Related
I wrote small script under Debian Linux 11 that should check how many instances of application is currently running and what is power usage of GPU cards.
I save it under name test , and she is started every time I access instance over SSH
#!/bin/sh
clear
a=$(nvidia-smi -q -i 0 | grep "Power Draw" | cut -c45-50)
b=$(nvidia-smi -q -i 1 | grep "Power Draw" | cut -c45-50)
c=$(nvidia-smi -q -i 2 | grep "Power Draw" | cut -c45-50)
d=$(nvidia-smi -q -i 3 | grep "Power Draw" | cut -c45-50)
zet=$( echo "$a" + "$b" + "$c" + "$d" | bc -l )
echo "SYSTEM DRAW:" "$zet"
if [ "${zet}" -gt 150 ]; then
echo WARRNING - SYSTEM DRAW LOW
else
echo OK
fi
sleep 8
exit
All I need to add is this line
x=${x%.*}
That convert decimal number in number without decimals and script works perfect.
You could add set -x right before the part you want to debug, which will show you a debug of what is happening in bash. and stop it by inserting after that set +x
like:
set -x
power=$150
echo "SYSTEM DRAW :" $total
if [ $total \> $power ] ; then # escape > otherwise it redirects output
I do not think you are setting the value of $150
The script might be failing if one of the compared values is not set.. so you should initialize your variables to be let' say equal to 0 as a default at the beginning of your script, or via bash
like:
power=${150:-10} # if $150 does not have a value or empty, the default value of $power will be set to `10`
So many possible issues but you can compare possibly decimal values using bc:
if [ "$(echo "$total > $power" | bc)" = 1 ]; then
One problem is that [ (and [[) do string comparisons, and you want a numeric comparison. For that you want to use ((, so something like
if (( $total > 150 )); then
echo "WARNING..."
else
echo "OK"
fi
will work better. Otherwise a total of 1000 would print ok, and 90 would give a warning.
Other problems:
$150 gives you the value of a variable called 150 -- you probably want to remove the $
Outside of special forms like [[ and ((, a > will do an output redirection, rather than being a 'normal' argument to a command such as [.
As the comments recommend, use shellcheck, however, I think your intention is not what you wrote.
Try this, create a script (i.e. myscripy)
#! /bin/bash
power=$150
echo "power=$power"
then run it
./myscript abc
and prints
power=abc50
which is probably very different than what you expect.
That is because power will take the first argument's value ($1) and append 50.
If you wanted argument number 150 (also very unlikely), you should write
power=${150}
but if you want just the number
power=150
edit
based on the comment, use
zet=$(bc <<<"$a+$b+$c+$d")
to calculate zet if the values are floating points.
For the comparison use
if [ "$(bc <<<"$zet>150")" == 1 ]; then
...
fi
How do I iterate over a range of numbers in Bash when the range is given by a variable?
I know I can do this (called "sequence expression" in the Bash documentation):
for i in {1..5}; do echo $i; done
Which gives:
1
2
3
4
5
Yet, how can I replace either of the range endpoints with a variable? This doesn't work:
END=5
for i in {1..$END}; do echo $i; done
Which prints:
{1..5}
for i in $(seq 1 $END); do echo $i; done
edit: I prefer seq over the other methods because I can actually remember it ;)
The seq method is the simplest, but Bash has built-in arithmetic evaluation.
END=5
for ((i=1;i<=END;i++)); do
echo $i
done
# ==> outputs 1 2 3 4 5 on separate lines
The for ((expr1;expr2;expr3)); construct works just like for (expr1;expr2;expr3) in C and similar languages, and like other ((expr)) cases, Bash treats them as arithmetic.
discussion
Using seq is fine, as Jiaaro suggested. Pax Diablo suggested a Bash loop to avoid calling a subprocess, with the additional advantage of being more memory friendly if $END is too large. Zathrus spotted a typical bug in the loop implementation, and also hinted that since i is a text variable, continuous conversions to-and-fro numbers are performed with an associated slow-down.
integer arithmetic
This is an improved version of the Bash loop:
typeset -i i END
let END=5 i=1
while ((i<=END)); do
echo $i
…
let i++
done
If the only thing that we want is the echo, then we could write echo $((i++)).
ephemient taught me something: Bash allows for ((expr;expr;expr)) constructs. Since I've never read the whole man page for Bash (like I've done with the Korn shell (ksh) man page, and that was a long time ago), I missed that.
So,
typeset -i i END # Let's be explicit
for ((i=1;i<=END;++i)); do echo $i; done
seems to be the most memory-efficient way (it won't be necessary to allocate memory to consume seq's output, which could be a problem if END is very large), although probably not the “fastest”.
the initial question
eschercycle noted that the {a..b} Bash notation works only with literals; true, accordingly to the Bash manual. One can overcome this obstacle with a single (internal) fork() without an exec() (as is the case with calling seq, which being another image requires a fork+exec):
for i in $(eval echo "{1..$END}"); do
Both eval and echo are Bash builtins, but a fork() is required for the command substitution (the $(…) construct).
