ios_system: Drop-in replacement for system() in iOS programs
When porting Unix utilities to iOS (vim, TeX, python...), sometimes the source code executes system commands, using system() calls. These calls are rejected at compile time, with: error: 'system' is unavailable: not available on iOS.
This project provides a drop-in replacement for system(). Simply add the following lines at the beginning of you header file:
extern int ios_system(char* cmd);
#define system ios_system
link with the ios_system.framework, and your calls to system() will be handled by this framework.
Commands available: shell commands (ls, cp, rm...), archive commands (curl, scp, sftp, tar, gzip, compress...) plus a few interpreted languages (python, lua, TeX). Scripts written in one of the interpreted languages are also executed, if they are in the $PATH.
The commands available are defined in two dictionaries, Resources/commandDictionary.plist and Resources/extraCommandsDictionary.plist. At startup time, ios_system loads these dictionaries and enables the commands defined inside. You will need to add these two dictionaries to the "Copy Bundle Resources" step in your Xcode project.
Each command is defined inside a framework. The framework is loaded when the command is called, and released after the command exits. Frameworks for small commands are in this project. Frameworks for interpreted languages are larger, and available separately: python, lua and TeX.
Network-based commands (nslookup, dig, host, ping, telnet) are also available as a separate framework, network_ios. Place the compiled library with the other libraries and add it to the embedded libraries of your application.
This ios_system framework has been successfully integrated into four shells, Blink, OpenTerm, Pisth and LibTerm, an editor, iVim and a TeX-writing app, TeXable. Each time, it provides a Unix look-and-feel (well, mostly feel).
Issues: In iOS, you cannot write in the ~ directory, only in ~/Documents/, ~/Library/ and ~/tmp. Most Unix programs assume the configuration files are in $HOME. So either you redefine $HOME to ~/Documents/ or you set configuration variables (using setenv) to some other place. This is done in the initializeEnvironment() function.
Here's what I have:
setenv PATH = $PATH:~/Library/bin:~/Documents/bin
setenv PYTHONHOME = $HOME/Library/
setenv SSH_HOME = $HOME/Documents/
setenv CURL_HOME = $HOME/Documents/
setenv HGRCPATH = $HOME/Documents/.hgrc/
setenv SSL_CERT_FILE = $HOME/Documents/cacert.pem
Your Mileage May Vary. Note that iOS already defines $HOME and $PATH.
Installation:
The easy way: (Xcode 12 and above) ios_system is available as a set of binary frameworks. Add this project as "Swift Package dependency", and link and embed the frameworks as you need them.
The semi-hard way:
Type swift run --package-path xcfs build. This will download all the requirements (libssh2 and openssl) and build all the ios_system XcFrameworks, in the .build directory.
The hard way:
- Open the Xcode project
ios_system.xcodeprojand hit build. This will create theios_systemframework, ready to be included in your own projects. - Compile the other targets as well:
files,tar,curl,awk,shell,text,ssh_cmd. This will create the corresponding frameworks. - Alternatively, type
xcodebuild -project ios_system.xcodeproj -alltargets -sdk iphoneos -configuration Release -quietto build all the frameworks. - If you need python, lua, TeX or network_ios, download the corresponding projects and compile them. All these projects need the
ios_systemframework to compile.
Integration with your app:
- Link your application with the
ios_system.frameworkframework. - Embed (but don't link) the frameworks corresponding to the commands you need (
libtar.dylibif you needtar,libfiles.dylibfor cp, rm, mv...). - Add the two dictionaries,
Resources/commandDictionary.plistandResources/extraCommandsDictionary.plistto the "Copy Bundle Resources" step in your Xcode project.
Basic commands:
The simplest way to integrate ios_system into your app is to just replace all calls to system() with calls to ios_system(). If you need more control and information, the following functions are available:
initializeEnvironment()sets environment variables to sensible defaults.ios_executable(char* inputCmd)returns true ifinputCmdis one of the commands defined insideios_system.NSArray* commandsAsArray()returns an array with all the commands available, if you need them for helping users.NSString* commandsAsString()same, but with aNSString*.NSString* getoptString(NSString* command)returns a string containing all accepted flags for a given command ("dfiPRrvW" for "rm", for example). Letters are followed by ":" if the flag cannot be combined with others.NSString* operatesOn(NSString* command)tells you what this command expects as arguments, so you can auto-complete accordingly. Return values are "file", "directory" or "no". For example, "cd" returns "directory".int ios_setMiniRoot(NSString* mRoot)lets you set the sandbox directory, so users are not exposed to files outside the sandbox. The argument is the path to a directory. It will not be possible tocdto directories above this one. Returns 1 if succesful, 0 if not.FILE* ios_popen(const char* inputCmd, const char* type)opens a pipe between the current command andinputCmd. (drop-in replacement forpopen).
More advance control:
replaceCommand: replaceCommand(NSString* commandName, int (*newFunction)(int argc, char *argv[]), bool allOccurences) lets you replace an existing command implementation with your own, or add new commands without editing the source.
