I received some command line instructions:
ROOT=`cmd1 | cmd2 | cmd3`
echo ROOT=$ROOT
...
It seems that this $ROOT stores the output of the three pipelined cmds. But what is $ROOT? Is it an environment variable? Is it a register? How should I call it? Can I have other variables like it? For example $APPLE? I hope to learn more about $ROOT :)
ROOT is an environment variable, plain and simple. $ROOT is simply the expansion/evaluation of the ROOT environment variable.
For example:
xyzzy=plugh
echo xyzzy # gives xyzzy
echo $xyzzy # gives plugh
What the ROOT environment variable is used for may be important (or not) but the actual naming of it is not in doubt - it's an environment variable called ROOT.
Root is an environment variable. Variables are expanded during execution, so when echo echoes $ROOT, it is expanded to the contents of the variable. In this case, the backquotes mean "execute command", so the output of cmd1 | cmd2 | cmd3 is emitted. What is confusing about your example is that the echo command uses the variable name as text as well, perhaps this makes more sense:
ROOT=`cmd1 | cmd2 | cmd3`
echo The ROOT variable is: $ROOT
Related
This question already has answers here:
PowerShell: Setting an environment variable for a single command only
(10 answers)
Closed 4 years ago.
I know that you can pass environment variables to docker-compose.
docker-compose.yml
. . .
mysql:
image: mariadb:10.2
ports:
- "${DB_PORT}:3306"
. . .
$ DB_PORT=3396 docker-compose up
However this only works using bash. I am using PowerShell and am trying to find an equivalent that is only a one line command.
PS> $env:DB_PORT:3306 docker-compose up does not work. Neither does
multiline
$env:DB_PORT=3396 `
>> docker-compose -up
The error I get is
Unexpected token 'docker-compose' in expression or statement.
If I do it one at a time it does work...
PS> $env:DB_PORT=3396
PS> docker-compose -up
Is there not way to do this in PowerShell when the equivalent in bash is ridiculously simple?
POSIX-like shells such as bash offer a way to set environment variables in a command-scoped way, simply by prepending <varName>=<value> pairs directly to a command, as the following example demonstrates:
$ foo=bar bash -c 'echo "[$foo]"'; echo "[$foo]"
[bar]
[]
foo=bar defines environment variable foo for the bash -c '...' child process only; the next command - echo ... - does not see this variable.
PowerShell has NO equivalent construct.
The best you can do is to define the environment variable of interest first, in a separate statement, using ;, PowerShell's statement separator. Any external utility you invoke thereafter - which invariably runs in a child process - will see it, but note that the environment variable will remain in effect in the current PowerShell session, unless you manually remove it:
# Set the env. variable, call the command that should see it,
# remove it afterwards.
PS> $env:foo = 'bar'; bash -c 'echo "[$foo]"'; $env:foo = $null
[bar]
Note how $env:foo = $null i.e., setting the environment variable to $null is the same as removing it; alternatively, you could all Remove-Item env:foo
If you also want to restore a pre-existing value afterwards:
$env:foo = 'original'
# Temporarily change $env:foo to a different value, invoke the
# program that should see it, then restore the previous value.
& { $org, $env:foo = $env:foo, 'bar'; bash -c 'echo "[$foo]"'; $env:foo = $org }
$env:foo
The above yields:
[bar]
original
showing that while the bash process saw the temporary value, bar, the original value of $env:foo was restored afterwards.
Also note another important difference:
In POSIX-like shells, environment variables are implicitly surfaced as shell variables - they share the one and only namespace the shell has for variables.
By contrast, PowerShell surfaces environment variables only via the $env:<varName> namespace (e.g., $env:foo), which is distinct from the (prefix-less) namespace for PowerShell's own variables (e.g., $foo).
This question already has answers here:
PowerShell: Setting an environment variable for a single command only
(10 answers)
Closed 4 years ago.
