Set variable conditionally on command line [duplicate]
up vote
3
down vote
favorite
This question already has an answer here:
How are parentheses interpreted at the command line?
2 answers
I want to set a variable if my condition is true on my Ubuntu system.
This proves that my if-statement is correct:
$ (if [ 1 == 1 ]; then echo "hi there"; fi);
hi there
This proves that I can set variables:
$ a=1
$ echo $a
1
This shows that setting a variable in the if-statement DOES NOT work:
$ (if [ 1 == 1 ]; then a=2; fi);
$ echo $a
1
Any ideas why? All my google research indicates that it should work like this...
shell command-line
marked as duplicate by RalfFriedl, Thomas, mosvy, Jeff Schaller, muru Nov 21 at 1:27
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
add a comment |
up vote
3
down vote
favorite
This question already has an answer here:
How are parentheses interpreted at the command line?
2 answers
I want to set a variable if my condition is true on my Ubuntu system.
This proves that my if-statement is correct:
$ (if [ 1 == 1 ]; then echo "hi there"; fi);
hi there
This proves that I can set variables:
$ a=1
$ echo $a
1
This shows that setting a variable in the if-statement DOES NOT work:
$ (if [ 1 == 1 ]; then a=2; fi);
$ echo $a
1
Any ideas why? All my google research indicates that it should work like this...
shell command-line
marked as duplicate by RalfFriedl, Thomas, mosvy, Jeff Schaller, muru Nov 21 at 1:27
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
add a comment |
up vote
3
down vote
favorite
up vote
3
down vote
favorite
This question already has an answer here:
How are parentheses interpreted at the command line?
2 answers
I want to set a variable if my condition is true on my Ubuntu system.
This proves that my if-statement is correct:
$ (if [ 1 == 1 ]; then echo "hi there"; fi);
hi there
This proves that I can set variables:
$ a=1
$ echo $a
1
This shows that setting a variable in the if-statement DOES NOT work:
$ (if [ 1 == 1 ]; then a=2; fi);
$ echo $a
1
Any ideas why? All my google research indicates that it should work like this...
shell command-line
This question already has an answer here:
How are parentheses interpreted at the command line?
2 answers
I want to set a variable if my condition is true on my Ubuntu system.
This proves that my if-statement is correct:
$ (if [ 1 == 1 ]; then echo "hi there"; fi);
hi there
This proves that I can set variables:
$ a=1
$ echo $a
1
This shows that setting a variable in the if-statement DOES NOT work:
$ (if [ 1 == 1 ]; then a=2; fi);
$ echo $a
1
Any ideas why? All my google research indicates that it should work like this...
This question already has an answer here:
How are parentheses interpreted at the command line?
2 answers
shell command-line
shell command-line
edited Nov 20 at 16:19
Barmar
6,8381122
6,8381122
asked Nov 20 at 13:33
Ron
1214
1214
marked as duplicate by RalfFriedl, Thomas, mosvy, Jeff Schaller, muru Nov 21 at 1:27
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
marked as duplicate by RalfFriedl, Thomas, mosvy, Jeff Schaller, muru Nov 21 at 1:27
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
add a comment |
add a comment |
3 Answers
3
active
oldest
votes
up vote
10
down vote
accepted
The (...)
part of your command is your problem. The parentheses create a separate subshell. The subshell will inherit the environment from its parent shell, but variables set inside it will not retain their new values once the subshell exits. This also goes for any other changes to the environment inside the subshell, including changing directories, setting shell options etc.
Therefore, remove the subshell:
if [ 1 = 1 ]; then a=2; fi
echo "$a"
add a comment |
up vote
5
down vote
This proves that setting a variable in a sub-shell has no lasting effect
(if [ 1 == 1 ]; then a=2; fi);
echo $a
produces
1
same as
(a=2)
echo $a
produces
1
Solution remove the parenthesis.
if [ 1 == 1 ]; then a=2; fi;
echo $a
produces
2
or if you need a sub-shell
(
if [ 1 == 1 ]; then a=2; fi;
echo $a
)
produces
2
add a comment |
up vote
-2
down vote
It done by below method and it worked fine
_example ~]# if [[ 1 == 1 ]]; then echo "praveen"; a=2; echo $a; fi| sed '1i================================n output'
================================
output
praveen
2
4
Can you explain what thesed
command is doing and why you are using it?
– Kusalananda
Nov 20 at 18:49
1
What does this add beyond Kusalananda's very similar, earlier answer and ctrl-alt-delor's earlier answer ?
– Jeff Schaller
Nov 20 at 22:37
add a comment |
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
10
down vote
accepted
The (...)
part of your command is your problem. The parentheses create a separate subshell. The subshell will inherit the environment from its parent shell, but variables set inside it will not retain their new values once the subshell exits. This also goes for any other changes to the environment inside the subshell, including changing directories, setting shell options etc.
Therefore, remove the subshell:
if [ 1 = 1 ]; then a=2; fi
echo "$a"
add a comment |
up vote
10
down vote
accepted
The (...)
part of your command is your problem. The parentheses create a separate subshell. The subshell will inherit the environment from its parent shell, but variables set inside it will not retain their new values once the subshell exits. This also goes for any other changes to the environment inside the subshell, including changing directories, setting shell options etc.
Therefore, remove the subshell:
if [ 1 = 1 ]; then a=2; fi
echo "$a"
add a comment |
up vote
10
down vote
accepted
up vote
10
down vote
accepted
The (...)
part of your command is your problem. The parentheses create a separate subshell. The subshell will inherit the environment from its parent shell, but variables set inside it will not retain their new values once the subshell exits. This also goes for any other changes to the environment inside the subshell, including changing directories, setting shell options etc.
