Question about order of variable and pathname expansion in Bash
Bash expands variables before expanding pathnames (globbing). Assuming that it did it the other way round (i.e. expand pathnames before expanding variables), is it possible to write a bash script that uses both variable expansion and path expansion, such that the result is the same irrespective of the order of expansion.
I have come up with the following simple bash script.
#!/bin/bash
# Create a variable called myvar1
myvar1=~
echo $myvar1
# Use both path expansion and variable expansion with the ls command
# Assuming the variable was expanded first, the out should be the same as if the path was expanded first. Is my assumption correct?
ls $myvar1/*.txt
Can someone verify that my script answers the question satisfactorily?
bash scripting variable
edited yesterday
Rui F Ribeiro
38.8k1479128
asked Mar 23 '17 at 23:00
Automation Zombie
172
add a comment |
Bash expands variables before expanding pathnames (globbing). Assuming that it did it the other way round (i.e. expand pathnames before expanding variables), is it possible to write a bash script that uses both variable expansion and path expansion, such that the result is the same irrespective of the order of expansion.
I have come up with the following simple bash script.
#!/bin/bash
# Create a variable called myvar1
myvar1=~
echo $myvar1
# Use both path expansion and variable expansion with the ls command
# Assuming the variable was expanded first, the out should be the same as if the path was expanded first. Is my assumption correct?
ls $myvar1/*.txt
Can someone verify that my script answers the question satisfactorily?
bash scripting variable
edited yesterday
Rui F Ribeiro
38.8k1479128
asked Mar 23 '17 at 23:00
Automation Zombie
172
1
Not quite: consider a directory containing a file called$myvar1, and you dols *. This will expand to potentially many files, one of which is$myvar1. Then (in this hypothetical world) this variable will be expanded to~and you will see the contents of your home directory (assuming~expansion still occurs after that, otherwise it will attempt to show a file literally called~)
– Fox
Mar 23 '17 at 23:07
add a comment |
Bash expands variables before expanding pathnames (globbing). Assuming that it did it the other way round (i.e. expand pathnames before expanding variables), is it possible to write a bash script that uses both variable expansion and path expansion, such that the result is the same irrespective of the order of expansion.
I have come up with the following simple bash script.
#!/bin/bash
# Create a variable called myvar1
myvar1=~
echo $myvar1
# Use both path expansion and variable expansion with the ls command
# Assuming the variable was expanded first, the out should be the same as if the path was expanded first. Is my assumption correct?
ls $myvar1/*.txt
Can someone verify that my script answers the question satisfactorily?
bash scripting variable
edited yesterday
Rui F Ribeiro
38.8k1479128
asked Mar 23 '17 at 23:00
Automation Zombie
172
Bash expands variables before expanding pathnames (globbing). Assuming that it did it the other way round (i.e. expand pathnames before expanding variables), is it possible to write a bash script that uses both variable expansion and path expansion, such that the result is the same irrespective of the order of expansion.
I have come up with the following simple bash script.
#!/bin/bash
# Create a variable called myvar1
myvar1=~
echo $myvar1
# Use both path expansion and variable expansion with the ls command
# Assuming the variable was expanded first, the out should be the same as if the path was expanded first. Is my assumption correct?
ls $myvar1/*.txt
Can someone verify that my script answers the question satisfactorily?
bash scripting variable
bash scripting variable
edited yesterday
Rui F Ribeiro
38.8k1479128
asked Mar 23 '17 at 23:00
Automation Zombie
172
edited yesterday
Rui F Ribeiro
38.8k1479128
asked Mar 23 '17 at 23:00
Automation Zombie
172
edited yesterday
Rui F Ribeiro
38.8k1479128
edited yesterday
Rui F Ribeiro
38.8k1479128
edited yesterday
Rui F Ribeiro
38.8k1479128
38.8k1479128
asked Mar 23 '17 at 23:00
Automation Zombie
172
asked Mar 23 '17 at 23:00
Automation Zombie
172
asked Mar 23 '17 at 23:00
Automation Zombie
172
172
1
Not quite: consider a directory containing a file called$myvar1, and you dols *. This will expand to potentially many files, one of which is$myvar1. Then (in this hypothetical world) this variable will be expanded to~and you will see the contents of your home directory (assuming~expansion still occurs after that, otherwise it will attempt to show a file literally called~)
– Fox
Mar 23 '17 at 23:07
add a comment |
1
Not quite: consider a directory containing a file called$myvar1, and you dols *. This will expand to potentially many files, one of which is$myvar1. Then (in this hypothetical world) this variable will be expanded to~and you will see the contents of your home directory (assuming~expansion still occurs after that, otherwise it will attempt to show a file literally called~)
– Fox
Mar 23 '17 at 23:07
1
1
Not quite: consider a directory containing a file called
$myvar1, and you do ls *. This will expand to potentially many files, one of which is $myvar1. Then (in this hypothetical world) this variable will be expanded to ~ and you will see the contents of your home directory (assuming ~ expansion still occurs after that, otherwise it will attempt to show a file literally called ~)– Fox
Mar 23 '17 at 23:07
Not quite: consider a directory containing a file called
$myvar1, and you do ls *. This will expand to potentially many files, one of which is $myvar1. Then (in this hypothetical world) this variable will be expanded to ~ and you will see the contents of your home directory (assuming ~ expansion still occurs after that, otherwise it will attempt to show a file literally called ~)– Fox
Mar 23 '17 at 23:07
add a comment |
1 Answer
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ls $myvar1/*.txt
If pathnames were expanded before variables, this would look for filenames matching *.txt in a directory called $myvar1, which you probably wouldn't have. After that, it would expand $myvar1 to the path to your home directory, leaving /home/username/*.txt, which is not the same as expanding the path first and then looking for filenames ending in .txt in the home directory.
