execute a line from a script in the current shell environment - shell

I'd like to execute a range of lines from a shell script from a running shell and using the current shell's environment.
For example, in a script that sets a variable to something I don't want to type (say an API key or MAC address):
line
1 #!/bin/zsh
2
3 # Set the MAC address:
4 MAC="12-34-56-78-90-ab-cd"
...
...and in my shell, I'd like to grab line 4 above, run it, and have $MAC exist in that environment
I've tried sed -n '4p' script.zsh | zsh but that doesn't affect the current shell I'm working in:
$ MAC="this is not a MAC address"
$ sed -n '4p' script.zsh | zsh
$ echo $MAC
---
this is not a MAC address
I could just copy and paste, but I'd like a solution I can use without touching my mouse - or when I don't have a mouse available.

You could combine your sed command with a process substitution and source it:
source <(sed -n '4p' script.zsh)
though you might want to use a pattern match for the print line in case the line numbers shift.
source <(sed -n '/^MAC=/p' script.zsh)

Related

Reading the second (or subsequent) line in Korn Shell

My OS is AIX (7200-05-03-2136) and my Korn Shell version is ksh88 (Version M-11/16/88f), but I think my question doesn't depend on versions.
Consider a single-line output of a command. I can easily put this into a variable via "read":
command | read variable
Now, suppose the command would have a two-line output. Is there a way to capture only the second line into a variable? It would be easy to use some external program like, i.e.:
command | sed '1d' | read variable
But I would like to avoid that and find a pure shell-solution. I have tried the following variations:
command | { read -r junk ; read -r variable }
command | { IFS=\n read junk ; read variable }
command | IFS='\n' read junk variable
But all these won't work.
Assign everything to a variable first. Next you can print or assign the second line.
variable=$(echo "line 1
line 2")
echo "${variable#*$'\n'}"
# or
variable="${variable#*$'\n'}"

Bash get the command that is piping into a script

Take the following example:
ls -l | grep -i readme | ./myscript.sh
What I am trying to do is get ls -l | grep -i readme as a string variable in myscript.sh. So essentially I am trying to get the whole command before the last pipe to use inside myscript.sh.
Is this possible?
No, it's not possible.
At the OS level, pipelines are implemented with the mkfifo(), dup2(), fork() and execve() syscalls. This doesn't provide a way to tell a program what the commands connected to its stdin are. Indeed, there's not guaranteed to be a string representing a pipeline of programs being used to generate stdin at all, even if your stdin really is a FIFO connected to another program's stdout; it could be that that pipeline was generated by programs calling execve() and friends directly.
The best available workaround is to invert your process flow.
It's not what you asked for, but it's what you can get.
#!/usr/bin/env bash
printf -v cmd_str '%q ' "$#" # generate a shell command representing our arguments
while IFS= read -r line; do
printf 'Output from %s: %s\n' "$cmd_str" "$line"
done < <("$#") # actually run those arguments as a command, and read from it
...and then have your script start the things it reads input from, rather than receiving them on stdin.
...thereafter, ./yourscript ls -l, or ./yourscript sh -c 'ls -l | grep -i readme'. (Of course, never use this except as an example; see ParsingLs).
It can't be done generally, but using the history command in bash it can maybe sort of be done, provided certain conditions are met:
history has to be turned on.
Only one shell has been running, or accepting new commands, (or failing that, running myscript.sh), since the start of myscript.sh.
Since command lines with leading spaces are, by default, not saved to the history, the invoking command for myscript.sh must have no leading spaces; or that default must be changed -- see Get bash history to remember only the commands run with space prefixed.
The invoking command needs to end with a &, because without it the new command line wouldn't be added to the history until after myscript.sh was completed.
The script needs to be a bash script, (it won't work with /bin/dash), and the calling shell needs a little prep work. Sometime before the script is run first do:
shopt -s histappend
PROMPT_COMMAND="history -a; history -n"
...this makes the bash history heritable. (Code swiped from unutbu's answer to a related question.)
Then myscript.sh might go:
#!/bin/bash
history -w
printf 'calling command was: %s\n' \
"$(history | rev |
grep "$0" ~/.bash_history | tail -1)"
Test run:
echo googa | ./myscript.sh &
Output, (minus the "&" associated cruft):
calling command was: echo googa | ./myscript.sh &
The cruft can be halved by changing "&" to "& fg", but the resulting output won't include the "fg" suffix.
I think you should pass it as one string parameter like this
./myscript.sh "$(ls -l | grep -i readme)"
I think that it is possible, have a look at this example:
#!/bin/bash
result=""
while read line; do
result=$result"${line}"
done
echo $result
Now run this script using a pipe, for example:
ls -l /etc | ./script.sh
I hope that will be helpful for you :)

