After reading the Bash man pages and with respect to this post, I am still having trouble understanding what exactly the eval command does and which would be its typical uses.
For example, if we do:
$ set -- one two three # Sets $1 $2 $3
$ echo $1
one
$ n=1
$ echo ${$n} ## First attempt to echo $1 using brackets fails
bash: ${$n}: bad substitution
$ echo $($n) ## Second attempt to echo $1 using parentheses fails
bash: 1: command not found
$ eval echo \${$n} ## Third attempt to echo $1 using 'eval' succeeds
one
What exactly is happening here and how do the dollar sign and the backslash tie into the problem?
eval takes a string as its argument, and evaluates it as if you'd typed that string on a command line. (If you pass several arguments, they are first joined with spaces between them.)
${$n} is a syntax error in bash. Inside the braces, you can only have a variable name, with some possible prefix and suffixes, but you can't have arbitrary bash syntax and in particular you can't use variable expansion. There is a way of saying “the value of the variable whose name is in this variable”, though:
echo ${!n}
one
$(…) runs the command specified inside the parentheses in a subshell (i.e. in a separate process that inherits all settings such as variable values from the current shell), and gathers its output. So echo $($n) runs $n as a shell command, and displays its output. Since $n evaluates to 1, $($n) attempts to run the command 1, which does not exist.
eval echo \${$n} runs the parameters passed to eval. After expansion, the parameters are echo and ${1}. So eval echo \${$n} runs the command echo ${1}.
Note that most of the time, you must use double quotes around variable substitutions and command substitutions (i.e. anytime there's a $): "$foo", "$(foo)". Always put double quotes around variable and command substitutions, unless you know you need to leave them off. Without the double quotes, the shell performs field splitting (i.e. it splits value of the variable or the output from the command into separate words) and then treats each word as a wildcard pattern. For example:
$ ls
file1 file2 otherfile
$ set -- 'f* *'
$ echo "$1"
f* *
$ echo $1
file1 file2 file1 file2 otherfile
$ n=1
$ eval echo \${$n}
file1 file2 file1 file2 otherfile
$eval echo \"\${$n}\"
f* *
$ echo "${!n}"
f* *
eval is not used very often. In some shells, the most common use is to obtain the value of a variable whose name is not known until runtime. In bash, this is not necessary thanks to the ${!VAR} syntax. eval is still useful when you need to construct a longer command containing operators, reserved words, etc.
Simply think of eval as "evaluating your expression one additional time before execution"
eval echo \${$n} becomes echo $1 after the first round of evaluation. Three changes to notice:
The \$ became $ (The backslash is needed, otherwise it tries to evaluate ${$n}, which means a variable named {$n}, which is not allowed)
$n was evaluated to 1
The eval disappeared
In the second round, it is basically echo $1 which can be directly executed.
So eval <some command> will first evaluate <some command> (by evaluate here I mean substitute variables, replace escaped characters with the correct ones etc.), and then run the resultant expression once again.
eval is used when you want to dynamically create variables, or to read outputs from programs specifically designed to be read like this. See Eval command and security issues for examples. The link also contains some typical ways in which eval is used, and the risks associated with it.
In my experience, a "typical" use of eval is for running commands that generate shell commands to set environment variables.
Perhaps you have a system that uses a collection of environment variables, and you have a script or program that determines which ones should be set and their values. Whenever you run a script or program, it runs in a forked process, so anything it does directly to environment variables is lost when it exits. But that script or program can send the export commands to standard output.
Without eval, you would need to redirect standard output to a temporary file, source the temporary file, and then delete it. With eval, you can just:
eval "$(script-or-program)"
Note the quotes are important. Take this (contrived) example:
# activate.sh
echo 'I got activated!'
# test.py
print("export foo=bar/baz/womp")
print(". activate.sh")
$ eval $(python test.py)
bash: export: `.': not a valid identifier
bash: export: `activate.sh': not a valid identifier
$ eval "$(python test.py)"
I got activated!
