I'm wondering is there anyway to use for example , or ^ or % and so on, from variables in Bash ?
in instance I have three variables
var1='hello world'
var2=${var1:3}
var3='^'
I want to do this in bash ! please attention to my question I know it's very simple in other ways but how about this ?
echo ${var1:0:3}${var2$var3} # instead of echo ${var1:0:3}${var2^}
and finally output is :
heLlo world
In theory, eval can do execute arbitrary code, but has many security issues, so it should be used as last resort. Use it only when you trust the input 100%.
var1='hello world'
var2=${var1:3}
var3='^'
eval echo '${var1:0:3}${var2'$var3'}'
Related
How can I retrieve a bash variable value if I have the variable name as string? var1="this is the real value"
a="var1"
Do something to get value of var1 just using variable a.
Context:
I have some AMI's (Amazon Machine Image) and I want to fire up a few instances of each AMI. As soon as they finish booting, I want to setup each instance according to its AMI type. I don't want to bake lots of scripts or secret keys inside any AMI so I prepared a generalized startup script and I put it on S3 with a publicly accessible link. In rc.local I put small piece of code which fetches the startup script and executes it. This is all I have in the AMIs. Then each AMI accesses a common configuration script which is applicable to all AMIs and special setup scripts for each. These scripts are private and require a signed URL to access them.
So now, when I fire an instance of an AMI (my_private_ami_1), I pass a signed URL for one more file presented on S3 which contains signed URL for all private scripts in terms of key/value pair.config_url="http://s3.amazo.../config?signature"
my_private_ami_1="http://s3.amazo.../ami_1?signature"
...
When the startup script runs, it downloads the above file and source's it. Then it checks for its AMI type and picks the correct setup script for itself.
ami\_type=GET AMI TYPE #ex: sets ami\_type to my\_private\_ami\_1
setup\_url=GET THE SETUP FILE URL BASED ON AMI\_TYPE # this is where this problem arises
So now I can have a generic code which can fire instances irrespective of their AMI types and instances can take care of themselves.
You can use ${!a}:
var1="this is the real value"
a="var1"
echo "${!a}" # outputs 'this is the real value'
This is an example of indirect parameter expansion:
The basic form of parameter expansion is ${parameter}. The value of
parameter is substituted.
If the first character of parameter is an exclamation point (!), it
introduces a level of variable indirection. Bash uses the value of the
variable formed from the rest of parameter as the name of the
variable; this variable is then expanded and that value is used in the
rest of the substitution, rather than the value of parameter itself.
X=foo
Y=X
eval "Z=\$$Y"
sets Z to foo.
For my fellow zsh users, the way to accomplish the same thing as the accepted answer is to use:
echo ${(P)a} # outputs 'this is the real value'
It is appropriately called Parameter name replacement
This forces the value of the parameter name to be interpreted as a
further parameter name, whose value will be used where appropriate.
Note that flags set with one of the typeset family of commands (in
particular case transformations) are not applied to the value of name
used in this fashion.
If used with a nested parameter or command substitution, the result of
that will be taken as a parameter name in the same way. For example,
if you have ‘foo=bar’ and ‘bar=baz’, the strings ${(P)foo},
${(P)${foo}}, and ${(P)$(echo bar)} will be expanded to ‘baz’.
Likewise, if the reference is itself nested, the expression with the
flag is treated as if it were directly replaced by the parameter name.
It is an error if this nested substitution produces an array with more
than one word. For example, if ‘name=assoc’ where the parameter assoc
is an associative array, then ‘${${(P)name}[elt]}’ refers to the
element of the associative subscripted ‘elt’.
Modified my search keywords and Got it :). eval a=\$$a Thanks for your time.