Why doesn't LIMIT=\`ulimit -u\` work in bash?

In my program I need to know the maximum number of process I can run. So I write a script. It works when I run it in shell but but when in program using system("./limit.sh"). I work in bash.
Here is my code:
#/bin/bash
LIMIT=\`ulimit -u\`
ACTIVE=\`ps -u | wc -l \`
echo $LIMIT > limit.txt
echo $ACTIVE >> limit.txt
Anyone can help?
Why The Original Fails
Command substitution syntax doesn't work if escaped. When you run:
LIMIT=\`ulimit -u\`
...what you're doing is running a command named
-u`
...with the environment variable named LIMIT containing the value
`ulimit
...and unless you actually have a command that starts with -u and contains a backtick in its name, this can be expected to fail.
This is because using backticks makes characters which would otherwise be syntax into literals, and running a command with one or more var=value pairs preceding it treats those pairs as variables to export in the environment for the duration of that single command.
Doing It Better
#!/bin/bash
limit=$(ulimit -u)
active=$(ps -u | wc -l)
printf '%s\n' "$limit" "$active" >limit.txt
Leave off the backticks.
Use modern $() command substitution syntax.
Avoid multiple redirections.
Avoid all-caps names for your own variables (these names are used for variables with meaning to the OS or system; lowercase names are reserved for application use).
Doing It Right
#!/bin/bash
exec >limit.txt # open limit.txt as output for the rest of the script
ulimit -u # run ulimit -u, inheriting that FD for output
ps -u | wc -l # run your pipeline, likewise with output to the existing FD
You have a typo on the very first line: #/bin/bash should be #!/bin/bash - this is often known as a "shebang" line, for "hash" (#) + "bang" (!)
Without that syntax written correctly, the script is run through the system's default shell, which will see that line as just a comment.
As pointed out in comments, that also means only the standardised options available to the builtin ulimit command, which doesn't include -u.