Here is why the original expression didn't work.
From man bash:
Brace expansion is performed before
any other expansions, and any
characters special to other
expansions are preserved in the
result. It is strictly textual. Bash
does not apply any syntactic
interpretation to the context of
the expansion or the text between the
braces.
So, brace expansion is something done early as a purely textual macro operation, before parameter expansion.
Shells are highly optimized hybrids between macro processors and more formal programming languages. In order to optimize the typical use cases, the language is made rather more complex and some limitations are accepted.
Recommendation
I would suggest sticking with Posix1 features. This means using for i in <list>; do, if the list is already known, otherwise, use while or seq, as in:
#!/bin/sh
limit=4
i=1; while [ $i -le $limit ]; do
echo $i
i=$(($i + 1))
done
# Or -----------------------
for i in $(seq 1 $limit); do
echo $i
done
1. Bash is a great shell and I use it interactively, but I don't put bash-isms into my scripts. Scripts might need a faster shell, a more secure one, a more embedded-style one. They might need to run on whatever is installed as /bin/sh, and then there are all the usual pro-standards arguments. Remember shellshock, aka bashdoor?
The POSIX way
If you care about portability, use the example from the POSIX standard:
i=2
end=5
while [ $i -le $end ]; do
echo $i
i=$(($i+1))
done
Output:
2
3
4
5
Things which are not POSIX:
(( )) without dollar, although it is a common extension as mentioned by POSIX itself.
[[. [ is enough here. See also: What is the difference between single and double square brackets in Bash?
for ((;;))
seq (GNU Coreutils)
{start..end}, and that cannot work with variables as mentioned by the Bash manual.
let i=i+1: POSIX 7 2. Shell Command Language does not contain the word let, and it fails on bash --posix 4.3.42
the dollar at i=$i+1 might be required, but I'm not sure. POSIX 7 2.6.4 Arithmetic Expansion says:
If the shell variable x contains a value that forms a valid integer constant, optionally including a leading plus or minus sign, then the arithmetic expansions "$((x))" and "$(($x))" shall return the same value.
but reading it literally that does not imply that $((x+1)) expands since x+1 is not a variable.
You can use
for i in $(seq $END); do echo $i; done
Another layer of indirection:
for i in $(eval echo {1..$END}); do
∶
I've combined a few of the ideas here and measured performance.
TL;DR Takeaways:
seq and {..} are really fast
for and while loops are slow
$( ) is slow
for (( ; ; )) loops are slower
$(( )) is even slower
Worrying about N numbers in memory (seq or {..}) is silly (at least up to 1 million.)
These are not conclusions. You would have to look at the C code behind each of these to draw conclusions. This is more about how we tend to use each of these mechanisms for looping over code. Most single operations are close enough to being the same speed that it's not going to matter in most cases. But a mechanism like for (( i=1; i<=1000000; i++ )) is many operations as you can visually see. It is also many more operations per loop than you get from for i in $(seq 1 1000000). And that may not be obvious to you, which is why doing tests like this is valuable.
Demos
# show that seq is fast
$ time (seq 1 1000000 | wc)
1000000 1000000 6888894
real 0m0.227s
user 0m0.239s
sys 0m0.008s
# show that {..} is fast
$ time (echo {1..1000000} | wc)
1 1000000 6888896
real 0m1.778s
user 0m1.735s
sys 0m0.072s
# Show that for loops (even with a : noop) are slow
$ time (for i in {1..1000000} ; do :; done | wc)
0 0 0
real 0m3.642s
user 0m3.582s
sys 0m0.057s
# show that echo is slow
$ time (for i in {1..1000000} ; do echo $i; done | wc)
1000000 1000000 6888896
real 0m7.480s
user 0m6.803s
sys 0m2.580s
$ time (for i in $(seq 1 1000000) ; do echo $i; done | wc)
1000000 1000000 6888894
real 0m7.029s
user 0m6.335s
sys 0m2.666s
# show that C-style for loops are slower
$ time (for (( i=1; i<=1000000; i++ )) ; do echo $i; done | wc)
1000000 1000000 6888896
real 0m12.391s
user 0m11.069s
sys 0m3.437s
# show that arithmetic expansion is even slower
$ time (i=1; e=1000000; while [ $i -le $e ]; do echo $i; i=$(($i+1)); done | wc)
1000000 1000000 6888896
real 0m19.696s
user 0m18.017s
sys 0m3.806s
$ time (i=1; e=1000000; while [ $i -le $e ]; do echo $i; ((i=i+1)); done | wc)
1000000 1000000 6888896
real 0m18.629s
user 0m16.843s
sys 0m3.936s
$ time (i=1; e=1000000; while [ $i -le $e ]; do echo $((i++)); done | wc)
1000000 1000000 6888896
real 0m17.012s
user 0m15.319s
sys 0m3.906s
# even a noop is slow
$ time (i=1; e=1000000; while [ $((i++)) -le $e ]; do :; done | wc)
