${var} ${var:-default_value} etc
# to allow arithmetic expansion or command substitution uncomment one of following lines:
# line=${line//\\\$\(/\$\(} # de-escape $( and $(( - allows $(( 1 + 2 )) or $( command ) - UNSECURE
# line=${line//\\\$\(\(/\$\(\(} # de-escape $(( - allows $(( 1 + 2 ))
eval "echo \"${line}\"";
done < "$1"
}
This is the pure bash function adjustable to your liking, used in production and should not break on any input.
If it breaks - let me know.
You can also use bashible (which internally uses the evaluating approach described above/below).
There is an example, how to generate a HTML from multiple parts:
https://github.com/mig1984/bashible/tree/master/examples/templates
Look at simple variables substitution python script here: https://github.com/jeckep/vsubst
It is very simple to use:
python subst.py --props secure.properties --src_path ./templates --dst_path ./dist
Here's a bash function that preserves whitespace:
# Render a file in bash, i.e. expand environment variables. Preserves whitespace.
function render_file () {
while IFS='' read line; do
eval echo \""${line}"\"
done < "${1}"
}
Here's a modified perl script based on a few of the other answers:
perl -pe 's/([^\\]|^)\$\{([a-zA-Z_][a-zA-Z_0-9]*)\}/$1.$ENV{$2}/eg' -i template
Features (based on my needs, but should be easy to modify):
Skips escaped parameter expansions (e.g. \${VAR}).
Supports parameter expansions of the form ${VAR}, but not $VAR.
Replaces ${VAR} with a blank string if there is no VAR envar.
Only supports a-z, A-Z, 0-9 and underscore characters in the name (excluding digits in the first position).
You can also use printf to fill a template.
#!/bin/bash
IFS='' read -rd '' TEMPL <<-'EOB'
The number is %d
The word is "%s"
Birds of Massachusetts:
%s
EOB
N=12
WORD="Bird"
MULTILINE="Eastern Bluebirds
Common Grackles"
echo "START"
printf "${TEMPL}" ${N} ${WORD} "${MULTILINE}"
echo "END"
Here's the output, with quotes and whitespace intact:
START
The number is 12
The word is "Bird"
Birds of Massachusetts:
Eastern Bluebirds
Common Grackles
END