In general, since file names can contain $ signs, I think the order of expansion will always matter. Consider any script using a glob like * in a directory containing a file called $PATH.
The opposite, variables containing glob characters, is easier to work around, since you control the values of your variables.
answered Mar 24 '17 at 10:37
ilkkachu
55.3k782150
Makes sense. Thanks!
– Automation Zombie
Mar 27 '17 at 16:55
add a comment |
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ls $myvar1/*.txt
If pathnames were expanded before variables, this would look for filenames matching *.txt in a directory called $myvar1, which you probably wouldn't have. After that, it would expand $myvar1 to the path to your home directory, leaving /home/username/*.txt, which is not the same as expanding the path first and then looking for filenames ending in .txt in the home directory.
In general, since file names can contain $ signs, I think the order of expansion will always matter. Consider any script using a glob like * in a directory containing a file called $PATH.
The opposite, variables containing glob characters, is easier to work around, since you control the values of your variables.
answered Mar 24 '17 at 10:37
ilkkachu
55.3k782150
Makes sense. Thanks!
– Automation Zombie
Mar 27 '17 at 16:55
add a comment |
ls $myvar1/*.txt
If pathnames were expanded before variables, this would look for filenames matching *.txt in a directory called $myvar1, which you probably wouldn't have. After that, it would expand $myvar1 to the path to your home directory, leaving /home/username/*.txt, which is not the same as expanding the path first and then looking for filenames ending in .txt in the home directory.
In general, since file names can contain $ signs, I think the order of expansion will always matter. Consider any script using a glob like * in a directory containing a file called $PATH.
The opposite, variables containing glob characters, is easier to work around, since you control the values of your variables.
answered Mar 24 '17 at 10:37
ilkkachu
55.3k782150
Makes sense. Thanks!
– Automation Zombie
Mar 27 '17 at 16:55
add a comment |
ls $myvar1/*.txt
If pathnames were expanded before variables, this would look for filenames matching *.txt in a directory called $myvar1, which you probably wouldn't have. After that, it would expand $myvar1 to the path to your home directory, leaving /home/username/*.txt, which is not the same as expanding the path first and then looking for filenames ending in .txt in the home directory.
In general, since file names can contain $ signs, I think the order of expansion will always matter. Consider any script using a glob like * in a directory containing a file called $PATH.
The opposite, variables containing glob characters, is easier to work around, since you control the values of your variables.
answered Mar 24 '17 at 10:37
ilkkachu
55.3k782150
ls $myvar1/*.txt
If pathnames were expanded before variables, this would look for filenames matching *.txt in a directory called $myvar1, which you probably wouldn't have. After that, it would expand $myvar1 to the path to your home directory, leaving /home/username/*.txt, which is not the same as expanding the path first and then looking for filenames ending in .txt in the home directory.
In general, since file names can contain $ signs, I think the order of expansion will always matter. Consider any script using a glob like * in a directory containing a file called $PATH.
The opposite, variables containing glob characters, is easier to work around, since you control the values of your variables.
answered Mar 24 '17 at 10:37
ilkkachu
55.3k782150
answered Mar 24 '17 at 10:37
ilkkachu
55.3k782150
answered Mar 24 '17 at 10:37
ilkkachu
55.3k782150
answered Mar 24 '17 at 10:37
ilkkachu
55.3k782150
55.3k782150
Makes sense. Thanks!
– Automation Zombie
Mar 27 '17 at 16:55
add a comment |
Makes sense. Thanks!
– Automation Zombie
Mar 27 '17 at 16:55
Makes sense. Thanks!
– Automation Zombie
Mar 27 '17 at 16:55
Makes sense. Thanks!
– Automation Zombie
Mar 27 '17 at 16:55
add a comment |
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1
Not quite: consider a directory containing a file called
$myvar1, and you dols *. This will expand to potentially many files, one of which is$myvar1. Then (in this hypothetical world) this variable will be expanded to~and you will see the contents of your home directory (assuming~expansion still occurs after that, otherwise it will attempt to show a file literally called~)– Fox
Mar 23 '17 at 23:07