0 0 0
real 0m12.679s
user 0m11.658s
sys 0m1.004s
If you need it prefix than you might like this
for ((i=7;i<=12;i++)); do echo `printf "%2.0d\n" $i |sed "s/ /0/"`;done
that will yield
07
08
09
10
11
12
If you're on BSD / OS X you can use jot instead of seq:
for i in $(jot $END); do echo $i; done
This works fine in bash:
END=5
i=1 ; while [[ $i -le $END ]] ; do
echo $i
((i = i + 1))
done
There are many ways to do this, however the ones I prefer is given below
Using seq
Synopsis from man seq
$ seq [-w] [-f format] [-s string] [-t string] [first [incr]] last
Syntax
Full command
seq first incr last
first is starting number in the sequence [is optional, by default:1]
incr is increment [is optional, by default:1]
last is the last number in the sequence
Example:
$ seq 1 2 10
1 3 5 7 9
Only with first and last:
$ seq 1 5
1 2 3 4 5
Only with last:
$ seq 5
1 2 3 4 5
Using {first..last..incr}
Here first and last are mandatory and incr is optional
Using just first and last
$ echo {1..5}
1 2 3 4 5
Using incr
$ echo {1..10..2}
1 3 5 7 9
You can use this even for characters like below
$ echo {a..z}
a b c d e f g h i j k l m n o p q r s t u v w x y z
I know this question is about bash, but - just for the record - ksh93 is smarter and implements it as expected:
$ ksh -c 'i=5; for x in {1..$i}; do echo "$x"; done'
1
2
3
4
5
$ ksh -c 'echo $KSH_VERSION'
Version JM 93u+ 2012-02-29
$ bash -c 'i=5; for x in {1..$i}; do echo "$x"; done'
{1..5}
This is another way:
end=5
for i in $(bash -c "echo {1..${end}}"); do echo $i; done
If you want to stay as close as possible to the brace-expression syntax, try out the range function from bash-tricks' range.bash.
For example, all of the following will do the exact same thing as echo {1..10}:
source range.bash
one=1
ten=10
range {$one..$ten}
range $one $ten
range {1..$ten}
range {1..10}
It tries to support the native bash syntax with as few "gotchas" as possible: not only are variables supported, but the often-undesirable behavior of invalid ranges being supplied as strings (e.g. for i in {1..a}; do echo $i; done) is prevented as well.
The other answers will work in most cases, but they all have at least one of the following drawbacks:
Many of them use subshells, which can harm performance and may not be possible on some systems.
Many of them rely on external programs. Even seq is a binary which must be installed to be used, must be loaded by bash, and must contain the program you expect, for it to work in this case. Ubiquitous or not, that's a lot more to rely on than just the Bash language itself.
Solutions that do use only native Bash functionality, like #ephemient's, will not work on alphabetic ranges, like {a..z}; brace expansion will. The question was about ranges of numbers, though, so this is a quibble.
Most of them aren't visually similar to the {1..10} brace-expanded range syntax, so programs that use both may be a tiny bit harder to read.
#bobbogo's answer uses some of the familiar syntax, but does something unexpected if the $END variable is not a valid range "bookend" for the other side of the range. If END=a, for example, an error will not occur and the verbatim value {1..a} will be echoed. This is the default behavior of Bash, as well--it is just often unexpected.
Disclaimer: I am the author of the linked code.
These are all nice but seq is supposedly deprecated and most only work with numeric ranges.
If you enclose your for loop in double quotes, the start and end variables will be dereferenced when you echo the string, and you can ship the string right back to BASH for execution. $i needs to be escaped with \'s so it is NOT evaluated before being sent to the subshell.
RANGE_START=a
RANGE_END=z
echo -e "for i in {$RANGE_START..$RANGE_END}; do echo \\${i}; done" | bash
This output can also be assigned to a variable:
VAR=`echo -e "for i in {$RANGE_START..$RANGE_END}; do echo \\${i}; done" | bash`
The only "overhead" this should generate should be the second instance of bash so it should be suitable for intensive operations.
Replace {} with (( )):
tmpstart=0;
tmpend=4;
for (( i=$tmpstart; i<=$tmpend; i++ )) ; do
echo $i ;
done
Yields:
0
1
2
3
4
If you're doing shell commands and you (like I) have a fetish for pipelining, this one is good:
seq 1 $END | xargs -I {} echo {}
if you don't wanna use 'seq' or 'eval' or jot or arithmetic expansion format eg. for ((i=1;i<=END;i++)), or other loops eg. while, and you don't wanna 'printf' and happy to 'echo' only, then this simple workaround might fit your budget:
a=1; b=5; d='for i in {'$a'..'$b'}; do echo -n "$i"; done;' echo "$d" | bash
PS: My bash doesn't have 'seq' command anyway.
