Mixing
Running a Test with expression in a string
Clash Royale CLAN TAG#URR8PPP
up vote
2
down vote
favorite
I am trying to run the test command from a string. The string contains the expression.
TEST="! -e ~/bin/xyz"
if [ `echo "$TEST"` ]; then
echo running "$TEST";
fi
However, the above if condition does evaluate to true but if I plug in the command directly (as below), it evaluates to false.
if [ ! -e ~/bin/xyz ]; then
echo running;
fi
The second snippet's behavior is correct. Can someone help me understand why there is a difference and also how I can correct the first snippet to give me the right result?
linux shell test
edited 4 hours ago
jimmij
29.9k867102
asked 5 hours ago
Ashwin
212
New contributor
Ashwin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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add a comment |Â
up vote
2
down vote
favorite
I am trying to run the test command from a string. The string contains the expression.
TEST="! -e ~/bin/xyz"
if [ `echo "$TEST"` ]; then
echo running "$TEST";
fi
However, the above if condition does evaluate to true but if I plug in the command directly (as below), it evaluates to false.
if [ ! -e ~/bin/xyz ]; then
echo running;
fi
The second snippet's behavior is correct. Can someone help me understand why there is a difference and also how I can correct the first snippet to give me the right result?
linux shell test
edited 4 hours ago
jimmij
29.9k867102
asked 5 hours ago
Ashwin
212
New contributor
Ashwin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
1
I would not recommend storing test expressions in strings, as there isn't really a safe way to handle this. I'd recommend backing up and asking if there's a better way to do it. What's the actual problem you're trying to solve by storing test expressions in a string?
– Gordon Davisson
3 hours ago
add a comment |Â
up vote
2
down vote
favorite
up vote
2
down vote
favorite
I am trying to run the test command from a string. The string contains the expression.
TEST="! -e ~/bin/xyz"
if [ `echo "$TEST"` ]; then
echo running "$TEST";
fi
However, the above if condition does evaluate to true but if I plug in the command directly (as below), it evaluates to false.
if [ ! -e ~/bin/xyz ]; then
echo running;
fi
The second snippet's behavior is correct. Can someone help me understand why there is a difference and also how I can correct the first snippet to give me the right result?
linux shell test
edited 4 hours ago
jimmij
29.9k867102
asked 5 hours ago
Ashwin
212
New contributor
Ashwin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
I am trying to run the test command from a string. The string contains the expression.
TEST="! -e ~/bin/xyz"
if [ `echo "$TEST"` ]; then
echo running "$TEST";
fi
However, the above if condition does evaluate to true but if I plug in the command directly (as below), it evaluates to false.
if [ ! -e ~/bin/xyz ]; then
echo running;
fi
The second snippet's behavior is correct. Can someone help me understand why there is a difference and also how I can correct the first snippet to give me the right result?
linux shell test
linux shell test
edited 4 hours ago
jimmij
29.9k867102
asked 5 hours ago
Ashwin
212
New contributor
Ashwin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 4 hours ago
jimmij
29.9k867102
asked 5 hours ago
Ashwin
212
New contributor
Ashwin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 4 hours ago
jimmij
29.9k867102
edited 4 hours ago
jimmij
29.9k867102
edited 4 hours ago
jimmij
29.9k867102
29.9k867102
asked 5 hours ago
Ashwin
212
New contributor
Ashwin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked 5 hours ago
Ashwin
212
asked 5 hours ago
Ashwin
212
212
New contributor
Ashwin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Ashwin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
Ashwin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
1
I would not recommend storing test expressions in strings, as there isn't really a safe way to handle this. I'd recommend backing up and asking if there's a better way to do it. What's the actual problem you're trying to solve by storing test expressions in a string?
– Gordon Davisson
3 hours ago
add a comment |Â
1
I would not recommend storing test expressions in strings, as there isn't really a safe way to handle this. I'd recommend backing up and asking if there's a better way to do it. What's the actual problem you're trying to solve by storing test expressions in a string?
– Gordon Davisson
3 hours ago
1
1
I would not recommend storing test expressions in strings, as there isn't really a safe way to handle this. I'd recommend backing up and asking if there's a better way to do it. What's the actual problem you're trying to solve by storing test expressions in a string?
– Gordon Davisson
3 hours ago
I would not recommend storing test expressions in strings, as there isn't really a safe way to handle this. I'd recommend backing up and asking if there's a better way to do it. What's the actual problem you're trying to solve by storing test expressions in a string?
– Gordon Davisson
3 hours ago
add a comment |Â
1 Answer
1
active
oldest
votes
up vote
2
down vote
The problem with first example is that you are testing whether the string that echo returns has non-zero length. One solution to this problem is to evaluate tested expression:
TEST="! -e ~/bin/xyz"
if eval "[ $TEST ]"; then
echo running "$TEST";
fi
Notice, that brackets are inside eval, because [ is a command, so we evaluate this command together with variable $TEST as its argument.
edited 3 hours ago
Kusalananda
113k15216344
answered 4 hours ago
jimmij
29.9k867102
1
Warning: as always,evalis an open invitation to weird bugs. It blurs the distinction between data and executable code (even more than it usually is in the shell), so you risk having something you thought was just data (e.g. a filename) get parsed & maybe executed as code. Basically,evalis only safe if you have full control over the string being evaluated (and if you have full control over it, you really shouldn't needeval).