In bash 4.3+, you can use declare -n:
#!/usr/bin/env bash
var="this is the real value"
var_name="var"
declare -n var_ref=$var_name
echo "${var_ref}"
Had the same issue with arrays, here is how to do it if you're manipulating arrays too :
array_name="ARRAY_NAME"
ARRAY_NAME=("Val0" "Val1" "Val2")
ARRAY=$array_name[#]
echo "ARRAY=${ARRAY}"
ARRAY=("${!ARRAY}")
echo "ARRAY=${ARRAY[#]}"
echo "ARRAY[0]=${ARRAY[0]}"
echo "ARRAY[1]=${ARRAY[1]}"
echo "ARRAY[2]=${ARRAY[2]}"
This will output :
ARRAY=ARRAY_NAME[#]
ARRAY=Val0 Val1 Val2
ARRAY[0]=Val0
ARRAY[1]=Val1
ARRAY[2]=Val2
In bash 4.3, the '-v' test for set variables was introduced. At the same time, 'nameref' declaration was added. These two features together with the indirection operator (!) enable a simplified version of the previous example:
get_value()
{
declare -n var_name=$1
if [[ -v var_name ]]
then
echo "${var_name}"
else
echo "variable with name <${!var_name}> is not set"
fi
}
test=123
get_value test
123
test="\$(echo \"something nasty\")"
get_value test
$(echo "something nasty")
unset test
get_value test
variable with name <test> is not set
As this approach eliminates the need for 'eval', it is safer.
This code checked under bash 5.0.3(1).
modern shells already support arrays( and even associative arrays). So please do use them, and use less of eval.
var1="this is the real value"
array=("$var1")
# or array[0]="$var1"
then when you want to call it , echo ${array[0]}
Based on the answer: https://unix.stackexchange.com/a/111627
###############################################################################
# Summary: Returns the value of a variable given it's name as a string.
# Required Positional Argument:
# variable_name - The name of the variable to return the value of
# Returns: The value if variable exists; otherwise, empty string ("").
###############################################################################
get_value_of()
{
variable_name=$1
variable_value=""
if set | grep -q "^$variable_name="; then
eval variable_value="\$$variable_name"
fi
echo "$variable_value"
}
test=123
get_value_of test
# 123
test="\$(echo \"something nasty\")"
get_value_of test
# $(echo "something nasty")
VALUE=$(eval "echo \$$SOME_VAR_NAME")
Example
SSH_KEY_FILE_PATH_FOO="/tmp/key"
SSH_KEY_FILE_PATH_VAR_NAME_PREFIX="SSH_KEY_FILE_PATH"
SSH_KEY_FILE_PATH_VAR_NAME_SUFFIX="FOO"
SSH_KEY_FILE_PATH=$(eval "echo \$${SSH_KEY_FILE_PATH_VAR_NAME_PREFIX}_${SSH_KEY_FILE_PATH_VAR_NAME_SUFFIX}")
echo "$SSH_KEY_FILE_PATH"
/tmp/key
I'm working with a growing bash script and within this script I have a number of functions. One of these functions is supposed to return a variables value, but I am running into some issues with the syntax. Below is an example of the code.
ShowTags() {
local tag=0
read tag
echo "$tag"
}
selected_tag=$(ShowTags)
echo "$selected_tag"
pulled this code from a Linux Journal article, but the problem is it doesn't seem to work, or perhaps it does and im missing something. Essentially whenever the function is called the script hangs up and does not output anything, I need to CTRL+C to drop back to CLI.
The article in question is below.
http://www.linuxjournal.com/content/return-values-bash-functions
So my question is this the proper way to return a value? Is there a better or more dependable way of doing this? And if there is please give me an example so I can figure this out without using global variables.
EDIT:
The behavior of this is really getting to me now. I am using the following script.
ShowTags() {
echo "hi"
local tag=0
read tag
echo "$tag"
}
selected_tag=$(ShowTags)
echo "$selected_tag
Basically what happens is bash will act as if the read command is taking place before the echo tag at the top of the function. As soon as I pass something to read though it will run the top echo, and complete the rest of the script. I am not sure why this is happening. This is exactly what is happening in my main script.
Change echo "hi" to echo "hi" >/dev/tty.