The eval statement tells the shell to take eval’s arguments as commands and run them through the command-line. It is useful in a situation like below:
In your script if you are defining a command into a variable and later on you want to use that command then you should use eval:
a="ls | more"
$a
Output:
bash: command not found: ls | more
The above command didn't work as ls tried to list file with name pipe (|) and more. But these files are not there:
eval $a
Output:
file.txt
mailids
remote_cmd.sh
sample.txt
tmp
Update: Some people say one should -never- use eval. I disagree. I think the risk arises when corrupt input can be passed to eval. However there are many common situations where that is not a risk, and therefore it is worth knowing how to use eval in any case. This stackoverflow answer explains the risks of eval and alternatives to eval. Ultimately it is up to the user to determine if/when eval is safe and efficient to use.
The bash eval statement allows you to execute lines of code calculated or acquired, by your bash script.
Perhaps the most straightforward example would be a bash program that opens another bash script as a text file, reads each line of text, and uses eval to execute them in order. That's essentially the same behavior as the bash source statement, which is what one would use, unless it was necessary to perform some kind of transformation (e.g. filtering or substitution) on the content of the imported script.
I rarely have needed eval, but I have found it useful to read or write variables whose names were contained in strings assigned to other variables. For example, to perform actions on sets of variables, while keeping the code footprint small and avoiding redundancy.
eval is conceptually simple. However, the strict syntax of the bash language, and the bash interpreter's parsing order can be nuanced and make eval appear cryptic and difficult to use or understand. Here are the essentials:
The argument passed to eval is a string expression that is calculated at runtime. eval will execute the final parsed result of its argument as an actual line of code in your script.
Syntax and parsing order are stringent. If the result isn't an executable line of bash code, in scope of your script, the program will crash on the eval statement as it tries to execute garbage.
When testing you can replace the eval statement with echo and look at what is displayed. If it is legitimate code in the current context, running it through eval will work.
The following examples may help clarify how eval works...
Example 1:
eval statement in front of 'normal' code is a NOP
$ eval a=b
$ eval echo $a
b
In the above example, the first eval statements has no purpose and can be eliminated. eval is pointless in the first line because there is no dynamic aspect to the code, i.e. it already parsed into the final lines of bash code, thus it would be identical as a normal statement of code in the bash script. The 2nd eval is pointless too, because, although there is a parsing step converting $a to its literal string equivalent, there is no indirection (e.g. no referencing via string value of an actual bash noun or bash-held script variable), so it would behave identically as a line of code without the eval prefix.
Example 2:
Perform var assignment using var names passed as string values.
$ key="mykey"
$ val="myval"
$ eval $key=$val
$ echo $mykey
myval
If you were to echo $key=$val, the output would be:
mykey=myval
That, being the final result of string parsing, is what will be executed by eval, hence the result of the echo statement at the end...
Example 3:
Adding more indirection to Example 2
$ keyA="keyB"
$ valA="valB"
$ keyB="that"
$ valB="amazing"
$ eval eval \$$keyA=\$$valA
$ echo $that
amazing
The above is a bit more complicated than the previous example, relying more heavily on the parsing-order and peculiarities of bash. The eval line would roughly get parsed internally in the following order (note the following statements are pseudocode, not real code, just to attempt to show how the statement would get broken down into steps internally to arrive at the final result).
eval eval \$$keyA=\$$valA # substitution of $keyA and $valA by interpreter
eval eval \$keyB=\$valB # convert '$' + name-strings to real vars by eval
eval $keyB=$valB # substitution of $keyB and $valB by interpreter
eval that=amazing # execute string literal 'that=amazing' by eval
If the assumed parsing order doesn't explain what eval is doing enough, the third example may describe the parsing in more detail to help clarify what is going on.
