While writing BASH scripts, I generally use the which command of a Linux machine (where Linux Machine refers to Desktop based Linux OS like Ubuntu, Fedora, OpenSUSE) for finding path or availability of other binaries. I understand that which can search for binaries (commands) which are present in the PATH variable set.
Now, I am unable to understand how to proceed in case the which command itself is not present on that machine.
My intention is to create a shell script (BASH) which can be run on a machine and in case the environment is not adequate (like some command being used in script is missing), it should be able to exit gracefully.
Does any one has any suggestions in this regard. I understand there can be ways like using locate or find etc - but again, what if even they are not available. Another option which I already know is that I look for existence of a which binary on standard path like /usr/bin/ or /bin/ or /usr/local/bin/. Is there any other possibility as well?
Thanks in advance.
type which
type is a bash built-in command, so it's always available in bash. See man bash for details on it.
Note, that this will also recognize aliases:
$ alias la='ls -l -a'
$ type la
la is aliased to 'ls -l -a'
(More of a comment because Boldewyn answered perfectly, but it is another take on the question that may be of interest to some.)
If you are worried that someone may have messed with your bash installation and somehow removed which, then I suppose in theory, when you actually invoked the command you would get an exit code of 127.
Consider
$ sdgsdg
-bash: sdgsdg: command not found
$ echo $?
127
Exit codes in bash: http://tldp.org/LDP/abs/html/exitcodes.html
Of course, if someone removed which, then I wouldn't trust the exit codes, either.
Related
This is a reduced example of a makefile which illustrates my problem:
exec:
time (ls > ls.txt; echo $$? > code) 2> time.txt
make exec runs fine under one Linux installation:
Linux-2.6.32-642.4.2.el6.x86_64-x86_64-with-centos-6.8-Final
but it fails under my Ubuntu installation:
Linux-4.4.0-64-generic-x86_64-with-Ubuntu-16.04-xenial
and produces the message:
/bin/sh: 1: Syntax error: word unexpected (expecting ")")
No problems if I run the command time directly from the terminal.
Are there different versions of the command in different Linux installations? I need the version which allows a sequence of commands.
Make always invokes /bin/sh to run the recipe. On some systems, /bin/sh is an alias for bash which has a lot of extra extensions to the standard POSIX shell (sh). On other systems (like Ubuntu), /bin/sh is an alias for dash which is a smaller, simpler, closer to plain POSIX shell.
Bash has a built-in time operation which accepts an entire pipeline and shows the time taken for it (run help time at a bash shell command prompt to see documentation). Other shells like dash don't have a built-in time, so when you run it you get the program /usr/bin/time; run man time to see documentation. As a separate program it of course cannot time an entire pipeline (because a pipeline is a feature of the shell); it can only time one individual command.
You have various options:
You can force your makefile to always use bash as its shell by adding:
SHELL := /bin/bash
to it. I recommend adding a comment there as well describing why bash specifically is needed.
Or you can modify your rule to work in a portable way by making the shell invocation explicit so that time only has one command to invoke:
exec:
time /bin/sh -c 'ls > ls.txt; echo $$? > code' 2>/time.txt
Put a semicolon in front of "time". As is, make is trying to parse your command as a list of dependencies.
The only suggestion that worked is to force bash in my makefile:
SHELL := /bin/bash
I checked: on my Ubuntu machine, /bin/sh is really /bin/dash whereas on the CentOS machine it is /bin/bash!
Thanks!
when I type the command stats.awk size.cvs, it says command not found on my virtual machine linux. Can someone help me get through sice Im new to linux and linux coding
It looks like you are not trying to run any standard Linux tool, but a script written by someone else called "stats.awk"... and the file you want to operate on is called "size.cvs" (it is more likely called size.csv -- as this is likely a "Comma Separated Value" file). Most Linux implementations do not allow you to just run a script unless the script is in your PATH and has the right permissions.
You could try "fully qualifying" the path of your awk script, but probably easiest is to just feed it into awk. Try:
awk -f stats.awk size.cvs
To find out more information about your shell run:
man $SHELL
Or just Google for a tutorial on your shell (which is likely bash). To find more about awk, again, try Googling for a tutorial on awk.
In the middle of my perl script I want to execute a bash command. The script takes a long time, so at the beginning of the script I want to see if the command exists. This answer says to just try and run it and this other answer suggests some bash commands to test if the program exists.
Is the latter option the best solution? Are there any better ways to do this check in perl?
My best guess is that you want to check for existence of an executable file that you want to run using system or qx//
But if you want your command line to behave the same way as the shell, then you can probably use File::Which
What if we assume that we don't know the command's location?
This means that syck's answer won't work, and zdim's answer is incomplete.
Try this function in perl:
sub check_exists_command {
my $check = `sh -c 'command -v $_[0]'`;
return $check;
}
# two examples
check_exists_command 'pgrep' or die "$0 requires pgrep";
check_exists_command 'readlink' or die "$0 requires readlink";
I just tested it, because I just wrote it.