I know that you can pass environment variables to docker-compose.
docker-compose.yml
. . .
mysql:
image: mariadb:10.2
ports:
- "${DB_PORT}:3306"
. . .
$ DB_PORT=3396 docker-compose up
However this only works using bash. I am using PowerShell and am trying to find an equivalent that is only a one line command.
PS> $env:DB_PORT:3306 docker-compose up does not work. Neither does
multiline
$env:DB_PORT=3396 `
>> docker-compose -up
The error I get is
Unexpected token 'docker-compose' in expression or statement.
If I do it one at a time it does work...
PS> $env:DB_PORT=3396
PS> docker-compose -up
Is there not way to do this in PowerShell when the equivalent in bash is ridiculously simple?
POSIX-like shells such as bash offer a way to set environment variables in a command-scoped way, simply by prepending <varName>=<value> pairs directly to a command, as the following example demonstrates:
$ foo=bar bash -c 'echo "[$foo]"'; echo "[$foo]"
[bar]
[]
foo=bar defines environment variable foo for the bash -c '...' child process only; the next command - echo ... - does not see this variable.
PowerShell has NO equivalent construct.
The best you can do is to define the environment variable of interest first, in a separate statement, using ;, PowerShell's statement separator. Any external utility you invoke thereafter - which invariably runs in a child process - will see it, but note that the environment variable will remain in effect in the current PowerShell session, unless you manually remove it:
# Set the env. variable, call the command that should see it,
# remove it afterwards.
PS> $env:foo = 'bar'; bash -c 'echo "[$foo]"'; $env:foo = $null
[bar]
Note how $env:foo = $null i.e., setting the environment variable to $null is the same as removing it; alternatively, you could all Remove-Item env:foo
If you also want to restore a pre-existing value afterwards:
$env:foo = 'original'
# Temporarily change $env:foo to a different value, invoke the
# program that should see it, then restore the previous value.
& { $org, $env:foo = $env:foo, 'bar'; bash -c 'echo "[$foo]"'; $env:foo = $org }
$env:foo
The above yields:
[bar]
original
showing that while the bash process saw the temporary value, bar, the original value of $env:foo was restored afterwards.
Also note another important difference:
In POSIX-like shells, environment variables are implicitly surfaced as shell variables - they share the one and only namespace the shell has for variables.
By contrast, PowerShell surfaces environment variables only via the $env:<varName> namespace (e.g., $env:foo), which is distinct from the (prefix-less) namespace for PowerShell's own variables (e.g., $foo).
I wrote a perl script that prints a path to a very specific file. I want to define a personal environment variables (by using setenv in .aliases file) that gives the output of this script.
For example, let's say that the file "myscript.pl" prints the path "/home/files/reports/file". Let's call the variable (that I want to define in .aliases file) 'myoutput'. I want that when I type "most $myoutput" in Unix, this file will be opened by most, and when I type "echo $myoutput", Unix will print the path.
How can I define a personal variable which value is determined by a script?
If you use bash, you can put the following to your .bashrc:
export myoutput=$(perl /path/to/myscript.pl)
For tcsh, use .cshrc instead, and modify the line to
setenv myoutput `perl /path/to/myscript.pl`
You need to start a new session to make the variable exist.
myoutput=$(perl myscript.pl)
When your script prints more, select the correct line:
myoutput=$(perl myscript.pl | grep /home)
I often see this command in node.js programs: NODE_ENV=test node app.js which sets the NODE_ENV variable to test and works. I also read here https://en.wikipedia.org/wiki/Environment_variable that this should work for any shell command, but running some tests on my own, here is what I see
$ HELLO="WORLD"
$ HELLO="MARS" echo "$HELLO"
WORLD
$
I would expect this to print MARS. Is there something I am missing here?
The syntax VAR=value command means that the command will be invoked with the environment variable VAR set to VALUE, and this will apply only for the scope of that command.