Example 4:
Discover whether vars, whose names are contained in strings, themselves contain string values.
a="User-provided"
b="Another user-provided optional value"
c=""
myvarname_a="a"
myvarname_b="b"
myvarname_c="c"
for varname in "myvarname_a" "myvarname_b" "myvarname_c"; do
eval varval=\$$varname
if [ -z "$varval" ]; then
read -p "$varname? " $varname
fi
done
In the first iteration:
varname="myvarname_a"
Bash parses the argument to eval, and eval sees literally this at runtime:
eval varval=\$$myvarname_a
The following pseudocode attempts to illustrate how bash interprets the above line of real code, to arrive at the final value executed by eval. (the following lines descriptive, not exact bash code):
1. eval varval="\$" + "$varname" # This substitution resolved in eval statement
2. .................. "$myvarname_a" # $myvarname_a previously resolved by for-loop
3. .................. "a" # ... to this value
4. eval "varval=$a" # This requires one more parsing step
5. eval varval="User-provided" # Final result of parsing (eval executes this)
Once all the parsing is done, the result is what is executed, and its effect is obvious, demonstrating there is nothing particularly mysterious about eval itself, and the complexity is in the parsing of its argument.
varval="User-provided"
The remaining code in the example above simply tests to see if the value assigned to $varval is null, and, if so, prompts the user to provide a value.
I originally intentionally never learned how to use eval, because most people will recommend to stay away from it like the plague. However I recently discovered a use case that made me facepalm for not recognizing it sooner.
If you have cron jobs that you want to run interactively to test, you might view the contents of the file with cat, and copy and paste the cron job to run it. Unfortunately, this involves touching the mouse, which is a sin in my book.
Lets say you have a cron job at /etc/cron.d/repeatme with the contents:
*/10 * * * * root program arg1 arg2
You cant execute this as a script with all the junk in front of it, but we can use cut to get rid of all the junk, wrap it in a subshell, and execute the string with eval
eval $( cut -d ' ' -f 6- /etc/cron.d/repeatme)
The cut command only prints out the 6th field of the file, delimited by spaces. Eval then executes that command.
I used a cron job here as an example, but the concept is to format text from stdout, and then evaluate that text.
The use of eval in this case is not insecure, because we know exactly what we will be evaluating before hand.
I've recently had to use eval to force multiple brace expansions to be evaluated in the order I needed. Bash does multiple brace expansions from left to right, so
xargs -I_ cat _/{11..15}/{8..5}.jpg
expands to
xargs -I_ cat _/11/8.jpg _/11/7.jpg _/11/6.jpg _/11/5.jpg _/12/8.jpg _/12/7.jpg _/12/6.jpg _/12/5.jpg _/13/8.jpg _/13/7.jpg _/13/6.jpg _/13/5.jpg _/14/8.jpg _/14/7.jpg _/14/6.jpg _/14/5.jpg _/15/8.jpg _/15/7.jpg _/15/6.jpg _/15/5.jpg
but I needed the second brace expansion done first, yielding
xargs -I_ cat _/11/8.jpg _/12/8.jpg _/13/8.jpg _/14/8.jpg _/15/8.jpg _/11/7.jpg _/12/7.jpg _/13/7.jpg _/14/7.jpg _/15/7.jpg _/11/6.jpg _/12/6.jpg _/13/6.jpg _/14/6.jpg _/15/6.jpg _/11/5.jpg _/12/5.jpg _/13/5.jpg _/14/5.jpg _/15/5.jpg
The best I could come up with to do that was
xargs -I_ cat $(eval echo _/'{11..15}'/{8..5}.jpg)
This works because the single quotes protect the first set of braces from expansion during the parsing of the eval command line, leaving them to be expanded by the subshell invoked by eval.
There may be some cunning scheme involving nested brace expansions that allows this to happen in one step, but if there is I'm too old and stupid to see it.
You asked about typical uses.
One common complaint about shell scripting is that you (allegedly) can't pass by reference to get values back out of functions.
But actually, via "eval", you can pass by reference. The callee can pass back a list of variable assignments to be evaluated by the caller. It is pass by reference because the caller can allowed to specify the name(s) of the result variable(s) - see example below. Error results can be passed back standard names like errno and errstr.