Tested on Mac OSX 10.6.8, Bash 3.2.48
This works in Bash and Korn, also can go from higher to lower numbers. Probably not fastest or prettiest but works well enough. Handles negatives too.
function num_range {
# Return a range of whole numbers from beginning value to ending value.
# >>> num_range start end
# start: Whole number to start with.
# end: Whole number to end with.
typeset s e v
s=${1}
e=${2}
if (( ${e} >= ${s} )); then
v=${s}
while (( ${v} <= ${e} )); do
echo ${v}
((v=v+1))
done
elif (( ${e} < ${s} )); then
v=${s}
while (( ${v} >= ${e} )); do
echo ${v}
((v=v-1))
done
fi
}
function test_num_range {
num_range 1 3 | egrep "1|2|3" | assert_lc 3
num_range 1 3 | head -1 | assert_eq 1
num_range -1 1 | head -1 | assert_eq "-1"
num_range 3 1 | egrep "1|2|3" | assert_lc 3
num_range 3 1 | head -1 | assert_eq 3
num_range 1 -1 | tail -1 | assert_eq "-1"
}
How do I iterate over a range of numbers in Bash when the range is given by a variable?
I know I can do this (called "sequence expression" in the Bash documentation):
for i in {1..5}; do echo $i; done
Which gives:
1
2
3
4
5
Yet, how can I replace either of the range endpoints with a variable? This doesn't work:
END=5
for i in {1..$END}; do echo $i; done
Which prints:
{1..5}
for i in $(seq 1 $END); do echo $i; done
edit: I prefer seq over the other methods because I can actually remember it ;)
The seq method is the simplest, but Bash has built-in arithmetic evaluation.
END=5
for ((i=1;i<=END;i++)); do
echo $i
done
# ==> outputs 1 2 3 4 5 on separate lines
The for ((expr1;expr2;expr3)); construct works just like for (expr1;expr2;expr3) in C and similar languages, and like other ((expr)) cases, Bash treats them as arithmetic.
discussion
Using seq is fine, as Jiaaro suggested. Pax Diablo suggested a Bash loop to avoid calling a subprocess, with the additional advantage of being more memory friendly if $END is too large. Zathrus spotted a typical bug in the loop implementation, and also hinted that since i is a text variable, continuous conversions to-and-fro numbers are performed with an associated slow-down.
integer arithmetic
This is an improved version of the Bash loop:
typeset -i i END
let END=5 i=1
while ((i<=END)); do
echo $i
…
let i++
done
If the only thing that we want is the echo, then we could write echo $((i++)).
ephemient taught me something: Bash allows for ((expr;expr;expr)) constructs. Since I've never read the whole man page for Bash (like I've done with the Korn shell (ksh) man page, and that was a long time ago), I missed that.
So,
typeset -i i END # Let's be explicit
for ((i=1;i<=END;++i)); do echo $i; done
seems to be the most memory-efficient way (it won't be necessary to allocate memory to consume seq's output, which could be a problem if END is very large), although probably not the “fastest”.
the initial question
eschercycle noted that the {a..b} Bash notation works only with literals; true, accordingly to the Bash manual. One can overcome this obstacle with a single (internal) fork() without an exec() (as is the case with calling seq, which being another image requires a fork+exec):
for i in $(eval echo "{1..$END}"); do
Both eval and echo are Bash builtins, but a fork() is required for the command substitution (the $(…) construct).
Here is why the original expression didn't work.
From man bash:
Brace expansion is performed before
any other expansions, and any
characters special to other
expansions are preserved in the
result. It is strictly textual. Bash
does not apply any syntactic
interpretation to the context of
the expansion or the text between the
braces.
So, brace expansion is something done early as a purely textual macro operation, before parameter expansion.
Shells are highly optimized hybrids between macro processors and more formal programming languages. In order to optimize the typical use cases, the language is made rather more complex and some limitations are accepted.
Recommendation
I would suggest sticking with Posix1 features. This means using for i in <list>; do, if the list is already known, otherwise, use while or seq, as in:
#!/bin/sh
limit=4
i=1; while [ $i -le $limit ]; do
echo $i
i=$(($i + 1))
done
# Or -----------------------
for i in $(seq 1 $limit); do
echo $i
done
1. Bash is a great shell and I use it interactively, but I don't put bash-isms into my scripts. Scripts might need a faster shell, a more secure one, a more embedded-style one. They might need to run on whatever is installed as /bin/sh, and then there are all the usual pro-standards arguments. Remember shellshock, aka bashdoor?
The POSIX way
If you care about portability, use the example from the POSIX standard:
i=2
end=5
while [ $i -le $end ]; do
echo $i
i=$(($i+1))
done
Output:
2
3
4
5
Things which are not POSIX:
(( )) without dollar, although it is a common extension as mentioned by POSIX itself.