Therefore, remove the subshell:
if [ 1 = 1 ]; then a=2; fi
echo "$a"
The (...)
part of your command is your problem. The parentheses create a separate subshell. The subshell will inherit the environment from its parent shell, but variables set inside it will not retain their new values once the subshell exits. This also goes for any other changes to the environment inside the subshell, including changing directories, setting shell options etc.
Therefore, remove the subshell:
if [ 1 = 1 ]; then a=2; fi
echo "$a"
edited Nov 20 at 18:50
answered Nov 20 at 13:44
Kusalananda
118k16222360
118k16222360
add a comment |
add a comment |
up vote
5
down vote
This proves that setting a variable in a sub-shell has no lasting effect
(if [ 1 == 1 ]; then a=2; fi);
echo $a
produces
1
same as
(a=2)
echo $a
produces
1
Solution remove the parenthesis.
if [ 1 == 1 ]; then a=2; fi;
echo $a
produces
2
or if you need a sub-shell
(
if [ 1 == 1 ]; then a=2; fi;
echo $a
)
produces
2
add a comment |
up vote
5
down vote
This proves that setting a variable in a sub-shell has no lasting effect
(if [ 1 == 1 ]; then a=2; fi);
echo $a
produces
1
same as
(a=2)
echo $a
produces
1
Solution remove the parenthesis.
if [ 1 == 1 ]; then a=2; fi;
echo $a
produces
2
or if you need a sub-shell
(
if [ 1 == 1 ]; then a=2; fi;
echo $a
)
produces
2
add a comment |
up vote
5
down vote
up vote
5
down vote
This proves that setting a variable in a sub-shell has no lasting effect
(if [ 1 == 1 ]; then a=2; fi);
echo $a
produces
1
same as
(a=2)
echo $a
produces
1
Solution remove the parenthesis.
if [ 1 == 1 ]; then a=2; fi;
echo $a
produces
2
or if you need a sub-shell
(
if [ 1 == 1 ]; then a=2; fi;
echo $a
)
produces
2
This proves that setting a variable in a sub-shell has no lasting effect
(if [ 1 == 1 ]; then a=2; fi);
echo $a
produces
1
same as
(a=2)
echo $a
produces
1
Solution remove the parenthesis.
if [ 1 == 1 ]; then a=2; fi;
echo $a
produces
2
or if you need a sub-shell
(
if [ 1 == 1 ]; then a=2; fi;
echo $a
)
produces
2
edited Nov 20 at 14:12
Zanna
2,5061023
2,5061023
answered Nov 20 at 13:42
ctrl-alt-delor
10.2k41955
10.2k41955
add a comment |
add a comment |
up vote
-2
down vote
It done by below method and it worked fine
_example ~]# if [[ 1 == 1 ]]; then echo "praveen"; a=2; echo $a; fi| sed '1i================================n output'
================================
output
praveen
2
4
Can you explain what thesed
command is doing and why you are using it?
– Kusalananda
Nov 20 at 18:49
1
What does this add beyond Kusalananda's very similar, earlier answer and ctrl-alt-delor's earlier answer ?
– Jeff Schaller
Nov 20 at 22:37
add a comment |
up vote
-2
down vote
It done by below method and it worked fine
_example ~]# if [[ 1 == 1 ]]; then echo "praveen"; a=2; echo $a; fi| sed '1i================================n output'
================================
output
praveen
2
4
Can you explain what thesed
command is doing and why you are using it?
– Kusalananda
Nov 20 at 18:49
1
What does this add beyond Kusalananda's very similar, earlier answer and ctrl-alt-delor's earlier answer ?
– Jeff Schaller
Nov 20 at 22:37
add a comment |
up vote
-2
down vote
up vote
-2
down vote
It done by below method and it worked fine
_example ~]# if [[ 1 == 1 ]]; then echo "praveen"; a=2; echo $a; fi| sed '1i================================n output'
================================
output
praveen
2
It done by below method and it worked fine
_example ~]# if [[ 1 == 1 ]]; then echo "praveen"; a=2; echo $a; fi| sed '1i================================n output'
================================
output
praveen
2
edited Nov 20 at 22:54
ilkkachu
54.1k782147
54.1k782147
answered Nov 20 at 18:19
Praveen Kumar BS
1,136138
1,136138
4
Can you explain what thesed
command is doing and why you are using it?
– Kusalananda
Nov 20 at 18:49
1
What does this add beyond Kusalananda's very similar, earlier answer and ctrl-alt-delor's earlier answer ?
– Jeff Schaller
Nov 20 at 22:37
add a comment |
4
Can you explain what thesed
command is doing and why you are using it?
– Kusalananda
Nov 20 at 18:49
1
What does this add beyond Kusalananda's very similar, earlier answer and ctrl-alt-delor's earlier answer ?
– Jeff Schaller
Nov 20 at 22:37
4
4
Can you explain what the
sed
command is doing and why you are using it?– Kusalananda
Nov 20 at 18:49
Can you explain what the
sed
command is doing and why you are using it?– Kusalananda
Nov 20 at 18:49
1
1
What does this add beyond Kusalananda's very similar, earlier answer and ctrl-alt-delor's earlier answer ?
– Jeff Schaller
Nov 20 at 22:37
What does this add beyond Kusalananda's very similar, earlier answer and ctrl-alt-delor's earlier answer ?
– Jeff Schaller
Nov 20 at 22:37
add a comment |