Shellcheck rewrites (#9597)

* Fix SC2001: See if you can use ${variable//search/replace} instead.

* Fix SC2010: Don't use ls | grep. Use a glob or a for loop with a condition to allow non-alphanumeric filenames.

* Fix SC2012: Use find instead of ls to better handle non-alphanumeric filenames.

* Fix SC2015: Note that A && B || C is not if-then-else. C may run when A is true.

* Fix SC2028: echo may not expand escape sequences. Use printf.

* Fix SC2034: variable appears unused. Verify use (or export if used externally).

* Fix SC2035: Use ./*glob* or -- *glob* so names with dashes won't become options.

* Fix SC2071: > is for string comparisons. Use -gt instead.

* Fix SC2154: variable is referenced but not assigned

* Fix SC2164: Use 'cd ... || exit' or 'cd ... || return' in case cd fails.

* Fix SC2188: This redirection doesn't have a command. Move to its command (or use 'true' as no-op).

* Fix SC2236: Use -n instead of ! -z.

* Fix SC2242: Can only exit with status 0-255. Other data should be written to stdout/stderr.

* Fix SC2086: Double quote to prevent globbing and word splitting.

Co-authored-by: Mehrdad <noreply@github.com>

doc/examples/plot_example-lm.rst

@@ -266,7 +266,7 @@ To start training, run `following commands <https://github.com/ray-project/ray/t
   WARMUP_UPDATES=10000       # Warmup the learning rate over this many updates
   PEAK_LR=0.0005             # Peak learning rate, adjust as needed
   TOKENS_PER_SAMPLE=512      # Max sequence length
-  MAX_POSITIONS=512          # Num. positional embeddings (usually same as above)
+  #MAX_POSITIONS=512         # Num. positional embeddings (usually same as above)
   MAX_SENTENCES=8            # Number of sequences per batch on one GPU (batch size)
   FIX_BATCH_SIZE=2048        # Number of batch size in total (max_sentences * update_freq * n_gpus)
   SAVE_INTERVAL_UPDATES=1000 # save a checkpoint every N updates