Sample use: replaceCommand(@"ls", gnu_ls_main, true);: Replaces all calls to ls to calls to gnu_ls_main. The last argument tells whether you want to replace only the function associated with ls (if false) or all the commands that used the function previously associated with ls(if true). For example, compress and uncompress are both done with the same function, compress_main (and the actual behaviour depends on argv[0]). Only you can know whether your replacement function handles both roles, or only one of them.
If the command does not already exist, your command is simply added to the list.
addCommandList: NSError* addCommandList(NSString* fileLocation) loads several commands at once, and adds them to the list of existing commands. fileLocation points to a plist file, with the same syntax as Resources/extraCommandsDictionary.plist: the key is the command name, and is followed by an Array of 4 Strings: name of the framework, name of the function to call, list of options (in getopt() format) and what the command expects as argument (file, directory, nothing). The last two can be used for autocomplete. The name of the framework can be MAIN if your command is defined in your main program (equivalent to the RTLD_MAIN_ONLY option for dlsym()), or SELF if it is defined inside ios_system.framework (equivalent to RTLD_SELF).
Example:
<key>rlogin</key>
<array>
<string>network_ios.framework/network_ios</string>
<string>rlogin_main</string>
<string>468EKLNS:X:acde:fFk:l:n:rs:uxy</string>
<string>no</string>
</array>
ios_execv(const char path, char const argv[]): executes the command in argv[0] with the arguments argv (it doesn't use path). It is not a drop-in replacement for execv because it does not terminate the current process. execv is usually called after fork(), and execv terminates the child process. This is not possible in iOS. If dup2 was called before execv to set stdin and stdout, ios_execv tries to do the right thing and pass these streams to the process started by execv.
ios_execve also exists, and stores the environment.
Adding more commands:
ios_system is OpenSource; you can extend it in any way you want. Keep in mind the intrinsic limitations:
- Sandbox and API limitations still apply. Commands that require root privilege (like
traceroute) are impossible. - Inside terminals we have limited interaction. Apps that require user input are unlikely to get it, or with no visual feedback. That could be solved, but it is hard.
To add a command:
- (Optional) create an issue: https://github.com/holzschu/ios_system/issues That will let others know you're working on it, and possibly join forces with you (that's the beauty of OpenSource).
- find the source code for the command, preferrably with BSD license. Apple OpenSource is a good place to start. Compile it first for OSX, to see if it works, and go through configuration.
- make the following changes to the code:
- change the
main()function intocommand_main(). - include
ios_error.h. - link with
ios_system.framework; this will replace most function calls byios_systemversion (exit,warn,err,errx,warnx,printf,write...) - replace calls to
isatty()with calls toios_isatty(). - usually, this is enough for your command to compile, and sometimes to run. Check that it works.
- if you have no output: find where the output happens. Within
ios_system, standard output must go tothread_stout.libc_replacement.cintercepts most of the output functions, but not all. - if you have issues with input: find where it happens. Standard input comes from
thread_stdin. - make sure you initialize all variables at startup, and release all memory on exit.
- make all global variables thread-local with
__thread, make sure local variables are marked withstatic. - make sure your code doesn't use commands that don't work in a sandbox:
fork,exec,system,popen,isExecutableFileAtPath,access... (some of these fail at compile time, others fail silently at run time). - compile the digital library, add it to the embedded frameworks of your app.
- Edit the
Resources/extraCommandsDictionary.plistto add your command, and run. - That's it.
- Test a lot. Side effects can appear after several launches.
- change the
Frequently asked commands: here is a list of commands that are often requested, and my experience with them:
ping,nslookup,telnet: now provided in the network_ios package.tracerouteand most network analysis tools: require root privilege, so impossible inside a sandbox.unzip: usetar -xz.sh,bash,zsh: shells are hard to compile, even without the sandbox/API limitations. They also tend to take a lot of memory, which is a limited asset.git: WorkingCopy does it very well, and you can transfer directories to your app, then transfer back to WorkingCopy. Also difficult to compile.
Licensing:
ios_system itself is released under the Revised BSD License (3-clause BSD license). Foe the other tools, I've used the BSD version as often as possible:
- awk: OpenSource license.
- curl, scp, sftp: MIT/X derivate license.
- lua: MIT License.
- python: Python license.
- libssh2: Revised BSD License (a.k.a. 3-clause BSD license).
- egrep, fgrep, grep, gzip, gunzip, cat, chflag, compress, cp, date, echo, env, link, ln, printenv, pwd, ed, sed, tar, uncompress, uptime, chgrp, chksum, chmod, chown, df, du, groups, id, ls, mkdir, mv, readlink, rm, rmdir, stat, sum, touch, tr, uname, wc, whoami: Revised BSD License (a.k.a. 3-clause BSD license).
- pdftex, luatex and all TeX-based programs: GNU General Public License.
Using BSD versions has consequences on the flags and how they work. For example, there are two versions of sed, the BSD version and the GNU version. They have roughly the same behaviour, but differ on -i (in place): the GNU version overwrites the file if you don't provide an extension, the BSD version won't work unless you provide the extension to use on the backup file (and will backup the input file with that extension).
The problem may be in line 1963 of ios_system.m. There is something wrong with the function pointer of the signal handler.
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