Preserving whitespaces in a string as a command line argument

I'm facing a small problem here, I want to pass a string containing whitespaces , to another program such that the whole string is treated as a command line argument.
In short I want to execute a command of the following structure through a bash shell script:
command_name -a arg1 -b arg2 -c "arg with whitespaces here"
But no matter how I try, the whitespaces are not preserved in the string, and is tokenized by default. A solution please,
edit: This is the main part of my script:
#!/bin/bash
#-------- BLACKRAY CONFIG ---------------#
# Make sure the current user is in the sudoers list
# Running all instances with sudo
BLACKRAY_BIN_PATH='/opt/blackray/bin'
BLACKRAY_LOADER_DEF_PATH='/home/crozzfire'
BLACKRAY_LOADER_DEF_NAME='load.xml'
BLACKRAY_CSV_PATH='/home/crozzfire'
BLACKRAY_END_POINT='default -p 8890'
OUT_FILE='/tmp/out.log'
echo "The current binary path is $BLACKRAY_BIN_PATH"
# Starting the blackray 0.9.0 server
sudo "$BLACKRAY_BIN_PATH/blackray_start"
# Starting the blackray loader utility
BLACKRAY_INDEX_CMD="$BLACKRAY_BIN_PATH/blackray_loader -c $BLACKRAY_LOADER_DEF_PATH/$BLACKRAY_LOADER_DEF_NAME -d $BLACKRAY_CSV_PATH -e "\"$BLACKRAY_END_POINT\"""
sudo time $BLACKRAY_INDEX_CMD -a $OUT_FILE
#--------- END BLACKRAY CONFIG ---------#
You're running into this problem because you store the command in a variable, then expand it later; unless there's a good reason to do this, don't:
sudo time $BLACKRAY_BIN_PATH/blackray_loader -c $BLACKRAY_LOADER_DEF_PATH/$BLACKRAY_LOADER_DEF_NAME -d $BLACKRAY_CSV_PATH -e "$BLACKRAY_END_POINT" -a $OUT_FILE
If you really do need to store the command and use it later, there are several options; the bash-hackers.org wiki has a good page on the subject. It looks to me like the most useful one here is to put the command in an array rather than a simple variable:
BLACKRAY_INDEX_CMD=($BLACKRAY_BIN_PATH/blackray_loader -c $BLACKRAY_LOADER_DEF_PATH/$BLACKRAY_LOADER_DEF_NAME -d $BLACKRAY_CSV_PATH -e "$BLACKRAY_END_POINT")
sudo time "${BLACKRAY_INDEX_CMD[#]}" -a $OUT_FILE
This avoids the whole confusion between spaces-separating-words and spaces-within-words because words aren't separated by spaces -- they're in separate elements of the array. Expanding the array in double-quotes with the [#] suffix preserves that structure.
(BTW, another option would be to use escaped quotes rather like you're doing, then run the command with eval. Don't do this; it's a good way to introduce weird parsing bugs.)
Edit:
Try:
BLACKRAY_END_POINT="'default -p 8890'"
or
BLACKRAY_END_POINT='"default -p 8890"'
or
BLACKRAY_END_POINT="default\ -p\ 8890"
or
BLACKRAY_END_POINT='default\ -p\ 8890'
and
BLACKRAY_INDEX_CMD="$BLACKRAY_BIN_PATH/blackray_loader -c $BLACKRAY_LOADER_DEF_PATH/$BLACKRAY_LOADER_DEF_NAME -d $BLACKRAY_CSV_PATH -e $BLACKRAY_END_POINT"
Original answer:
Is blackray_loader a shell script?
Here is a demonstration that you have to deal with this issue both when specifying the parameter and when handling it:
A text file called "test.txt" (include the line numbers):
1 two words
2 two words
3 two
4 words
A script called "spacetest":
#!/bin/bash
echo "No quotes in script"
echo $1
grep $1 test.txt
echo
echo "With quotes in script"
echo "$1"
grep "$1" test.txt
echo
Running it with ./spacetest "two--------words" (replace the hyphens with spaces):
No quotes in script
two words
grep: words: No such file or directory
test.txt:1 two words
test.txt:2 two words
test.txt:3 two
With quotes in script
two words
2 two words
You can see that in the "No quotes" section it tried to do grep two words test.txt which interpreted "words" as a filename in addition to "test.txt". Also, the echo dropped the extra spaces.
When the parameter is quoted, as in the second section, grep saw it as one argument (including the extra spaces) and handled it correctly. And echo preserved the extra spaces.
I used the extra spaces, by the way, merely to aid in the demonstration.
I have a suggestion:
# iterate through the passed arguments, save them to new properly quoted ARGS string
while [ -n "$1" ]; do
ARGS="$ARGS '$1'"
shift
done
# invoke the command with properly quoted arguments
my_command $ARGS
probably you need to surround the argument by double quotes (e.g. "${6}").
Following OP comment it should be "$BLACKRAY_END_POINT"
Below is my example of restarting a script via exec su USER or exec su - USER. It accommodates:
being called from a relative path or current working directory
spaces in script name and arguments
single and double-quotes in arguments, without crazy escapes like: \\"
#
# This script should always be run-as a specific user
#
user=jimbob
if [ $(whoami) != "$user" ]; then
exec su -c "'$(readlink -f "$0")' $(printf " %q" "$#")" - $user
exit $?
fi
A post on other blog saved me for this whitespaces problem: http://logbuffer.wordpress.com/2010/09/23/bash-scripting-preserve-whitespaces-in-variables/
By default, whitespaces are trimed:
bash> VAR1="abc def gh ijk"
bash> echo $VAR1
abc def gh ijk
bash>
"The cause of this behaviour is the internal shell variable $IFS (Internal Field Separator), that defaults to whitespace, tab and newline.
To preserve all contiguous whitespaces you have to set the IFS to something different"
With IFS bypass:
bash> IFS='%'
bash> echo $VAR1
abc def gh ijk
bash>unset IFS
bash>
It works wonderfully for my command case:
su - user1 -c 'test -r "'${filepath}'"; ....'
Hope this helps.

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