[[. [ is enough here. See also: What is the difference between single and double square brackets in Bash?
for ((;;))
seq (GNU Coreutils)
{start..end}, and that cannot work with variables as mentioned by the Bash manual.
let i=i+1: POSIX 7 2. Shell Command Language does not contain the word let, and it fails on bash --posix 4.3.42
the dollar at i=$i+1 might be required, but I'm not sure. POSIX 7 2.6.4 Arithmetic Expansion says:
If the shell variable x contains a value that forms a valid integer constant, optionally including a leading plus or minus sign, then the arithmetic expansions "$((x))" and "$(($x))" shall return the same value.
but reading it literally that does not imply that $((x+1)) expands since x+1 is not a variable.
You can use
for i in $(seq $END); do echo $i; done
Another layer of indirection:
for i in $(eval echo {1..$END}); do
∶
I've combined a few of the ideas here and measured performance.
TL;DR Takeaways:
seq and {..} are really fast
for and while loops are slow
$( ) is slow
for (( ; ; )) loops are slower
$(( )) is even slower
Worrying about N numbers in memory (seq or {..}) is silly (at least up to 1 million.)
These are not conclusions. You would have to look at the C code behind each of these to draw conclusions. This is more about how we tend to use each of these mechanisms for looping over code. Most single operations are close enough to being the same speed that it's not going to matter in most cases. But a mechanism like for (( i=1; i<=1000000; i++ )) is many operations as you can visually see. It is also many more operations per loop than you get from for i in $(seq 1 1000000). And that may not be obvious to you, which is why doing tests like this is valuable.
Demos
# show that seq is fast
$ time (seq 1 1000000 | wc)
1000000 1000000 6888894
real 0m0.227s
user 0m0.239s
sys 0m0.008s
# show that {..} is fast
$ time (echo {1..1000000} | wc)
1 1000000 6888896
real 0m1.778s
user 0m1.735s
sys 0m0.072s
# Show that for loops (even with a : noop) are slow
$ time (for i in {1..1000000} ; do :; done | wc)
0 0 0
real 0m3.642s
user 0m3.582s
sys 0m0.057s
# show that echo is slow
$ time (for i in {1..1000000} ; do echo $i; done | wc)
1000000 1000000 6888896
real 0m7.480s
user 0m6.803s
sys 0m2.580s
$ time (for i in $(seq 1 1000000) ; do echo $i; done | wc)
1000000 1000000 6888894
real 0m7.029s
user 0m6.335s
sys 0m2.666s
# show that C-style for loops are slower
$ time (for (( i=1; i<=1000000; i++ )) ; do echo $i; done | wc)
1000000 1000000 6888896
real 0m12.391s
user 0m11.069s
sys 0m3.437s
# show that arithmetic expansion is even slower
$ time (i=1; e=1000000; while [ $i -le $e ]; do echo $i; i=$(($i+1)); done | wc)
1000000 1000000 6888896
real 0m19.696s
user 0m18.017s
sys 0m3.806s
$ time (i=1; e=1000000; while [ $i -le $e ]; do echo $i; ((i=i+1)); done | wc)
1000000 1000000 6888896
real 0m18.629s
user 0m16.843s
sys 0m3.936s
$ time (i=1; e=1000000; while [ $i -le $e ]; do echo $((i++)); done | wc)
1000000 1000000 6888896
real 0m17.012s
user 0m15.319s
sys 0m3.906s
# even a noop is slow
$ time (i=1; e=1000000; while [ $((i++)) -le $e ]; do :; done | wc)
0 0 0
real 0m12.679s
user 0m11.658s
sys 0m1.004s
If you need it prefix than you might like this
for ((i=7;i<=12;i++)); do echo `printf "%2.0d\n" $i |sed "s/ /0/"`;done
that will yield
07
08
09
10
11
12
If you're on BSD / OS X you can use jot instead of seq:
for i in $(jot $END); do echo $i; done
This works fine in bash:
END=5
i=1 ; while [[ $i -le $END ]] ; do
echo $i
((i = i + 1))
done
There are many ways to do this, however the ones I prefer is given below
Using seq
Synopsis from man seq
$ seq [-w] [-f format] [-s string] [-t string] [first [incr]] last
Syntax
Full command
seq first incr last
first is starting number in the sequence [is optional, by default:1]
incr is increment [is optional, by default:1]
last is the last number in the sequence
Example:
$ seq 1 2 10
1 3 5 7 9
Only with first and last:
$ seq 1 5
1 2 3 4 5
Only with last:
$ seq 5
1 2 3 4 5
Using {first..last..incr}
Here first and last are mandatory and incr is optional
Using just first and last
$ echo {1..5}
1 2 3 4 5
Using incr
$ echo {1..10..2}
1 3 5 7 9
You can use this even for characters like below
$ echo {a..z}
a b c d e f g h i j k l m n o p q r s t u v w x y z
I know this question is about bash, but - just for the record - ksh93 is smarter and implements it as expected:
$ ksh -c 'i=5; for x in {1..$i}; do echo "$x"; done'
1
2
3
4
5
$ ksh -c 'echo $KSH_VERSION'
Version JM 93u+ 2012-02-29
$ bash -c 'i=5; for x in {1..$i}; do echo "$x"; done'
{1..5}
This is another way:
end=5
for i in $(bash -c "echo {1..${end}}"); do echo $i; done
If you want to stay as close as possible to the brace-expression syntax, try out the range function from bash-tricks' range.bash.
