A basic search for a plain text string in a file would be something like:

$ r2 -q -c "/ lib" /bin/ls
Searching 3 bytes from 0x00400000 to 0x0041ae08: 6c 69 62 
hits: 9
0x00400239 hit0_0 "lib64/ld-linux-x86-64.so.2"
0x00400f19 hit0_1 "libselinux.so.1"
0x00400fae hit0_2 "librt.so.1"
0x00400fc7 hit0_3 "libacl.so.1"
0x00401004 hit0_4 "libc.so.6"
0x004013ce hit0_5 "libc_start_main"
0x00416542 hit0_6 "libs/"
0x00417160 hit0_7 "lib/xstrtol.c"
0x00417578 hit0_8 "lib"

As can be seen from the output above, radare2 generates a "hit" flag for every entry found. You can then use the ps command to see the strings stored at the offsets marked by the flags in this group, and they will have names of the form hit0_<index>:

[0x00404888]> / ls
...
[0x00404888]> ps @ hit0_0
lseek

You can search for wide-char strings (e.g., unicode letters) using the /w command:

[0x00000000]> /w Hello
0 results found.

To perform a case-insensitive search for strings use /i:

[0x0040488f]> /i Stallman
Searching 8 bytes from 0x00400238 to 0x0040488f: 53 74 61 6c 6c 6d 61 6e
[# ]hits: 004138 < 0x0040488f  hits = 0

It is possible to specify hexadecimal escape sequences in the search string by prepending them with \x:

[0x00000000]> / \x7FELF

if, instead, you are searching for a string of hexadecimal values, you're probably better of using the /x command:

[0x00000000]> /x 7F454C46

If you want to mask some nibble during the search you can use the symbol . to allow any nibble value to match:

[0x00407354]> /x 80..80
0x0040d4b6 hit3_0 800080
0x0040d4c8 hit3_1 808080
0x004058a6 hit3_2 80fb80

You may not know some bit values of your hexadecimal pattern. Thus you may use a bit mask on your pattern. Each bit set to one in the mask indicates to search the bit value in the pattern. A bit set to zero in the mask indicates that the value of a matching value can be 0 or 1:

[0x00407354]> /x 808080:ff80ff
0x0040d4c8 hit4_0 808080
0x0040d7b0 hit4_1 808080
0x004058a6 hit4_2 80fb80

You can notice that the command /x 808080:ff00ff is equivalent to the command /x 80..80.

Once the search is done, the results are stored in the searches flag space.

[0x00000000]> fs
0    0 . strings
1    0 . symbols
2    6 . searches

[0x00000000]> f
0x00000135 512 hit0_0
0x00000b71 512 hit0_1
0x00000bad 512 hit0_2
0x00000bdd 512 hit0_3
0x00000bfb 512 hit0_4
0x00000f2a 512 hit0_5

To remove "hit" flags after you do not need them anymore, use the f- hit* command.

Often, during long search sessions, you will need to launch the latest search more than once. You can use the // command to repeat the last search.

[0x00000f2a]> //     ; repeat last search