With perl, you can test files for existence, readability, executability etc., take a look here.
Therefore just use
executeBashStuff() if -x $filename;
or stat it:
stat($filename);
executeBashStuff() if -x _;
To me a better check is to run the program at the beginning of the script (with -V say).
I'd use the same invocation as you use to run the job later (via shell or not, via execvp). Once at it, make sure to see whether it threw errors. This is also discussed in your link but I would in fact get the output back (not send it away) and check that. This is the surest way to see whether the thing actually runs out of your program and whether it is what you expect it to be.
Checking for the executable with -x (if you know the path) is useful, too, but it only tells you that a file with a given name is there and that it is executable.
The system's which seems to be beset with critism for its possible (mis)behavior, it may or may not be a shell-builtin (which complicates how exactly to use it), is an external utility, and its exact behavior is system dependent. The module File::Which pointed out in Borodin's answer would be better -- if it is indeed better than which. (What it may well be, I just don't know.)
Note. I am not sure what "bash command" means: a bash shell built-in, or the fact that you use bash when on terminal? Perl's qx and system use the sh shell, not bash (if they invoke the shell, which depends on how you use them). While sh is mostly a link, and often to bash, it may not be and there are differences, and you cannot rely on your shell configuration.
Can also actually run a shell, qx(/path/bash -c 'cmd args'), if you must. Mind the quotes. You may need to play with it to find the exact syntax on your system. See this page and links.
A predecessor of mine installed a crappy piece of software on an old machine (running Linux) which I've inherited. Said crappy piece of software installed flotsam all over the place, and also is sufficiently bloated that I want it off ASAP -- it no longer has any functional purpose since we've moved on to better software.
Vendor provided an uninstall script. Not trusting the crappy piece of software, I opened the uninstall script in an editor (a 200+ line Bash monster), and it starts off something like this:
SWROOT=`cat /etc/vendor/path.conf`
...
rm -rf $SWROOT/bin
...
It turns out that /etc/vendor/path.conf is missing. Don't know why, don't know how, but it is. If I had run this lovely little script, it would have deleted the /bin folder, which would have had rather amusing implications. Of course this script required root to run!
I've dealt with this issue by just manually running all the install commands (guh) where sensible. This kind of sucked because I had to interpolate all the commands manually. In general, is there some sort of way I can "dry run" a script to have it dump out all the commands it would execute, without it actually executing them?
bash does not offer dry-run functionality (and neither do ksh, zsh, or any other shell I know).
It seems to me that offering such a feature in a shell would be next to impossible: state changes would have to be simulated and any command invoked - whether built in or external - would have to be aware of these simulations.
The closest thing that bash, ksh, and zsh offer is the ability to syntax-check a script without executing it, via option -n:
bash -n someScript # syntax-check a script, without executing it.
If there are no syntax errors, there will be no output, and the exit code will be 0.
If there are syntax errors, analysis will stop at the first error, an error message including the line number is written to stderr, and the exit code will be:
2 in bash
3 in ksh
1 in zsh
Separately, bash, ksh, and zsh offer debugging options:
-v to print each raw source code line[1]
to stderr before it is executed.
-x to print each expanded simple command to stderr before it is executed (env. var. PS4 allows tweaking the output format).
Combining -n with -v and/or -x offers little benefit:
With -n specified, -x has no effect at all, because nothing is being executed.
With -n specified, -v will effectively simply print the source code.
If there is a syntax error, there may be benefit in the source code getting print up to the point where the error occurs; keep in mind, though that the error message produced by
-n always includes the offending line number.
[1] Typically, it is individual lines that are printed, but the true unit is however many lines a given command - which may be a compound command such as while or a command list (such as a pipeline) - spans.
You could try running the script under Kornshell. When you execute a script with ksh -D, it reads the commands and checks them for syntax, but doesn't execute them. Combine that with set -xv, and you'll print out the commands that will be executed.
You can also use set -n for the same effect. Kornshell and BASH are fairly compatible with each other. If it's a pure Bourne shell script, both Kornshell and BASH will execute it pretty much the same.
You can also run ksh -u which will cause unset shell variables to cause the script to fail. However, that wouldn't have caught the catless cat of a nonexistent file. In that case, the shell variable was set. It was set to null.
Of course, you could run the script under a restricted shell too, but that's probably not going to uninstall the package.
That's the best you can probably do.
After logging by PuTTY to Netgem n5200 device I have open in terminal kind of shell with about 40 commends, but no information about way how to close it and work directly on Linux.
Device has for 99% installed HardHat Linux distribution (MontaVista Linux).
From level of mentioned shell I can use some simple linux command like "! pwd", "! ls -l", "! ps".
Maybe it would be useful that I'm in tmp dir and I can not leave it at this moment.
I've already study Netgem SDK looking for information and I found nothing, the same result after searching in google, so that's why I came looking for help here.
Probably you can try to start the shell using
!sh
or
!bash
command.
If not, could you please provide the list of available commands (such as pwd, ls), may be some of them could be used to get a shell (e.g. vi).