Here is an example of passing by reference in bash:
#!/bin/bash
isint()
{
re='^[-]?[0-9]+$'
[[ $1 =~ $re ]]
}
#args 1: name of result variable, 2: first addend, 3: second addend
iadd()
{
if isint ${2} && isint ${3} ; then
echo "$1=$((${2}+${3}));errno=0"
return 0
else
echo "errstr=\"Error: non-integer argument to iadd $*\" ; errno=329"
return 1
fi
}
var=1
echo "[1] var=$var"
eval $(iadd var A B)
if [[ $errno -ne 0 ]]; then
echo "errstr=$errstr"
echo "errno=$errno"
fi
echo "[2] var=$var (unchanged after error)"
eval $(iadd var $var 1)
if [[ $errno -ne 0 ]]; then
echo "errstr=$errstr"
echo "errno=$errno"
fi
echo "[3] var=$var (successfully changed)"
The output looks like this:
[1] var=1
errstr=Error: non-integer argument to iadd var A B
errno=329
[2] var=1 (unchanged after error)
[3] var=2 (successfully changed)
There is almost unlimited band width in that text output! And there are more possibilities if the multiple output lines are used: e.g., the first line could be used for variable assignments, the second for continuous 'stream of thought', but that's beyond the scope of this post.
In the question:
who | grep $(tty | sed s:/dev/::)
outputs errors claiming that files a and tty do not exist. I understood this to mean that tty is not being interpreted before execution of grep, but instead that bash passed tty as a parameter to grep, which interpreted it as a file name.
There is also a situation of nested redirection, which should be handled by matched parentheses which should specify a child process, but bash is primitively a word separator, creating parameters to be sent to a program, therefore parentheses are not matched first, but interpreted as seen.
I got specific with grep, and specified the file as a parameter instead of using a pipe. I also simplified the base command, passing output from a command as a file, so that i/o piping would not be nested:
grep $(tty | sed s:/dev/::) <(who)
works well.
who | grep $(echo pts/3)
is not really desired, but eliminates the nested pipe and also works well.
In conclusion, bash does not seem to like nested pipping. It is important to understand that bash is not a new-wave program written in a recursive manner. Instead, bash is an old 1,2,3 program, which has been appended with features. For purposes of assuring backward compatibility, the initial manner of interpretation has never been modified. If bash was rewritten to first match parentheses, how many bugs would be introduced into how many bash programs? Many programmers love to be cryptic.
As clearlight has said, "(p)erhaps the most straightforward example would be a bash program that opens another bash script as a text file, reads each line of text, and uses eval to execute them in order". I'm no expert, but the textbook I'm currently reading (Shell-Programmierung by Jürgen Wolf) points to one particular use of this that I think would be a valuable addition to the set of potential use cases collected here.
For debugging purposes, you may want to go through your script line by line (pressing Enter for each step). You could use eval to execute every line by trapping the DEBUG signal (which I think is sent after every line):
trap 'printf "$LINENO :-> " ; read line ; eval $line' DEBUG
I like the "evaluating your expression one additional time before execution" answer, and would like to clarify with another example.
var="\"par1 par2\""
echo $var # prints nicely "par1 par2"
function cntpars() {
echo " > Count: $#"
echo " > Pars : $*"
echo " > par1 : $1"
echo " > par2 : $2"
if [[ $# = 1 && $1 = "par1 par2" ]]; then
echo " > PASS"
else
echo " > FAIL"
return 1
fi
}
# Option 1: Will Pass
echo "eval \"cntpars \$var\""
eval "cntpars $var"
# Option 2: Will Fail, with curious results
echo "cntpars \$var"
cntpars $var
The curious results in option 2 are that we would have passed two parameters as follows:
First parameter: "par1
Second parameter: par2"
How is that for counter intuitive? The additional eval will fix that.
It was adapted from another answer on How can I reference a file for variables using Bash?
This is almost the exact same question as in this post, except that I do not want to use eval.
Quick question short, I want to execute the command echo aaa | grep a by first storing it in a string variable Command='echo aaa | grep a', and then running it without using eval.
In the post above, the selected answer used eval. That works for me too. What concerns me a lot is that there are plenty of warnings about eval below, followed by some attempts to circumvent it. However, none of them are able to solve my problem (essentially the OP's). I have commented below their attempts, but since it has been there for a long time, I suppose it is better to post the question again with the restriction of not using eval.