Serialize a subset of environment variables

I'm trying to export some environment variables for use by a TomCat process.
There's a few ways to do this (I know how to solve the overall problem), but it bugged me that I didn't know how to do this particular shell task.
Tomcat recommends that all your environment customizations should be exported by "$CATALINA_HOME/bin/setenv.sh".
This whole thing is gonna be stuffed into a Docker container, so the only parameterizability will be via Docker env variables (let's assume for this task that I don't want to use volume mounts or create setenv.sh during the build process).
First, observe that docker run -e can be used to pass environment into the container:
🍔 docker run -eMY_VAR=SUP alpine env
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=a528b6fc264b
MY_VAR=SUP
no_proxy=*.local, 169.254/16
HOME=/root
If we wanted to copy all of that env into setenv.sh, it's as simple as:
SETENV="/usr/local/tomcat/bin/setenv.sh"
echo '#!/bin/sh' > "$SETENV"
echo 'export -p' >> "$SETENV"
env >> "$SETENV"
But copying everything somewhat defeats the point of setenv.sh -- which is, to give your tomcat process a clean environment, with only intentional customizations.
So, we can agree on a convention for "which env vars are ones that we want to pass through to setenv.sh". Everything prefixed with MY_.
And now we get to an interesting shell problem.
env | grep '^MY_' | sed 's/^MY_/EXPORT /'
This gets us pretty close. Output looks like:
🍔 docker run -e MY_VAR=hey alpine sh -c "env | grep '^MY_' | sed 's/^MY_/EXPORT /'"
EXPORT VAR=hey
So, we've selected from the env command: only env vars prefixed with MY_. And we can redirect that output to setenv.sh.
Why do I say "pretty close"? Looks like we're done, right?
Try this for size:
🍔 docker run -e MY_VAR='multi
quote> line
quote> string' alpine sh -c "env | grep '^MY_' | sed 's/^MY_/EXPORT /'"
EXPORT VAR=multi
The script only worked for a simple subset of possibilities. i.e. we only managed to export the first line of our multi-line string.
For your convenience: env output for multi-line strings looks like this:
🍔 docker run -e MY_VAR='multi
line
string' alpine env
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=0d0afaac6bec
MY_VAR=multi
line
string
no_proxy=*.local, 169.254/16
HOME=/root
I hesitate to try and tackle this using awk; there may be further string escaping complications that I have not considered.
I wonder whether there's a better way altogether to select & serialize a subset of exported environment?
EDIT: I negligently tagged this as a bash question, when really my intention was to pose an sh question. Specifically my intention is to get something that will work with no dependencies other than those that come with the alpine docker image. i.e. BusyBox sh, sed, grep, awk, env.
I've retained the bash tag so as not to punish the initial answer that was submitted when this was a bash-only question.
But I will give preference to an sh-compatible answer, and in particular to one that works with just the BusyBox UNIX utils.
So you need several things:
Enumerate the environment variables and select a subset.
For each selected environment variable, emit sh code that sets the variable to the desired value.
You can use export -p if you want to export all variables in a form that can be read back in, but parsing it to select only certain variables is harder. One way to make use of export -p is to unset the other variables. This only works if none of the environment variables is read-only, but you can work around that by running a separate shell instance (as opposed to a subshell).
To gather the list of variables to unset, you only need to get a superset of the list of all environment variables, and remove the ones you want to keep. You can easily do that by filtering the env output. I do that with a simple grep, you may want to use more complex code if your criteria for inclusion are more complex than “begins with a specific prefix”.
The occasional false positive due to a variable containing a newline followed by a valid variable name and an equal sign will only lead to calling unset on a non-existent variable, which does nothing. The desired variables are removed from the exclusion list, so the final output will never omit a desired variable.
excluded=$(env | LC_ALL=C sed -n 's/^\([A-Z_a-z][0-9A-Z_a-z]*\)=.*/\1/p' |
grep -v 'MY_')
sh -c 'unset $1; export -p' sh "$excluded" >setenv.sh
Dash prints an extra export PATH (with no value) if PATH was in the environment when it was invoked. If that bothers you, change sh -c … to (unset PATH; sh -c …).
Assuming GNU grep:
grep --null '^MY_' </proc/self/environ
...will emit your environment variables in NUL-delimited form (newlines intact).
Similarly, if you have bash:
while IFS= read -r -d '' vardef; do
[[ $vardef = MY_* ]] && printf '%s\0' "$vardef"
done </proc/self/environ
Note that if these variables were set in the same shell session, you may need to create a subprocess for /proc/self/environ to be updated:
(while IFS= read -r -d '' vardef; do
[[ $vardef = MY_* ]] && printf '%s\0' "$vardef"
done </proc/self/environ)
alpine image doesn't ship with bash.
You can use this script to extract all MY_* variables including newline variables:
docker run -e MY_FOO=bar -e MY_VAR="multi' export MY_INJECTED='val" -e MY_VAR2=$'multi
0MY_line=val
string' alpine sh -c "awk -v RS='\06' -F= '/^MY_/{k=\$1; sub(/^[^=]+=/, \"\");
gsub(/\047/, \"\047\\\\\\047\047\"); printf \"export %s=\047%s\047\n\", k, \$0
}' /proc/self/environ"
This will output:
export MY_FOO='bar'
export MY_VAR='multi'\'' export MY_INJECTED='\''val'
export MY_VAR2='multi
0MY_line=val
string'
Here is how awk works:
-v RS='\6': sets record separator as \6 works for nul byte as well (assuming you don't have \6 in value)
-F=: sets field separator as =
/^MY_/: Only process records starting with MY_
store variable name or $1 in variable k
Using sub function get part after = in $0
Using print format output so that it can be used in $CATALINA_HOME/bin/setenv.sh file.
\047 is for printing single quote
what about
declare -p ${!MY_*}
and
declare -p ${!MY_*} | sed -r 's/^declare (-[^ ]*)* MY_/export /'
or
declare -p ${!MY_*} | sed 's/^declare \(-[^ ]*\)* MY_/export /'
EDIT posix compliant version :
some env or printenv accept -0 option to end each output line by \0 rather than a newline. Thus
env -0 | perl -ne 'BEGIN{$/="\0";$\="\n";$q="\047"}next unless /^MY_/;chomp;s/$q/$q\\$q$q/;s/=/=$q/;s/$/$q/;print'
How it works
$/ : input record separator
$\ : output record separator
$q : variable to store single quote (\047) because of surrounding single quotes in command
next : to filter "MY_" variables
chomp : removes the input separator
s/// : quote substitution
EDIT: variation of perl version in posix shell
env -0 | xargs -0 sh -c 'for entry; do [[ $entry = MY_* ]] || continue; printf "%s=\047%s\047\n" "${entry%%=*}" "$(echo "${entry#*=}" | sed '\''s/\x27/\x27\\\x27\x27/g'\'' )"; done' -

