Testing Fgrep Gives Same Results As Grep
I'm trying to learn how fgrep differs from grep. However in my test results, there is no difference. Apparently fgrep matches on strings and ignores regex. So I put this to the test and there was absolutely nothing that a basic fgrep can do, that grep can't. So I can't move on, I need to understand why I have the results below, and what the difference between fgrep is, since I categorically cannot see any difference in any test result.
$ cat testfile
subscribe|unsubscribe
@lp1n3
$ grep 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
$ fgrep 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
$ grep '@lp1n3' testfile
@lp1n3
$ fgrep '@lp1n3' testfile
@lp1n3
grep
asked Feb 28 at 13:00
john smithjohn smith
1,06542041
add a comment |
I'm trying to learn how fgrep differs from grep. However in my test results, there is no difference. Apparently fgrep matches on strings and ignores regex. So I put this to the test and there was absolutely nothing that a basic fgrep can do, that grep can't. So I can't move on, I need to understand why I have the results below, and what the difference between fgrep is, since I categorically cannot see any difference in any test result.
$ cat testfile
subscribe|unsubscribe
@lp1n3
$ grep 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
$ fgrep 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
$ grep '@lp1n3' testfile
@lp1n3
$ fgrep '@lp1n3' testfile
@lp1n3
grep
asked Feb 28 at 13:00
john smithjohn smith
1,06542041
add a comment |
I'm trying to learn how fgrep differs from grep. However in my test results, there is no difference. Apparently fgrep matches on strings and ignores regex. So I put this to the test and there was absolutely nothing that a basic fgrep can do, that grep can't. So I can't move on, I need to understand why I have the results below, and what the difference between fgrep is, since I categorically cannot see any difference in any test result.
$ cat testfile
subscribe|unsubscribe
@lp1n3
$ grep 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
$ fgrep 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
$ grep '@lp1n3' testfile
@lp1n3
$ fgrep '@lp1n3' testfile
@lp1n3
grep
asked Feb 28 at 13:00
john smithjohn smith
1,06542041
I'm trying to learn how fgrep differs from grep. However in my test results, there is no difference. Apparently fgrep matches on strings and ignores regex. So I put this to the test and there was absolutely nothing that a basic fgrep can do, that grep can't. So I can't move on, I need to understand why I have the results below, and what the difference between fgrep is, since I categorically cannot see any difference in any test result.
$ cat testfile
subscribe|unsubscribe
@lp1n3
$ grep 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
$ fgrep 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
$ grep '@lp1n3' testfile
@lp1n3
$ fgrep '@lp1n3' testfile
@lp1n3
grep
grep
asked Feb 28 at 13:00
john smithjohn smith
1,06542041
asked Feb 28 at 13:00
john smithjohn smith
1,06542041
asked Feb 28 at 13:00
john smithjohn smith
1,06542041
asked Feb 28 at 13:00
john smithjohn smith
1,06542041
asked Feb 28 at 13:00
john smithjohn smith
1,06542041
1,06542041
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
You may find it helpful to add the -o switch to make it easier to see what's happening:
$ grep -o 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
Here we are using grep in Basic Regular Expression mode, where | is not special (it would need to be escaped | to mean OR), so the pattern is matched as a single string
$ grep -oE 'subscribe|unsubscribe' testfile
subscribe
unsubscribe
Here we have switched to Extended Regular Expression (ERE) mode, where | is a regex special character, so we are matching two patterns, subscribe and unsubscribe
$ grep -oF 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
Now we are matching in fixed string mode (like fgrep) and as expected | is not special - just like BRE.
fgrep is an (officially, deprecated) equivalent of grep -F so it behaves exactly the same:
$ fgrep -o 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
As far as I know, @ isn't special in either BRE or ERE, so will always give the same result as fgrep or grep -F.
answered Feb 28 at 13:12
steeldriversteeldriver
70.5k11114187
Not sure if I made my question clear. My question is what's the difference between fgrep and grep, because my testing shows that there is no difference.
– john smith
Feb 28 at 13:30
@johnsmith sorry I should have mentioned -fgrepis equivalent togrep -F(I have edited the answer to make that clearer)
– steeldriver
Feb 28 at 13:35
add a comment |
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1 Answer
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active
oldest
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1 Answer
1
active
oldest
votes
active
oldest
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active
oldest
votes
You may find it helpful to add the -o switch to make it easier to see what's happening:
$ grep -o 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
Here we are using grep in Basic Regular Expression mode, where | is not special (it would need to be escaped | to mean OR), so the pattern is matched as a single string
$ grep -oE 'subscribe|unsubscribe' testfile
subscribe
unsubscribe
Here we have switched to Extended Regular Expression (ERE) mode, where | is a regex special character, so we are matching two patterns, subscribe and unsubscribe
$ grep -oF 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
Now we are matching in fixed string mode (like fgrep) and as expected | is not special - just like BRE.
fgrep is an (officially, deprecated) equivalent of grep -F so it behaves exactly the same:
$ fgrep -o 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
As far as I know, @ isn't special in either BRE or ERE, so will always give the same result as fgrep or grep -F.
answered Feb 28 at 13:12
steeldriversteeldriver
70.5k11114187
Not sure if I made my question clear. My question is what's the difference between fgrep and grep, because my testing shows that there is no difference.