Concrete Example
What I want is a shell script that runs my command when I am happy:
#!/bin/bash
# This script run-this-if.sh runs the commands when I am happy
# Warning: the following script does not work (on nose)
if [ "$1" == "I-am-happy" ]; then
"$2"
fi
$ run-if.sh I-am-happy [insert-any-command]
Your sample usage can't ever work with an assignment, because assignments are scoped to the current process and its children. Because there's no reason to try to support assignments, things get suddenly far easier:
#!/bin/sh
if [ "$1" = "I-am-happy" ]; then
shift; "$#"
fi
This then can later use all the usual techniques to run shell pipelines, such as:
run-if-happy "$happiness" \
sh -c 'echo "$1" | grep "$2"' _ "$untrustedStringOne" "$untrustedStringTwo"
Note that we're passing the execve() syscall an argv with six elements:
sh (the shell to run; change to bash etc if preferred)
-c (telling the shell that the following argument is the code for it to run)
echo "$1" | grep "$2" (the code for sh to parse)
_ (a constant which becomes $0)
...whatever the shell variable untrustedStringOne contains... (which becomes $1)
...whatever the shell variable untrustedStringTwo contains... (which becomes $2)
Note here that echo "$1" | grep "$2" is a constant string -- in single-quotes, with no parameter expansions or command substitutions -- and that untrusted values are passed into the slots that fill in $1 and $2, out-of-band from the code being evaluated; this is essential to have any kind of increase in security over what eval would give you.
When my bash scripts start getting complex, I usually break them up into functions. This applies especially to complex pipes, as a sequence of complicated pipe commands (e.g. containing while-loops) can quickly become hard to read. Even more so when parallelization is wanted, where xargs is very helpful.
I know that I can export functions to a subshell with export -f, thus in a simple case I can do
export -f myfunction
some-command | xargs -Iline bash -c "myfunction 'line'"
but if the myfunction depends on other functions this becomes hard to maintain -- every time the function changes such that the functions needed by the subshell for executing myfunction change, the export statement would have to be changed -- that seems pretty error prone.
Is there some general way to export functions for use by subshells? I was thinking about something along the lines of an "export all defined functions" command, which would then allow a code structure like
main() { ... }
func1 () { ... }
func2 () { ... }
<export all functions>
main "$#"
Your question asks only about exporting functions. This is easy in bash, see below.
Your question title/subject implies using functions in xargs, as though they were a script;
I don't know that xargs can "call" a bash function directly, but you can of course wrap
your use of the exported function(s) in a script called by xargs, see below.
First, a function to list functions. User functions by default and -v to list all functions:
lsfns () {
case "$1" in
-v | v*)
# verbose:
set | grep '()' --color=always
;;
*)
declare -F | cut -d" " -f3 | egrep -v "^_"
;;
esac
}
Next a function to export all user functions:
exportfns () { export -f $(lsfns); }
or just put export -f $(lsfns) in your .bashrc.
Example script doit.sh:
#!/bin/bash
lsfns "$#" # make use of function exported by parent shell :)
Example command line (after chmod a+rx doit.sh):
echo -v | xargs doit.sh
Compare with
echo "" | xargs doit.sh
EDIT 1: responding further to kdb's comment/answer below ("running into situations where exporting functions does not work at all"):
Export of shell functions is not Posix compatible - i.e. it only works with Bash and presumably other shells such as Zsh, Ksh etc.
That is, in Dash, and "standard" Posix shells not providing "export -f", we cannot export functions, AND if we export a function say in Bash, then run a script which starts with the sh-bang e.g. "#!/bin/dash", that script will NOT be able to use the "exported" functions from the parent shell, since functions exported to the "process environment" by Bash, are not recognised by Dash.
EDIT 2: responding further to OP comment "but if the myfunction depends on other functions this becomes hard to maintain":
This is probably a situation where one could make good use of the shell source command (alias "."), e.g.:
. ~/etc/my-functions.sh; myMain ...