The reason you're not seeing it immediately is that $(ShowTags) captures all the standard output of the function, and that gets assigned to selected_tag. So you don't see any of it until you echo that variable.
By redirecting the prompt to /dev/tty, it's always displayed immediately on the terminal, not sent to the function's stdout, so it doesn't get captured by the command substitution.
You are trying to define a function with Name { ... ]. You have to use name() { ... }:
ShowTags() { # add ()
local tag=0
read tag
echo "$tag"
} # End with }
selected_tag=$(ShowTags)
echo "$selected_tag"
It now lets the user type in a string and have it written back:
$ bash myscript
hello world # <- my input
hello world # script's output
You can add a prompt with read -p "Enter tag: " tag to make it more obvious when to write your input.
As #thatotherguy pointed out, your function declaration syntax is off; but I suspect that's a transcription error, as if it was wrong in the script you'd get different problems. I think what's going on is that the read tag command in the function is trying to read a value from standard input (by default that's the terminal), and pausing until you type something in. I'm not sure what it's intended to do, but as written I'd expect it to pause indefinitely until something's typed in.
Solution: either type something in, or use something other than read. You could also add a prompt (read -p "Enter a tag: " tag) to make it more clear what's going on.
BTW, I have a couple of objections to the linux journal article you linked. These aren't relevant to your script, but things you should be aware of.
First, the function keyword is a nonstandard bashism, and I recommend against using it. myfunc() ... is sufficient to introduce a function definition.
Second, and more serious, the article recommends using eval in an unsafe way. Actually, it's really hard to use eval safely (see BashFAQ #48). You can improve it a great deal just by changing the quoting, and even more by not using eval at all:
eval $__resultvar="'$myresult'" # BAD, can evaluate parts of $myresult as executable code
eval $__resultvar='"$myresult"' # better, is only vulnerable to executing $__resultvar
declare $__resultvar="$myresult" # better still
See BashFAQ #6 for more options and discussion.
I'm interest in some thing : every time I echo $RANDOM , the show value difference . I guess the RANDOM is special (When I read it , it may call a function , set a variable flag and return the RANDOM number . ) . I want to create a variable like this , how I can do it ？ Every answer will be helpful .
The special behavior of $RANDOM is a built-in feature of bash. There is no mechanism for defining your own special variables.
You can write a function that prints a different value each time it's called, and then invoke it as $(func). For example:
now() {
date +%s
}
echo $(now)
Or you can set $PROMPT_COMMAND to a command that updates a specified variable. It runs just before printing each prompt.
i=0
PROMPT_COMMAND='((i++))'
This doesn't work in a script (since no prompt is printed), and it imposes an overhead whether you refer to the variable or not.
If you are BASH scripting there is a $RANDOM variable already internal to BASH.
This post explains the random variable $RANDOM:
http://tldp.org/LDP/abs/html/randomvar.html
It generates a number from 0 - 32767.
If you want to do different things then something like this:
case $RANDOM in
[1-10000])
Message="All is quiet."
;;
[10001-20000])
Message="Start thinking about cleaning out some stuff. There's a partition that is $space % full."
;;
[20001-32627])
Message="Better hurry with that new disk... One partition is $space % full."
;;
esac
I stumbled on this question a while ago and wasn't satisfied by the accepted answer: He wanted to create a variable just like $RANDOM (a variable with a dynamic value), thus I've wondered if we can do it without modifying bash itself.
Variables like $RANDOM are defined internally by bash using the dynamic_value field of the struct variable. If we don't want to patch bash to add our custom "dynamic values" variables, we still have few other alternatives.
An obscure feature of bash is loadable builtins (shell builtins loaded at runtime), providing a convenient way to dynamically load new symbols via the enable function:
$ enable --help|grep '\-f'
enable: enable [-a] [-dnps] [-f filename] [name ...]
-f Load builtin NAME from shared object FILENAME
-d Remove a builtin loaded with -f
We now have to write a loadable builtin providing the functions (written in C) that we want use as dynamic_value for our variables, then setting the dynamic_value field of our variables with a pointer to the chosen functions.