– john smith
Feb 28 at 13:30
@johnsmith sorry I should have mentioned -fgrepis equivalent togrep -F(I have edited the answer to make that clearer)
– steeldriver
Feb 28 at 13:35
add a comment |
You may find it helpful to add the -o switch to make it easier to see what's happening:
$ grep -o 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
Here we are using grep in Basic Regular Expression mode, where | is not special (it would need to be escaped | to mean OR), so the pattern is matched as a single string
$ grep -oE 'subscribe|unsubscribe' testfile
subscribe
unsubscribe
Here we have switched to Extended Regular Expression (ERE) mode, where | is a regex special character, so we are matching two patterns, subscribe and unsubscribe
$ grep -oF 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
Now we are matching in fixed string mode (like fgrep) and as expected | is not special - just like BRE.
fgrep is an (officially, deprecated) equivalent of grep -F so it behaves exactly the same:
$ fgrep -o 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
As far as I know, @ isn't special in either BRE or ERE, so will always give the same result as fgrep or grep -F.
answered Feb 28 at 13:12
steeldriversteeldriver
70.5k11114187
Not sure if I made my question clear. My question is what's the difference between fgrep and grep, because my testing shows that there is no difference.
– john smith
Feb 28 at 13:30
@johnsmith sorry I should have mentioned -fgrepis equivalent togrep -F(I have edited the answer to make that clearer)
– steeldriver
Feb 28 at 13:35
add a comment |
You may find it helpful to add the -o switch to make it easier to see what's happening:
$ grep -o 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
Here we are using grep in Basic Regular Expression mode, where | is not special (it would need to be escaped | to mean OR), so the pattern is matched as a single string
$ grep -oE 'subscribe|unsubscribe' testfile
subscribe
unsubscribe
Here we have switched to Extended Regular Expression (ERE) mode, where | is a regex special character, so we are matching two patterns, subscribe and unsubscribe
$ grep -oF 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
Now we are matching in fixed string mode (like fgrep) and as expected | is not special - just like BRE.
fgrep is an (officially, deprecated) equivalent of grep -F so it behaves exactly the same:
$ fgrep -o 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
As far as I know, @ isn't special in either BRE or ERE, so will always give the same result as fgrep or grep -F.
answered Feb 28 at 13:12
steeldriversteeldriver
70.5k11114187
You may find it helpful to add the -o switch to make it easier to see what's happening:
$ grep -o 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
Here we are using grep in Basic Regular Expression mode, where | is not special (it would need to be escaped | to mean OR), so the pattern is matched as a single string
$ grep -oE 'subscribe|unsubscribe' testfile
subscribe
unsubscribe
Here we have switched to Extended Regular Expression (ERE) mode, where | is a regex special character, so we are matching two patterns, subscribe and unsubscribe
$ grep -oF 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
Now we are matching in fixed string mode (like fgrep) and as expected | is not special - just like BRE.
fgrep is an (officially, deprecated) equivalent of grep -F so it behaves exactly the same:
$ fgrep -o 'subscribe|unsubscribe' testfile
subscribe|unsubscribe
As far as I know, @ isn't special in either BRE or ERE, so will always give the same result as fgrep or grep -F.
answered Feb 28 at 13:12
steeldriversteeldriver
70.5k11114187
edited Feb 28 at 13:34
answered Feb 28 at 13:12
steeldriversteeldriver
70.5k11114187
answered Feb 28 at 13:12
steeldriversteeldriver
70.5k11114187
answered Feb 28 at 13:12
steeldriversteeldriver
70.5k11114187
70.5k11114187
Not sure if I made my question clear. My question is what's the difference between fgrep and grep, because my testing shows that there is no difference.
– john smith
Feb 28 at 13:30
@johnsmith sorry I should have mentioned -fgrepis equivalent togrep -F(I have edited the answer to make that clearer)
– steeldriver
Feb 28 at 13:35
add a comment |
Not sure if I made my question clear. My question is what's the difference between fgrep and grep, because my testing shows that there is no difference.
– john smith
Feb 28 at 13:30
@johnsmith sorry I should have mentioned -fgrepis equivalent togrep -F(I have edited the answer to make that clearer)
– steeldriver
Feb 28 at 13:35
Not sure if I made my question clear. My question is what's the difference between fgrep and grep, because my testing shows that there is no difference.
– john smith
Feb 28 at 13:30
Not sure if I made my question clear. My question is what's the difference between fgrep and grep, because my testing shows that there is no difference.
– john smith
Feb 28 at 13:30
@johnsmith sorry I should have mentioned -
fgrep is equivalent to grep -F (I have edited the answer to make that clearer)– steeldriver
Feb 28 at 13:35
@johnsmith sorry I should have mentioned -
fgrep is equivalent to grep -F (I have edited the answer to make that clearer)– steeldriver
Feb 28 at 13:35
add a comment |
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