apps/system/grep: Add standalone grep with regexp support

[Junbo-Zheng]

Summary

Add a standalone grep application under apps/system/grep/, registered as a built-in NSH app via nuttx_add_application.

The implementation uses the NuttX libc regex library (regcomp / regexec) rather than substring matching, so it supports real regular expressions including anchors and character classes. Supported options:

• -i (case-insensitive), -n (line numbers), -v (invert match),
• -r (recursive directory search), -H (always print filename) and
• -h (never print filename), plus reading from stdin when no file is given. Exit codes follow POSIX grep: 0 on match, 1 on no match,
• 2 on error.

SYSTEM_GREP does select LIBC_REGEX, so enabling CONFIG_SYSTEM_GREP=y alone is sufficient -- the regex dependency and its ALLOW_MIT_COMPONENTS gate are pulled in automatically by Kconfig, avoiding a link-time undefined reference to regcomp if a user enables grep without the regex library.

Impact

• Users: Adds a new grep builtin command to NSH; no existing command or behavior is changed.
• Build: Adds a new apps/system/grep/ app discovered via the existing system/*/Make.defs wildcard; no shared build files are modified. New CONFIG_SYSTEM_GREP Kconfig option (default n).
• Hardware: None -- host-side sim and any board with NSH + a filesystem.
• Documentation: None -- usage follows standard grep conventions.
• Security & Compatibility: None -- opt-in (default n), no change to existing symbols or behavior.

Testing

Built and ran on sim:nsh (Ubuntu 22.04 host, gcc):

cd nuttx
tools/configure.sh sim:nsh
enable CONFIG_SYSTEM_GREP=y

make olddefconfig
make -j
./nuttx

help lists grep among the builtin apps:

Builtin Apps:

    dd  gcov  grep  nsh  sh

Functional matrix verified inside NSH against a 4-line test file:

command	result
grep error	case-sensitive match only
grep -i error	case-insensitive, both cases
grep -in error	matches with line numbers
grep -v error	inverted, non-matching lines
grep "^ERR"	regex anchor, line-head only
grep nomatch	no output, exit code 1

Pipe usage verified with real NSH commands -- grep works as a pipe consumer reading stdin from other commands:

nsh> 
nsh> 
nsh> 
nsh> 
nsh> ps
  TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK            STACK COMMAND
    0     0     0   0 FIFO     Kthread   - Ready              0000000000000000 0069584 Idle_Task
    1     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067448 sim_loop_wq 0x7f4d736003f0 0x7f4d73600470
    2     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067472 hpwork 0x40170fe0 0x40171060
    4     4     0 100 FIFO     Task      - Running            0000000000000000 0067496 nsh_main
nsh> 
nsh> 
nsh> 
nsh> ps | grep work
sh [7:100]
    2     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067472 hpwork 0x40170fe0 0x40171060
    8     8     4 100 FIFO     Task      - Ready              0000000000000000 0067488 grep work
nsh> 
nsh> 
nsh> 
nsh> ps | grep sim
sh [9:100]
    1     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067448 sim_loop_wq 0x7f4d736003f0 0x7f4d73600470
   10    10     4 100 FIFO     Task      - Ready              0000000000000000 0067488 grep sim
nsh> 
nsh> 
nsh> uname -a
NuttX 12.13.0 0790b52e64 Aug 20 2026 23:30:32 sim sim
nsh> 
nsh> help
help usage:  help [-v] [<cmd>]

    .           dirname     free        mount       set         umount      
    [           date        help        mv          kill        unset       
    ?           df          hexdump     pidof       pkill       uptime      
    alias       dmesg       losetup     poweroff    sleep       watch       
    unalias     echo        ln          quit        usleep      xd          
    basename    env         ls          printf      source      wait        
    break       exec        du          ps          test        
    cat         exit        mkdir       pwd         time        
    cd          expr        mkfatfs     readlink    true        
    cp          false       mkfifo      rm          truncate    
    cmp         fdinfo      mkrd        rmdir       uname       

Builtin Apps:
    dd           gcov         grep         nsh          sh           
    dumpstack    gpio         hello        ostest       
nsh> 

Signed-off-by: Junbo Zheng zhengjunbo1@xiaomi.com