How to get “FOO=bar.cpp meld a/$FOO b/$FOO” to work the way I expect it to?

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1
down vote

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As far as I understand, to compare the same file in separate directories I should be able to do this:



FOO=bar.cpp meld a/$FOO b/$FOO


However, it doesn't work, as if $FOO is an empty string. This not surprisingly works:



(FOO=bar.cpp && meld a/$FOO b/$FOO)


My question is why does the first example not work?




bash environment-variables




share|improve this question






















  • Are you sure FOO=bar.cpp echo a/$FOO b/$FOO give you a/$FOO b/$FOO? I think no
    – cuonglm
    Feb 27 at 3:59










  • Oops, I had set FOO globally whilst experimenting - I've deleted that example.
    – Ken Y-N
    Feb 27 at 4:03














up vote
1
down vote

favorite












As far as I understand, to compare the same file in separate directories I should be able to do this:



FOO=bar.cpp meld a/$FOO b/$FOO


However, it doesn't work, as if $FOO is an empty string. This not surprisingly works:



(FOO=bar.cpp && meld a/$FOO b/$FOO)


My question is why does the first example not work?




bash environment-variables




share|improve this question






















  • Are you sure FOO=bar.cpp echo a/$FOO b/$FOO give you a/$FOO b/$FOO? I think no
    – cuonglm
    Feb 27 at 3:59










  • Oops, I had set FOO globally whilst experimenting - I've deleted that example.
    – Ken Y-N
    Feb 27 at 4:03












up vote
1
down vote

favorite









up vote
1
down vote

favorite











As far as I understand, to compare the same file in separate directories I should be able to do this:



FOO=bar.cpp meld a/$FOO b/$FOO


However, it doesn't work, as if $FOO is an empty string. This not surprisingly works:



(FOO=bar.cpp && meld a/$FOO b/$FOO)


My question is why does the first example not work?




bash environment-variables




share|improve this question














As far as I understand, to compare the same file in separate directories I should be able to do this:



FOO=bar.cpp meld a/$FOO b/$FOO


However, it doesn't work, as if $FOO is an empty string. This not surprisingly works:



(FOO=bar.cpp && meld a/$FOO b/$FOO)


My question is why does the first example not work?





bash environment-variables





share|improve this question













share|improve this question




share|improve this question








edited Feb 27 at 4:02

























asked Feb 27 at 3:56









Ken Y-N

1375




1375











  • Are you sure FOO=bar.cpp echo a/$FOO b/$FOO give you a/$FOO b/$FOO? I think no
    – cuonglm
    Feb 27 at 3:59










  • Oops, I had set FOO globally whilst experimenting - I've deleted that example.
    – Ken Y-N
    Feb 27 at 4:03
















  • Are you sure FOO=bar.cpp echo a/$FOO b/$FOO give you a/$FOO b/$FOO? I think no
    – cuonglm
    Feb 27 at 3:59










  • Oops, I had set FOO globally whilst experimenting - I've deleted that example.
    – Ken Y-N
    Feb 27 at 4:03















Are you sure FOO=bar.cpp echo a/$FOO b/$FOO give you a/$FOO b/$FOO? I think no
– cuonglm
Feb 27 at 3:59




Are you sure FOO=bar.cpp echo a/$FOO b/$FOO give you a/$FOO b/$FOO? I think no
– cuonglm
Feb 27 at 3:59












Oops, I had set FOO globally whilst experimenting - I've deleted that example.
– Ken Y-N
Feb 27 at 4:03




Oops, I had set FOO globally whilst experimenting - I've deleted that example.
– Ken Y-N
Feb 27 at 4:03










1 Answer
1






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oldest

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up vote
3
down vote



accepted










In:



FOO=bar.cpp meld a/$FOO b/$FOO


It's a Simple Command, so FOO=bar.cpp isn't executed at the time $FOO was expanded.



In:



FOO=bar.cpp && meld a/$FOO b/$FOO


There're 2 commands:



FOO=bar.cpp


and:



meld a/$FOO b/$FOO


When parameter substitution performed in meld a/$FOO b/$FOO, FOO=bar.cpp was executed, so FOO was set to bar.cpp.






share|improve this answer






















  • So basically a Simple Command cannot do what I hoped it could?
    – Ken Y-N
    Feb 27 at 4:35










  • @KenY-N yes, (FOO=bar.cpp; meld "a/$FOO" "b/$FOO") is the way to go if you don't want to FOO retains after command finish. Note the double quotes. You can use any statement termination like ; above instead of &&.
    – cuonglm
    Feb 27 at 4:53










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1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes








up vote
3
down vote



accepted










In:



FOO=bar.cpp meld a/$FOO b/$FOO


It's a Simple Command, so FOO=bar.cpp isn't executed at the time $FOO was expanded.