The production-ready way of doing this is using an another loadable builtin crafted on purpose to do the heavy-lifting, but one may abuse gdb if the ptrace call is available to do the same.
I've made a little demo using gdb, answering "How to create a bash variable like $RANDOM?":
$ ./bashful RANDOM dynamic head -c 8 /dev/urandom > /dev/null
$ echo $RANDOM
L-{Sgf
I have a bash script I am having some issues with concatenating 2 variables to call a 3rd.
Here is a simplification of the script, but the syntax is eluding me after reading the docs.
server_list_all="server1 server2 server3";
var1 = "server";
var2 = "all";
echo $(($var1_list_$var2));
This is about as close as I get to the right answer, it acknowledges the string and tosses an error on tokenization.
syntax error in expression (error token is "server1 server2 server3....
Not really seeing anything in the docs for this, but it should be doable.
EDIT: Cleaned up a bit
The Bash Reference Manual explains how you can use a neat feature of parameter expansion to do some indirection. In your case, you're interested in finding the contents of a variable whose name is defined by two other variables:
server_list_all="server1 server2 server3"
var1=server
var2=all
combined=${var1}_list_${var2}
echo ${!combined}
The exclamation mark when referring to combined means "use the variable whose name is defined by the contents of combined"
The Advanced Bash Scripting Guide has the answer for you (http://tldp.org/LDP/abs/html/ivr.html). You have two options, the first is classic shell:
#!/bin/bash
server_list_all="server1 server2 server3";
var1="server";
var2="all";
server_var="${var1}_list_${var2}"
eval servers=\$$server_var;
echo $servers
Alternatively you can use the bash shortcut ${!var}
#!/bin/bash
server_list_all="server1 server2 server3";
var1="server";
var2="all";
server_var="${var1}_list_${var2}"
echo ${!server_var}
Either approach works.
How can I retrieve a bash variable value if I have the variable name as string? var1="this is the real value"
a="var1"
Do something to get value of var1 just using variable a.
Context:
I have some AMI's (Amazon Machine Image) and I want to fire up a few instances of each AMI. As soon as they finish booting, I want to setup each instance according to its AMI type. I don't want to bake lots of scripts or secret keys inside any AMI so I prepared a generalized startup script and I put it on S3 with a publicly accessible link. In rc.local I put small piece of code which fetches the startup script and executes it. This is all I have in the AMIs. Then each AMI accesses a common configuration script which is applicable to all AMIs and special setup scripts for each. These scripts are private and require a signed URL to access them.
So now, when I fire an instance of an AMI (my_private_ami_1), I pass a signed URL for one more file presented on S3 which contains signed URL for all private scripts in terms of key/value pair.config_url="http://s3.amazo.../config?signature"
my_private_ami_1="http://s3.amazo.../ami_1?signature"
...
When the startup script runs, it downloads the above file and source's it. Then it checks for its AMI type and picks the correct setup script for itself.
ami\_type=GET AMI TYPE #ex: sets ami\_type to my\_private\_ami\_1
setup\_url=GET THE SETUP FILE URL BASED ON AMI\_TYPE # this is where this problem arises
So now I can have a generic code which can fire instances irrespective of their AMI types and instances can take care of themselves.
You can use ${!a}:
var1="this is the real value"
a="var1"
echo "${!a}" # outputs 'this is the real value'
This is an example of indirect parameter expansion:
The basic form of parameter expansion is ${parameter}. The value of
parameter is substituted.
If the first character of parameter is an exclamation point (!), it
introduces a level of variable indirection. Bash uses the value of the
variable formed from the rest of parameter as the name of the
variable; this variable is then expanded and that value is used in the
rest of the substitution, rather than the value of parameter itself.
X=foo
Y=X
eval "Z=\$$Y"
sets Z to foo.