And similarly, if you "live" in functions rather than script files, e.g. by calling myMain when you need to, then the first line of this function can be to source your function library;
since this would be excess overhead in the "running a script regularly" case, myMain becomes the command-line stub function, which (re)loads your function library, and calls the actuallyDoit function (which would also be called from inside your script, if you have a script file).
Enjoy
Zenaan
This seems to work to print all the function names. It feels fragile, so test it out
declare -f | grep -oP '^\S+(?=\s*\(\))'
export -f $(compgen -A function)
Since receiving the answer, I found many cases where a different technique proofed preferable: Making the script invoke itself. A simple example would be
# Print hash and disk usage for each argument
if [[ $1 == run ]]; then
shift 1
printf "%s\0" "$#" | xargs -0 -n 1 -P "$NUMBER_OF_PROCESSORS" -- "$0" printpar
elif [[ $1 == printpar ]]; then
echo ":: $(cat "$2" | sha1sum) $(du -sh "$2")"
else
echo "Invalid first parameter '$1'"
exit 1
fi
In real-world examples I'd either make assumptions about the arguments (e.g. using __SUCH__ a shape for self-call keywords) or hide recursive invocations behind an undocumented --command-line-switch.
Exporting functions is generally more elegant, but for large numbers of function it can get prohibitively slow and I remember running into issues, where it failed entirely.
Consider the following ksh script "myquery.ksh"
#/usr/bin/env ksh -eu
PROCESS_TYP=$1
PROCESS_DT=$2
#Generate a query
makeSQL()
{
local numfiles=0
local query='SEL \\* FROM TABLE_1_'
case "$1" in
'ABC') query="${query}ABC" ; numfiles=1 ;;
'DEF') query="${query}DEF" ; numfiles=7 ;;
esac
query="${query}_V WHERE LOAD_DT='${2}';"
printf "$query\n"
eval $3="${query}"
eval $4=$numfiles
return 0
}
makeSQL $PROCESS_TYP $PROCESS_DT qry num_files
printf "QUERY: $qry\n"
printf "NUMFILES: $num_files\n"
In the above code, the eval $3="${query}" statement never works correctly. In all circumstances it seems to attempt to glob the "*" in the "local query="... statement.
./myquery.ksh ABC 2011-01-01
It always returns a message like this:
./myquery.ksh: line 17: \*: command not found
I am pretty sure this is my own user error with how I am applying eval in this situation but have tried nearly every alternate syntax construct for doing that eval $3 assignment but cannot manage to make this work.
I have tagged this as bash because I am pretty sure it would also behave identically there as well...
Did you mean:
eval "$3=\$query"
eval "$4=\$numfiles"
Explanation: in your example you want to pass variable names to the function, and these variables are then set. So the statements should look like this:
qry=$query
num_files=$numfiles
Now the question is how to put these strings together. The left hand side comes from the arguments, for example "$3". The right hand side contains a $ that must not be interpreted too early. Therefore I wrote it as \$.
I always pass a single string to the eval command, since that gives me the most reliable results. Not eval echo foo, but eval "echo foo" instead.
Alternatively you could have written:
eval "$3"='$query'
eval "$3="'$'"query"
But the first form I suggested seems the simplest to me.
Here is a problem:
In my bash scripts I want to source several file with some checks, so I have:
if [ -r foo ] ; then
source foo
else
logger -t $0 -p crit "unable to source foo"
exit 1
fi
if [ -r bar ] ; then
source bar
else
logger -t $0 -p crit "unable to source bar"
exit 1
fi
# ... etc ...
Naively I tried to create a function that do:
function safe_source() {
if [ -r $1 ] ; then
source $1
else
logger -t $0 -p crit "unable to source $1"
exit 1
fi
}
safe_source foo
safe_source bar
# ... etc ...
But there is a snag there.
If one of the files foo, bar, etc. have a global such as --
declare GLOBAL_VAR=42
-- it will effectively become:
function safe_source() {
# ...
declare GLOBAL_VAR=42
# ...
}
thus a global variable becomes local.
The question:
An alias in bash seems too weak for this, so must I unroll the above function, and repeat myself, or is there a more elegant approach?