However, when you are using the command line:
HELLO="MARS" echo "$HELLO"
The shell first interprets the "$HELLO" parameter, determines that it is WORLD, and then what it actually does is run:
HELLO="MARS" echo "WORLD"
So the echo may have the HELLO variable set, but it doesn't affect what it prints - it has already been interpreted before.
Doing
HELLO="MARS"; echo "$HELLO"
does something else entirely. First it sets HELLO to MARS in the current shell, and then it goes on to interpret the echo command. By this time HELLO contains MARS, not WORLD. But this is an entirely different effect - the variable HELLO stays with the value MARS, which is not the case in the command without the ;.
Your problem is that echo is just a poor choice for a demonstartion of this. You can do other demonstrations to prove that HELLO is changed properly:
HELLO="MARS" eval 'echo $HELLO'
In this case, the shell will not interpret the $HELLO because it is within a string in single quotes. It will first put MARS in HELLO, and then call the eval 'echo $HELLO' with that variable set. The eval command with then run echo $HELLO, and you'll get the output you were expecting.
This syntax is best used for things that don't use the given variable as part of the command line, but rather use it internally.
Other answers are correct, but here a refinement :
There are 2 cases in fact when defining a list of variable separated by spaces in bash whether it ends or not with a command.
VAR1=value1 VAR2=value2 ... VARn=valuen command arg1 arg2 ... argn
and
VAR1=value1 VAR2=value2 ... VARn=valuen
don't export VAR1 ... VARn the same way.
In first case VAR1 ... VARn will be set only for command and will then not be exported to current shell.
In second case VAR1 ... VARn will alter current shell.
then ( remark that ';' is very same of using a new line )
HELLO=WORLD
HELLO=MARS echo "i don't export HELLO."
echo "HELLO=$HELLO"
will display
i don't export HELLO.
HELLO=WORLD
and
HELLO=WORLD
HELLO=MARS ; echo "i did export HELLO."
echo "HELLO=$HELLO"
will display
i did export HELLO.
HELLO=MARS
I'm a newbie to PowerShell. What's wrong with my script below? It's not wanting to emit the value of $config. However, when I wrap that command in double quotes, everything looks okay.
param($config, $logfolder)
# Must run log analysis in chronological order.
ls $logfolder | Sort-Object LastWriteTime | % {
perl D:\Websites\_awstats\wwwroot\cgi-bin\awstats.pl -LogFile="$($_.FullName)" -config=$config update
}
# Execute with - .\regen-logs.ps1 webgenesis "C:\inetpub\logs\LogFiles\W3SVC5"
# Returns for each file - Error: Couldn't open config file "awstats.config.conf" nor "awstats.conf" after searching in path "D:\Websites\_awstats\wwwroot\cgi-bin,/etc/awstats,/usr/local/etc/awstats,/etc,/etc/opt/awstats": No such file or directory
As-is, what gets emitted and executed seems to have "-config=$config" passed as an argument. At least, that's my best guess. I don't know if $_ is working correctly either.
If I put quotes around the perl command like so, I get the command I do want to execute.
ls $logfolder | Sort-Object LastWriteTime | % {
"perl D:\Websites\_awstats\wwwroot\cgi-bin\awstats.pl -LogFile=`"$($_.FullName)`" -config=$config update"
}
# Outputs for each log file something like - perl D:\Websites\_awstats\wwwroot\cgi-bin\awstats.pl -LogFile="C:\inetpub\logs\LogFiles\W3SVC5\u_ex110602.log" -config=webgenesis update
If putting quotes around it produces the correct commandline, one way to execute the contents of a string is with Invoke-Expression (alias iex):
$v = "myexe -myarg1 -myarg2=$someVar"
iex $v
Put double quotes around "-config=$config". Without this, PowerShell will interpret -config=$config as one string argument that just happens to contain a $ sign in it.
I think you need to start your perl command out with & so that PowerShell interprets things as a command and not a string.
& perl D:\Websites\_awstats\wwwroot\cgi-bin\awstats.pl -LogFile=`"$($_.FullName)`" -config=$config update
Also, see: Run a program in a foreach