For my fellow zsh users, the way to accomplish the same thing as the accepted answer is to use:
echo ${(P)a} # outputs 'this is the real value'
It is appropriately called Parameter name replacement
This forces the value of the parameter name to be interpreted as a
further parameter name, whose value will be used where appropriate.
Note that flags set with one of the typeset family of commands (in
particular case transformations) are not applied to the value of name
used in this fashion.
If used with a nested parameter or command substitution, the result of
that will be taken as a parameter name in the same way. For example,
if you have ‘foo=bar’ and ‘bar=baz’, the strings ${(P)foo},
${(P)${foo}}, and ${(P)$(echo bar)} will be expanded to ‘baz’.
Likewise, if the reference is itself nested, the expression with the
flag is treated as if it were directly replaced by the parameter name.
It is an error if this nested substitution produces an array with more
than one word. For example, if ‘name=assoc’ where the parameter assoc
is an associative array, then ‘${${(P)name}[elt]}’ refers to the
element of the associative subscripted ‘elt’.
Modified my search keywords and Got it :). eval a=\$$a Thanks for your time.
In bash 4.3+, you can use declare -n:
#!/usr/bin/env bash
var="this is the real value"
var_name="var"
declare -n var_ref=$var_name
echo "${var_ref}"
Had the same issue with arrays, here is how to do it if you're manipulating arrays too :
array_name="ARRAY_NAME"
ARRAY_NAME=("Val0" "Val1" "Val2")
ARRAY=$array_name[#]
echo "ARRAY=${ARRAY}"
ARRAY=("${!ARRAY}")
echo "ARRAY=${ARRAY[#]}"
echo "ARRAY[0]=${ARRAY[0]}"
echo "ARRAY[1]=${ARRAY[1]}"
echo "ARRAY[2]=${ARRAY[2]}"
This will output :
ARRAY=ARRAY_NAME[#]
ARRAY=Val0 Val1 Val2
ARRAY[0]=Val0
ARRAY[1]=Val1
ARRAY[2]=Val2
In bash 4.3, the '-v' test for set variables was introduced. At the same time, 'nameref' declaration was added. These two features together with the indirection operator (!) enable a simplified version of the previous example:
get_value()
{
declare -n var_name=$1
if [[ -v var_name ]]
then
echo "${var_name}"
else
echo "variable with name <${!var_name}> is not set"
fi
}
test=123
get_value test
123
test="\$(echo \"something nasty\")"
get_value test
$(echo "something nasty")
unset test
get_value test
variable with name <test> is not set
As this approach eliminates the need for 'eval', it is safer.
This code checked under bash 5.0.3(1).
modern shells already support arrays( and even associative arrays). So please do use them, and use less of eval.
var1="this is the real value"
array=("$var1")
# or array[0]="$var1"
then when you want to call it , echo ${array[0]}
Based on the answer: https://unix.stackexchange.com/a/111627
###############################################################################
# Summary: Returns the value of a variable given it's name as a string.
# Required Positional Argument:
# variable_name - The name of the variable to return the value of
# Returns: The value if variable exists; otherwise, empty string ("").
###############################################################################
get_value_of()
{
variable_name=$1
variable_value=""
if set | grep -q "^$variable_name="; then
eval variable_value="\$$variable_name"
fi
echo "$variable_value"
}
test=123
get_value_of test
# 123
test="\$(echo \"something nasty\")"
get_value_of test
# $(echo "something nasty")
VALUE=$(eval "echo \$$SOME_VAR_NAME")
Example
SSH_KEY_FILE_PATH_FOO="/tmp/key"
SSH_KEY_FILE_PATH_VAR_NAME_PREFIX="SSH_KEY_FILE_PATH"
SSH_KEY_FILE_PATH_VAR_NAME_SUFFIX="FOO"
SSH_KEY_FILE_PATH=$(eval "echo \$${SSH_KEY_FILE_PATH_VAR_NAME_PREFIX}_${SSH_KEY_FILE_PATH_VAR_NAME_SUFFIX}")
echo "$SSH_KEY_FILE_PATH"
/tmp/key