– Gordon Davisson
3 hours ago
add a comment |Â
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
2
down vote
The problem with first example is that you are testing whether the string that echo returns has non-zero length. One solution to this problem is to evaluate tested expression:
TEST="! -e ~/bin/xyz"
if eval "[ $TEST ]"; then
echo running "$TEST";
fi
Notice, that brackets are inside eval, because [ is a command, so we evaluate this command together with variable $TEST as its argument.
edited 3 hours ago
Kusalananda
113k15216344
answered 4 hours ago
jimmij
29.9k867102
1
Warning: as always,evalis an open invitation to weird bugs. It blurs the distinction between data and executable code (even more than it usually is in the shell), so you risk having something you thought was just data (e.g. a filename) get parsed & maybe executed as code. Basically,evalis only safe if you have full control over the string being evaluated (and if you have full control over it, you really shouldn't needeval).
– Gordon Davisson
3 hours ago
add a comment |Â
up vote
2
down vote
The problem with first example is that you are testing whether the string that echo returns has non-zero length. One solution to this problem is to evaluate tested expression:
TEST="! -e ~/bin/xyz"
if eval "[ $TEST ]"; then
echo running "$TEST";
fi
Notice, that brackets are inside eval, because [ is a command, so we evaluate this command together with variable $TEST as its argument.
edited 3 hours ago
Kusalananda
113k15216344
answered 4 hours ago
jimmij
29.9k867102
1
Warning: as always,evalis an open invitation to weird bugs. It blurs the distinction between data and executable code (even more than it usually is in the shell), so you risk having something you thought was just data (e.g. a filename) get parsed & maybe executed as code. Basically,evalis only safe if you have full control over the string being evaluated (and if you have full control over it, you really shouldn't needeval).
– Gordon Davisson
3 hours ago
add a comment |Â
up vote
2
down vote
up vote
2
down vote
The problem with first example is that you are testing whether the string that echo returns has non-zero length. One solution to this problem is to evaluate tested expression:
TEST="! -e ~/bin/xyz"
if eval "[ $TEST ]"; then
echo running "$TEST";
fi
Notice, that brackets are inside eval, because [ is a command, so we evaluate this command together with variable $TEST as its argument.
edited 3 hours ago
Kusalananda
113k15216344
answered 4 hours ago
jimmij
29.9k867102
The problem with first example is that you are testing whether the string that echo returns has non-zero length. One solution to this problem is to evaluate tested expression:
TEST="! -e ~/bin/xyz"
if eval "[ $TEST ]"; then
echo running "$TEST";
fi
Notice, that brackets are inside eval, because [ is a command, so we evaluate this command together with variable $TEST as its argument.
edited 3 hours ago
Kusalananda
113k15216344
answered 4 hours ago
jimmij
29.9k867102
edited 3 hours ago
Kusalananda
113k15216344
edited 3 hours ago
Kusalananda
113k15216344
edited 3 hours ago
Kusalananda
113k15216344
113k15216344
answered 4 hours ago
jimmij
29.9k867102
answered 4 hours ago
jimmij
29.9k867102
answered 4 hours ago
jimmij
29.9k867102
29.9k867102
1
Warning: as always,evalis an open invitation to weird bugs. It blurs the distinction between data and executable code (even more than it usually is in the shell), so you risk having something you thought was just data (e.g. a filename) get parsed & maybe executed as code. Basically,evalis only safe if you have full control over the string being evaluated (and if you have full control over it, you really shouldn't needeval).
– Gordon Davisson
3 hours ago
add a comment |Â
1
Warning: as always,evalis an open invitation to weird bugs. It blurs the distinction between data and executable code (even more than it usually is in the shell), so you risk having something you thought was just data (e.g. a filename) get parsed & maybe executed as code. Basically,evalis only safe if you have full control over the string being evaluated (and if you have full control over it, you really shouldn't needeval).
– Gordon Davisson
3 hours ago
1
1
Warning: as always,
eval is an open invitation to weird bugs. It blurs the distinction between data and executable code (even more than it usually is in the shell), so you risk having something you thought was just data (e.g. a filename) get parsed & maybe executed as code. Basically, eval is only safe if you have full control over the string being evaluated (and if you have full control over it, you really shouldn't need eval).– Gordon Davisson
3 hours ago
Warning: as always,
eval is an open invitation to weird bugs. It blurs the distinction between data and executable code (even more than it usually is in the shell), so you risk having something you thought was just data (e.g. a filename) get parsed & maybe executed as code. Basically, eval is only safe if you have full control over the string being evaluated (and if you have full control over it, you really shouldn't need eval).– Gordon Davisson
3 hours ago
add a comment |Â
Ashwin is a new contributor. Be nice, and check out our Code of Conduct.
Ashwin is a new contributor. Be nice, and check out our Code of Conduct.
Ashwin is a new contributor. Be nice, and check out our Code of Conduct.
Ashwin is a new contributor. Be nice, and check out our Code of Conduct.
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1
I would not recommend storing test expressions in strings, as there isn't really a safe way to handle this. I'd recommend backing up and asking if there's a better way to do it. What's the actual problem you're trying to solve by storing test expressions in a string?
– Gordon Davisson
3 hours ago