For example, all of the following will do the exact same thing as echo {1..10}:
source range.bash
one=1
ten=10
range {$one..$ten}
range $one $ten
range {1..$ten}
range {1..10}
It tries to support the native bash syntax with as few "gotchas" as possible: not only are variables supported, but the often-undesirable behavior of invalid ranges being supplied as strings (e.g. for i in {1..a}; do echo $i; done) is prevented as well.
The other answers will work in most cases, but they all have at least one of the following drawbacks:
Many of them use subshells, which can harm performance and may not be possible on some systems.
Many of them rely on external programs. Even seq is a binary which must be installed to be used, must be loaded by bash, and must contain the program you expect, for it to work in this case. Ubiquitous or not, that's a lot more to rely on than just the Bash language itself.
Solutions that do use only native Bash functionality, like #ephemient's, will not work on alphabetic ranges, like {a..z}; brace expansion will. The question was about ranges of numbers, though, so this is a quibble.
Most of them aren't visually similar to the {1..10} brace-expanded range syntax, so programs that use both may be a tiny bit harder to read.
#bobbogo's answer uses some of the familiar syntax, but does something unexpected if the $END variable is not a valid range "bookend" for the other side of the range. If END=a, for example, an error will not occur and the verbatim value {1..a} will be echoed. This is the default behavior of Bash, as well--it is just often unexpected.
Disclaimer: I am the author of the linked code.
These are all nice but seq is supposedly deprecated and most only work with numeric ranges.
If you enclose your for loop in double quotes, the start and end variables will be dereferenced when you echo the string, and you can ship the string right back to BASH for execution. $i needs to be escaped with \'s so it is NOT evaluated before being sent to the subshell.
RANGE_START=a
RANGE_END=z
echo -e "for i in {$RANGE_START..$RANGE_END}; do echo \\${i}; done" | bash
This output can also be assigned to a variable:
VAR=`echo -e "for i in {$RANGE_START..$RANGE_END}; do echo \\${i}; done" | bash`
The only "overhead" this should generate should be the second instance of bash so it should be suitable for intensive operations.
Replace {} with (( )):
tmpstart=0;
tmpend=4;
for (( i=$tmpstart; i<=$tmpend; i++ )) ; do
echo $i ;
done
Yields:
0
1
2
3
4
If you're doing shell commands and you (like I) have a fetish for pipelining, this one is good:
seq 1 $END | xargs -I {} echo {}
if you don't wanna use 'seq' or 'eval' or jot or arithmetic expansion format eg. for ((i=1;i<=END;i++)), or other loops eg. while, and you don't wanna 'printf' and happy to 'echo' only, then this simple workaround might fit your budget:
a=1; b=5; d='for i in {'$a'..'$b'}; do echo -n "$i"; done;' echo "$d" | bash
PS: My bash doesn't have 'seq' command anyway.
Tested on Mac OSX 10.6.8, Bash 3.2.48
This works in Bash and Korn, also can go from higher to lower numbers. Probably not fastest or prettiest but works well enough. Handles negatives too.
function num_range {
# Return a range of whole numbers from beginning value to ending value.
# >>> num_range start end
# start: Whole number to start with.
# end: Whole number to end with.
typeset s e v
s=${1}
e=${2}
if (( ${e} >= ${s} )); then
v=${s}
while (( ${v} <= ${e} )); do
echo ${v}
((v=v+1))
done
elif (( ${e} < ${s} )); then
v=${s}
while (( ${v} >= ${e} )); do
echo ${v}
((v=v-1))
done
fi
}
function test_num_range {
num_range 1 3 | egrep "1|2|3" | assert_lc 3
num_range 1 3 | head -1 | assert_eq 1
num_range -1 1 | head -1 | assert_eq "-1"
num_range 3 1 | egrep "1|2|3" | assert_lc 3
num_range 3 1 | head -1 | assert_eq 3
num_range 1 -1 | tail -1 | assert_eq "-1"
}
My Syst admin prof just started teaching us bash and he wanted us to write a bash script using grep to find all 3-45 letter palindromes in the linux dictionary without using reverse. And im getting an error on my if statement saying im missing a '
UPDATED CODE:
front='\([a-z]\)'
front_s='\([a-z]\)'
numcheck=1
back='\1'
middle='[a-z]'
count=3
while [ $count -ne "45" ]; do
if [[ $(($count % 2)) == 0 ]]
then
front=$front$front_s
back=+"\\$numcheck$back"
grep "^$front$back$" /usr/share/dict/words
count=$((count+1))
else
grep "^$front$middle$back$" /usr/share/dict/words
numcheck=$((numcheck+1))
count=$((count+1))
fi
done
You have four obvious problems here:
First about a misplaced and unescaped backslash:
back="\\$numcheck$back" # and not back="$numcheck\$back"
Second is that you only want to increment numcheck if count is odd.