In:



FOO=bar.cpp && meld a/$FOO b/$FOO


There're 2 commands:



FOO=bar.cpp


and:



meld a/$FOO b/$FOO


When parameter substitution performed in meld a/$FOO b/$FOO, FOO=bar.cpp was executed, so FOO was set to bar.cpp.






share|improve this answer






















  • So basically a Simple Command cannot do what I hoped it could?
    – Ken Y-N
    Feb 27 at 4:35










  • @KenY-N yes, (FOO=bar.cpp; meld "a/$FOO" "b/$FOO") is the way to go if you don't want to FOO retains after command finish. Note the double quotes. You can use any statement termination like ; above instead of &&.
    – cuonglm
    Feb 27 at 4:53














up vote
3
down vote



accepted










In:



FOO=bar.cpp meld a/$FOO b/$FOO


It's a Simple Command, so FOO=bar.cpp isn't executed at the time $FOO was expanded.



In:



FOO=bar.cpp && meld a/$FOO b/$FOO


There're 2 commands:



FOO=bar.cpp


and:



meld a/$FOO b/$FOO


When parameter substitution performed in meld a/$FOO b/$FOO, FOO=bar.cpp was executed, so FOO was set to bar.cpp.






share|improve this answer






















  • So basically a Simple Command cannot do what I hoped it could?
    – Ken Y-N
    Feb 27 at 4:35










  • @KenY-N yes, (FOO=bar.cpp; meld "a/$FOO" "b/$FOO") is the way to go if you don't want to FOO retains after command finish. Note the double quotes. You can use any statement termination like ; above instead of &&.
    – cuonglm
    Feb 27 at 4:53












up vote
3
down vote



accepted







up vote
3
down vote



accepted






In:



FOO=bar.cpp meld a/$FOO b/$FOO


It's a Simple Command, so FOO=bar.cpp isn't executed at the time $FOO was expanded.



In:



FOO=bar.cpp && meld a/$FOO b/$FOO


There're 2 commands:



FOO=bar.cpp


and:



meld a/$FOO b/$FOO


When parameter substitution performed in meld a/$FOO b/$FOO, FOO=bar.cpp was executed, so FOO was set to bar.cpp.






share|improve this answer














In:



FOO=bar.cpp meld a/$FOO b/$FOO


It's a Simple Command, so FOO=bar.cpp isn't executed at the time $FOO was expanded.



In:



FOO=bar.cpp && meld a/$FOO b/$FOO


There're 2 commands:



FOO=bar.cpp


and:



meld a/$FOO b/$FOO


When parameter substitution performed in meld a/$FOO b/$FOO, FOO=bar.cpp was executed, so FOO was set to bar.cpp.







share|improve this answer














share|improve this answer



share|improve this answer








edited Feb 27 at 4:21

























answered Feb 27 at 4:12









cuonglm

97.3k21185278




97.3k21185278











  • So basically a Simple Command cannot do what I hoped it could?
    – Ken Y-N
    Feb 27 at 4:35










  • @KenY-N yes, (FOO=bar.cpp; meld "a/$FOO" "b/$FOO") is the way to go if you don't want to FOO retains after command finish. Note the double quotes. You can use any statement termination like ; above instead of &&.
    – cuonglm
    Feb 27 at 4:53
















  • So basically a Simple Command cannot do what I hoped it could?
    – Ken Y-N
    Feb 27 at 4:35










  • @KenY-N yes, (FOO=bar.cpp; meld "a/$FOO" "b/$FOO") is the way to go if you don't want to FOO retains after command finish. Note the double quotes. You can use any statement termination like ; above instead of &&.
    – cuonglm
    Feb 27 at 4:53















So basically a Simple Command cannot do what I hoped it could?
– Ken Y-N
Feb 27 at 4:35




So basically a Simple Command cannot do what I hoped it could?
– Ken Y-N
Feb 27 at 4:35












@KenY-N yes, (FOO=bar.cpp; meld "a/$FOO" "b/$FOO") is the way to go if you don't want to FOO retains after command finish. Note the double quotes. You can use any statement termination like ; above instead of &&.
– cuonglm
Feb 27 at 4:53




@KenY-N yes, (FOO=bar.cpp; meld "a/$FOO" "b/$FOO") is the way to go if you don't want to FOO retains after command finish. Note the double quotes. You can use any statement termination like ; above instead of &&.
– cuonglm
Feb 27 at 4:53












 

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