How to execute lines of text on the clipboard as bash commands

I'm working with Mac OS X's pbpaste command, which returns the clipboard's contents. I'd like to create a shell script that executes each line returned by pbpaste as a separate bash command. For example, let's say that the clipboard's contents consists of the following lines of text:
echo 1234 >~/a.txt
echo 5678 >~/b.txt
I would like a shell script that executes each of those lines, creating the two files a.txt and b.txt in my home folder. After a fair amount of searching and trial and error, I've gotten to the point where I'm able to assign individual lines of text to a variable in a while loop with the following construct:
pbpaste | egrep -o [^$]+ | while read l; do echo $l; done
which sends the following to standard out, as expected:
echo 1234 >~/a.txt
echo 5678 >~/b.txt
Instead of simply echoing each line of text, I then try to execute them with the following construct:
pbpaste | egrep -o [^$]+ | while read l; do $l; done
I thought that this would execute each line (thus creating two text files a.txt and b.txt in my home folder). Instead, the first term (echo) seems to be interpreted as the command, and the remaining terms (nnnn >~/...) seem to get lumped together as if they were a single parameter, resulting in the following being sent to standard out without any files being created:
1234 >~/a.txt
5678 >~/b.txt
I would be grateful for any help in understanding why my construct isn't working and what changes might get it to work.
[…] the remaining terms (nnnn >~/...) seem to get lumped together as if they were a single parameter, […]
Not exactly. The line actually gets split on whitespace (or whatever $IFS specifies), but the problem is that the redirection operator > cannot be taken from a shell variable. For example, this snippet:
gt='>'
echo $gt foo.txt
will print > foo.txt, rather than printing a newline to foo.txt.
And you'll have similar problems with various other shell metacharacters, such as quotation marks.
What you need is the eval builtin, which takes a string, parses it as a shell command, and runs it:
pbpaste | egrep -o [^$]+ | while IFS= read -r LINE; do eval "$LINE"; done
(The IFS= and -r and the double-quotes around $LINE are all to prevent any other processing besides the processing performed by eval, so that e.g. whitespace inside quotation marks will be preserved.)
Another possibility, depending on the details of what you need, is simply to pipe the commands into a new instance of Bash:
pbpaste | egrep -o [^$]+ | bash
Edited to add: For that matter, it occurs to me that you can pass everything to eval in a single batch; just as you can (per your comment) write pbpaste | bash, you can also write eval "$(pbpaste)". That will support multiline while-loops and so on, while still running in the current shell (useful if you want it to be able to reference shell parameters, to set environment variables, etc., etc.).

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