Third: in the line
front=$front$front
you're doubling the number of patterns in front! hey, that yields an exponential growth, hence the explosion Argument list too long. To fix this: add a variable, say, front_step:
front_step='\([a-z]\)'
front=$front_step
and when you increment front:
front=$front$front_step
With these fixed, you should be good!
The fourth flaw is that grep's back-references may only have one digit: from man grep:
Back References and Subexpressions
The back-reference \n, where n is a single digit, matches the substring
previously matched by the nth parenthesized subexpression of the
regular expression.
In your approach, we'll need up to 22 back-references. That's too much for grep. I doubt there are any such long palindromes, though.
Also, you're grepping the file 43 times… that's a bit too much.
Try this:
#!/bin/bash
for w in `grep -E "^[[:alnum:]]{3,45}$" /usr/share/dict/words`; do if [[ "$w" == "`echo $w|sed "s/\(.\)/\1\n/g"|tac|tr -d '\012'`" ]]; then echo "$w == is a palindrome"; fi; done
OR
#!/bin/bash
front='\([a-z]\)'
numcheck=1
back='\1'
middle='[a-z]'
count=3
while [ $count -ne "45" ]; do
if [[ $(($count % 2)) == 0 ]]
then
front=$front$front
back="\\$numcheck$back"
grep "^$front$back$" /usr/share/dict/words
else
grep "^$front$middle$back$" /usr/share/dict/words
## Thanks to gniourf for catching this.
numcheck=$((numcheck+1))
fi
count=$((count+1))
## Uncomment the following if you want to see one by one and run script using bash -x filename.sh
#echo Press any key to continue: ; read toratora;
done
I've reviewing some bash scripts written by other people at work and I found this line that I'm trying to understand
[[ $(awk 'BEGIN{print ('$CAPACITY'>=0.9)}') -eq 1 ]] && echo "Capacity at 90 Percent"
Is my understanding that this line is replacing an if statement. Could someone help me out explaining what this line really does. Thanks
This makes me very sad and pessimistic about the future of civilization...
Let's break this down:
[[ $(awk 'BEGIN{print ('$CAPACITY'>=0.9)}') -eq 1 ]] && echo "Capacity at 90 Percent"
Note the $(....). This tells the shell to execute the program inside, and replace the contents of $(...) with the value. For example:
$ file_name="/usr/local/bin/foo"
$ short_name="$(basename $file_name)"
$ echo $short_name
foo
In this the second line, we are running the command basename $file_name. This returns foo. Then, the shell will substitute foo for $(basename $filename) before assigning short_name. Here's the same thing with the debugger on:
$ set -xv
$ file_name="/usr/local/bin/foo"
foo=/usr/local/bin/foo
+ foo=/usr/local/bin/foo
$ short_name=$(basename $file_name)
short_name=$(basename $file_name)
basename $file_name
++ basename /usr/local/bin/foo
+ short_name=foo
$ echo $short_name
echo $short_name
+ echo foo
foo
$ set +xv # Turn off the debugger.
You can see how the shell executes $(...) and replaces it.
Thus, the user is actually running the program:
awk 'BEGIN{print ('$CAPACITY'>=0.9)}'
However, take a look at the quotation marks:
awk 'BEGIN{print ('$CAPACITY'>=0.9)}'
+++++++++++++ +++++++
The stuff with the pluses under it are part of the awk command inside single quotes and thus cannot be interpolated by the shell. HOWEVER, $CAPACITY is not in quotes. In other words, the value of $CAPACITY replaces that variable before the awk command is executed. Thus, if $CAPACITY is .8, the awk command will become:
awk 'BEGIN{print ('.8'>=0.9)}`
That's the very first part of the explanation.
Now on to the next part. How much do you know about awk?
Awk is a programming language that's usually part of Unix/Linux distributions. Awk normally works on files and assumes a loop around the file with each line being read in and operated on. For example:
$ awk '{print $1}` foo.txt
Let's assume that each line in foo.txt consists of several fields that are separated by spaces. The file foo.txt is read in and each line is passed through to the awk program and the awk program will print out the first field of each line.
However, there is no file for awk to operate on. This developer is using the special patter BEGIN. This is executed before any lines are read in. Since there is no file for awk to process, and there is no actual awk program (only a BEGIN statement), awk will execute this statement (assuming capacity is at 80%:
.8 >= .9
Like in Shell and other programming languages. This statement will evaluate as true or false. In awk, if this statement is true, it will a non-zero value (we hope 1). If it is false, it will equal zero. In this case, it will equal false.
Awk returns (like Perl) the last value it executes. Thus, if the capacity is at 80%, the awk statement .8 >= .9 will be false. Awk will return a zero.