... and yes, I agree that Python, Perl, Ruby would make my life easier, but when working with legacy system, one doesn't always have the privilege of choosing the best tool.
It's a bit late answer, but now declare supports a -g parameter, which makes a variable global (when used inside function). Same works in sourced file.
If you need a global (read exported) variable, use:
declare -g DATA="Hello World, meow!"
Yes, Bash's 'eval' command can make this work. 'eval' isn't very elegant, and it sometimes can be difficult to understand and debug code that uses it. I usually try to avoid it, but Bash often leaves you with no other choice (like the situation that prompted your question). You'll have to weigh the pros and cons of using 'eval' for yourself.
Some background on 'eval'
If you're not familiar with 'eval', it's a Bash built-in command that expects you to pass it a string as its parameter. 'eval' dynamically interprets and executes your string as a command in its own right, in the current shell context and scope. Here's a basic example of a common use (dynamic variable assignment):
$> a_var_name="color"
$> eval ${a_var_name}="blue"
$> echo -e "The color is ${color}."
The color is blue.
See the Advanced Bash Scripting Guide for more info and examples: http://tldp.org/LDP/abs/html/internal.html#EVALREF
Solving your 'source' problem
To make 'eval' handle your sourcing issue, you'd start by rewriting your function, 'safe_source()'. Instead of actually executing the command, 'safe_source()' should just PRINT the command as a string on STDOUT:
function safe_source() { echo eval " \
if [ -r $1 ] ; then \
source $1 ; \
else \
logger -t $0 -p crit \"unable to source $1\" ; \
exit 1 ; \
fi \
"; }
Also, you'll need to change your function invocations, slightly, to actually execute the 'eval' command:
`safe_source foo`
`safe_source bar`
(Those are backticks/backquotes, BTW.)
How it works
In short:
We converted the function into a command-string emitter.
Our new function emits an 'eval' command invocation string.
Our new backticks call the new function in a subshell context, returning the 'eval' command string output by the function back up to the main script.
The main script executes the 'eval' command string, captured by the backticks, in the main script context.
The 'eval' command string re-parses and executes the 'eval' command string in the main script context, running the whole if-then-else block, including (if the file exists) executing the 'source' command.
It's kind of complex. Like I said, 'eval' is not exactly elegant. In particular, there are a couple of special things you should notice about the changes we made:
The entire IF-THEN-ELSE block has becomes one whole double-quoted string, with backslashes at the end of each line "hiding" the newlines.
Some of the shell special characters like '"') have been backslash-escaped, while others ('$') have been left un-escaped.
'echo eval' has been prepended to the whole command string.
Extra semicolons have been appended to all of the lines where a command gets executed to terminate them, a role that the (now-hidden) newlines originally performed.
The function invocation has been wrapped in backticks.
Most of these changes are motived by the fact that 'eval' won't handle newlines. It can only deal with multiple commands if we combine them into a single line delimited by semicolons, instead. The new function's line breaks are purely a formatting convenience for the human eye.
If any of this is unclear, run your script with Bash's '-x' (debug execution) flag turned on, and that should give you a better picture of exactly what's happening. For instance, in the function context, the function actually produces the 'eval' command string by executing this command:
echo eval ' if [ -r <INCL_FILE> ] ; then source <INCL_FILE> ; else logger -t <SCRIPT_NAME> -p crit "unable to source <INCL_FILE>" ; exit 1 ; fi '
Then, in the main context, the main script executes this:
eval if '[' -r <INCL_FILE> ']' ';' then source <INCL_FILE> ';' else logger -t <SCRIPT_NAME> -p crit '"unable' to source '<INCL_FILE>"' ';' exit 1 ';' fi
Finally, again in the main context, the eval command executes these two commands if exists:
'[' -r <INCL_FILE> ']'
source <INCL_FILE>
Good luck.
declare inside a function makes the variable local to that function. export affects the environment of child processes not the current or parent environments.
You can set the values of your variables inside the functions and do the declare -r, declare -i or declare -ri after the fact.