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Old 06-23-2020, 06:32 AM   #1
czezz
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[BASH / SED] sed executed in script does not work


This is an example input file:
Code:
cat test.input
9fcfdec4-58a9-4612-b80a-741669bd485d\nfa17fd04-80e1-44c1-bd5e-418bdd332100\nfa17fd04-80e1-44c1-bd5e-418bdd332100
When I use following "sed" on it, it works as expected (each \n is replaced by .)
Code:
# cat test.input | sed -r 's/[\\n]+/./g'
9fcfdec4-58a9-4612-b80a-741669bd485d.fa17fd04-80e1-44c1-bd5e-418bdd332100.fa17fd04-80e1-44c1-bd5e-418bdd332100
BUT, if I try to execute it as script, it fails... what am I doing wrong???

Code:
cat script.sh
-------------
#!/bin/bash
X=`cat test.input | sed -r 's/[\n]+/./g'`
echo $X

# ./script.sh
9fcfdec4-58a9-4612-b80a-741669bd485d\nfa17fd04-80e1-44c1-bd5e-418bdd332100\nfa17fd04-80e1-44c1-bd5e-418bdd332100
 
Old 06-23-2020, 06:52 AM   #2
pan64
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you do not need that cat at all:
Code:
sed 's/\\n/./g' test.input
should work.
Would be better to use $( ) instead of backticks
Code:
X=$(sed 's/\\n/./g' test.input)
echo "$X"
 
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Old 06-23-2020, 07:32 AM   #3
teckk
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Code:
test="9fcfdec4-58a9-4612-b80a-741669bd485d\nfa17fd04-80e1-44c1-bd5e-418bdd332100\nfa17fd04-80e1-44c1-bd5e-418bdd332100"

a=`sed -r 's/[\n]+/./g' <<< "$test"`
b=$(sed -r 's/[\\n]+/./g' <<< "$test")

echo "$a"
9fcfdec4-58a9-4612-b80a-741669bd485d\nfa17fd04-80e1-44c1-bd5e-418bdd332100\nfa17fd04-80e1-44c1-bd5e-418bdd332100

echo "$b"
9fcfdec4-58a9-4612-b80a-741669bd485d.fa17fd04-80e1-44c1-bd5e-418bdd332100.fa17fd04-80e1-44c1-bd5e-418bdd332100
In your script you did not escape the \ in \n
 
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Old 06-23-2020, 07:33 AM   #4
crts
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Quote:
Originally Posted by czezz View Post
This is an example input file:
Code:
cat test.input
9fcfdec4-58a9-4612-b80a-741669bd485d\nfa17fd04-80e1-44c1-bd5e-418bdd332100\nfa17fd04-80e1-44c1-bd5e-418bdd332100
When I use following "sed" on it, it works as expected (each \n is replaced by .)
Code:
# cat test.input | sed -r 's/[\\n]+/./g'
9fcfdec4-58a9-4612-b80a-741669bd485d.fa17fd04-80e1-44c1-bd5e-418bdd332100.fa17fd04-80e1-44c1-bd5e-418bdd332100
BUT, if I try to execute it as script, it fails... what am I doing wrong???

Code:
cat script.sh
-------------
#!/bin/bash
X=`cat test.input | sed -r 's/[\n]+/./g'`
echo $X

# ./script.sh
9fcfdec4-58a9-4612-b80a-741669bd485d\nfa17fd04-80e1-44c1-bd5e-418bdd332100\nfa17fd04-80e1-44c1-bd5e-418bdd332100
From the Bash manpage (emphasis by me):
Code:
   Command Substitution
       Command substitution allows the output of a command to replace the com‐
       mand name.  There are two forms:

              $(command)
       or
              `command`

...

       When the old-style backquote form of substitution  is  used,  backslash
       retains  its  literal  meaning except when followed by $, `, or \.  The
       first backquote not preceded by a backslash terminates the command sub‐
       stitution.   When using the $(command) form, all characters between the
       parentheses make up the command; none are treated specially.
The command substition instructions $() and `` do not behave the same. Two backslashes \\ inside `` are replaced by Bash with a single backslash \. So the sed command
Code:
echo `sed -r 's/[\\n]+/./g' test.input`
that is executed inside `` (I assume you tested the command substitution with \\ as well) is:
Code:
sed -r 's/[\n]+/./g' test.input
If you only use a single backslash \ (as you did inside your script)
Code:
echo `sed -r 's/[\n]+/./g' test.input`
then it is not treated specially by Bash inside `` because it is not followed by either $, `, or \. So the sed command that is executed executed inside `` is same as before:
Code:
sed -r 's/[\n]+/./g' test.input
The character sequence \n, however, is interpreted as a newline by sed and NOT two literal, independent characters \ and n, as is the case in your input. Since there is no actual newline in your input sed will not replace the literal character sequence \ and n.

The backslash is not treated specially by Bash when it is used inside $(), i.e., Bash passes it to sed as is. So you have two possible choices to achieve your goal:
Code:
echo `sed -r 's/[\\\n]+/./g' test.input` # three backslash
or
Code:
echo $(sed -r 's/[\\n]+/./g' test.input) # two backslash
In both cases sed will see a double backslash \\ and interpret it as one literal backslash followed by an n; as is the case in your input.

Last edited by crts; 06-23-2020 at 07:41 AM.
 
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Old 06-23-2020, 07:54 AM   #5
shruggy
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You can do the same with tr though:
Code:
X=$(tr -s '\\n' . <test.input)
 
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Old 06-24-2020, 12:59 AM   #6
MadeInGermany
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Registered: Dec 2011
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The tr is equivalent to
Code:
sed 's/[\\n]/./g' test.input
Any \ or n characters are substituted.

While
Code:
sed 's/\\n/./g' test.input
substitutes any \n string. This cannot be done with tr.
 
  


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