Now, the entire $([[ $(awk 'BEGIN{print ('$CAPACITY'>=0.9)}') will be replaced with 0. Your [[ ... ]] test now becomes:
[[ 0 -eq 1 ]] && echo "Capacity at 90 Percent"
Well, [[ 0 -eq 1 ]] is false.
Now the final part.
The two commands && and || are list operators. Their name comes from the C programming operators of the same name, and the way C short circuits tests. For example,
if ( ( bar > 20 ) && ( foo < 30 ) ) {
is a typical C if statement. with foo and bar being variables. I am asking if bar is greater than 20 AND if foo is less than 30 to do something.
C will first evaluate bar > 20 and decide whether it is true or false. If bar > 20 is false, there's is no need to test foo < 30 because no matter what the results are, the statement is still false. What if bar is indeed greater than 20? You have to run the next part of the if statement.
Imagine this:
if ( ( bar > 20 ) || ( foo < 30 ) ) {
This says if bar is greater than 20 OR foo < 30. In this case, C will evaluate whether bar is greater than 20. If it is, there is no need to test whether or not foo is less than 30. The statement will be true no matter what the value of foo is. What if bar isn't greater than 20? Then, I have to test the value of foo.
So, if I have && and the first statement is false, don't do the second statement (the entire expression is false anyway). If the first statement is true, I have to run the second statement (because I don't know whether or not that entire statement is true or not).
If I have ||, the complete opposite happens. If the first statement is true, don't do the second statement (because the entire expression is true). If that first statement is false, I have to run the second statement.
The gist of this is:
[ "$foo" = "$bar" ] && echo "Foo equals bar"
is the same as:
if [ "$foo" = "$bar" ]
then
echo "Foo equals bar"
fi
Because if $foo does equal $bar, I have to execute the second part of the statement!
And, this:
[ "$foo" = "$bar" ] || echo "Foo and Bar are not equal"
is the same as:
if [ "$foo" != "$bar" ]
then
echo "Foo and Bar are not equal"
fi
So, first the shell substitutes in the value of the shell variable $CAPACITY into your little awk script.
Next the awk script runs testing whether or not the substituted value of $CAPACITY is greater than or equal to 0.9. Since there is no actual awk program, awk doesn't attempt to read in from STDIN.
Next, awk will assign a zero or non-zero value to that boolean statement (depending whether or not it's true). Then, the awk program will exit with the evaluated value of that boolean statement.
The shell now substitutes that zero or non-zero value for that entire $(...) phrase. This is run through a test to see if it is or isn't equal to 1.
Finally if that test statement is equal to 1, the && will tell the shell to evaluate that echo statement. Thus, if the shell variable $CAPACITY is .9 or greater, that echo statement will print.
That's a lot of machinations just to compare .8 (or whatever the capacity is) with .9, so why did the developer do this?
Probably because BASH shell can only do integer arithmetic. Since $CAPACITY is less than one, you can't do this:
if [[ $CAPACITY -le .9 ]]
then
echo "Capacity is at 90%"
fi
Instead of using awk, I would probably have used bc:
OVER_CAPACITY=$(bc <<<"$CAPACITY >= .9")
if [[ ! $OVER_CAPACITY -eq $(true) ]]
then
echo "Capacity is over 90%"
else
echo "Every thing is okay"
fi
It would have been a few more lines, but I hope it makes things a bit easier to understand and make the file easier to maintain.
The complete line can be thought of as [[ if something is true ]] &&=then do another thing
To understand what is going on in this code, turn on your mental shell parser, and find the innermost construct that will produce output. in this case
awk 'BEGIN{print ('$CAPACITY'>=0.9)}'
execute that on a cmd-line by itself. Obviously the variable CAPACITY has to be set with a value.
Then you can use the shell debug/trace facility (set -vx) to see every thing executing
CAPACITY=0.95
set -vx
[[ $(awk 'BEGIN{print ('$CAPACITY'>=0.9)}') -eq 1 ]] && echo "Capacity at 90 Percent"
+ awk 'BEGIN{print (0.95>=0.9)}'
+ [[ 1 -eq 1 ]]
+ echo 'Capacity at 90 Percent'
Capacity at 90 Percent
IHTH
It's not, the [[ and ]] are an improvement upon the test builtin and the && is an AND
So, what this line is doing equivalent to:
if [[ $(awk 'BEGIN{print ('$CAPACITY'>=0.9)}') -eq 1 ]] ; then
echo "Capacity at 90 Percent"
fi
In effect, the line is saying TEST this condition AND do this other thing only if it's TRUE
Similarly, you could do [[ something_to_test ]] || do this if something_to_test is false
which means, TEST this condition OR do this other thing
These are bash shell one-line shortcuts.
You got a lot of good explanations, now rewrite the whole thing as:
awk -v cap="$CAPACITY" 'BEGIN{ if (cap>=0.9) print "Capacity at 90 Percent" }'
for clarity and simplicity.
[[ .... ]] construct returns true or false.
so the exp in [[ .... ]] must be a logic operation